How to Measure a Window for Blinds: The 2026 UK Guide

You’ve picked the blind style, the colour works, and you’re ready to order. Then the tape measure comes out and everything slows down.

That hesitation is sensible. A made-to-measure blind only looks custom if the measurements are right. A few millimetres the wrong way and you can end up with a blind that rubs the frame, leaves a strip of daylight down one side, or won’t clear a handle properly.

British windows make this harder than many generic guides admit. UPVC frames, older timber casements, sash windows, shallow recesses, and openings that aren’t perfectly square all change how you should measure. That’s why learning how to measure a window for blinds properly is useful beyond blinds alone. The same habits matter for any bespoke window product, including fly screens.

The Foundation of a Perfect Fit

Most measuring mistakes happen before the first number is written down. The usual pattern is simple. Someone measures the obvious width once, measures the obvious drop once, and assumes the opening is square because it looks square.

It often isn’t.

In practice, the blind doesn’t care what the window looks like from across the room. It cares what the tightest point is, where the frame pinches, and whether the hardware has room to move. That’s why this job rewards patience more than confidence.

A good way to think about it is this. You are not measuring the glass. You are not measuring what “looks about right”. You are measuring the exact space the product must live in and move through.

Practical rule: Measure for operation, not appearance. A blind that only just fits on paper often becomes a blind that catches in real life.

That’s also why professional-looking results usually come from very ordinary habits. Use the right tape. Hold it flat. Measure more than once. Record immediately. Keep width and height in the right order. Those basics sound dull, but they prevent expensive mistakes.

If you want an extra reference point before you start, this expert DIY guide on how to measure windows for blinds gives a useful companion overview. Use it as a cross-check, not a substitute for measuring your own openings carefully.

The bigger point is that precision isn’t fussy for the sake of it. It’s what separates a smooth install from a reorder. The same discipline that gives you a blind that sits cleanly in a recess is what gives you a screen that seals neatly to a frame, clears handles, and doesn’t leave awkward gaps for insects or pollen to come through.

Preparing to Measure Your Windows

A lot of measuring errors happen before the tape comes out. Someone stands in front of a UPVC window, sees a tidy recess, and assumes an inside fit will work. Then they notice the handle projects too far, the top of the opening is tighter than the bottom, or the plaster line is out by a few millimetres. That is how a straightforward blind order turns into a poor fit, and the same mistake causes trouble with made-to-measure fly screens as well.

Before you record a single width or drop, decide where the product is going to sit and whether the opening will let it operate cleanly. If you want a useful second reference on how to measure windows for blinds, use one. Still check your own window on its own merits. British homes vary too much for template thinking, especially with older sash windows, trimmed reveals, and replacement UPVC frames fitted into less-than-perfect openings.

What to have beside you

Keep the setup simple, but use the right kit.

• Steel tape measure. A rigid tape gives straighter, repeatable readings than a fabric one.
• Pencil and pad or a tablet. Write each number down immediately. Memory is where mix-ups start.
• Step stool or ladder. You need a safe look at the head of the recess and the top corners.
• A decided mount type. Measure for the fit you are ordering.
• A helper for wider openings. It makes long measurements easier to hold level and read correctly.

Check depth before you commit to a recess fit. That point gets missed all the time. On many UK windows, the width and drop look fine, but the recess is too shallow for the blind mechanism, or a handle sits proud and steals the space the blind needs to move. With fly screens, the same principle applies. A frame can measure correctly on paper and still be wrong in practice if there is not enough clearance for hardware, beads, vents, or opening sashes.

Inside mount or outside mount

An inside mount sits within the recess. An outside mount fixes to the wall, trim, or face around the opening.

This is partly a style choice, but it is mostly a practical one. If the recess is square enough, deep enough, and free of obstructions, an inside mount usually gives the neatest finish. If the opening is uneven, shallow, or cluttered with handles, trickle vents, or stays, an outside mount is often the safer option.

Consideration	Inside Mount (Recess)	Outside Mount (Exact)
Look	Cleaner, built-in appearance	More visual coverage
Best for	Good recess depth, tidy frames	Shallow recesses, uneven frames, better light control
Tolerance for wonky openings	Lower	Higher
Effect on handles and vents	Can be an issue if they project	Easier to work around
Measuring style	Measure inside the opening	Measure the area you want to cover
Common reason to choose it	You want the blind tucked into the frame	You want to hide imperfections and reduce side light gaps

What usually works best

Choose inside mount if:

• The recess has enough depth for the blind and its brackets.
• The opening is reasonably true and not pinched in one corner.
• Handles and vents sit clear of the blind’s path.
• You want a fitted look within the frame.

Choose outside mount if:

• The recess is shallow.
• Handles, vents, or stays project into the opening.
• The plaster line or frame is visibly uneven.
• You want better coverage at the sides and a straighter visual line than the recess can give you.

If there is any doubt, outside mount is usually more forgiving. That is true for blinds, and it is often true for bespoke screens too. A slightly imperfect opening can still take a made-to-measure product well, but only if you choose the mounting position that suits the actual shape of the window.

Label each window before you start. “Kitchen rear sash” beats “small one by sink” every time. It saves ordering mistakes, especially in houses where several openings look similar but measure differently.

The Core Measurement Process

Good measuring work looks dull on paper. That is usually a good sign. Clear notes, repeatable steps, and no guesswork are what stop a made-to-measure blind, or a bespoke fly screen, arriving a few millimetres out and becoming an expensive lesson.

Keep every note in W x H in mm. Do not switch to centimetres for one window and fractions for another.

Measuring for an inside mount

For a recess fit, measure the opening itself, not the glass and not the outer frame. In UK homes, especially with older plaster returns, timber boxes, and some UPVC installs, the opening is often tighter in one spot than it first appears.

Start with width. Then check height. Then check depth.

Width

Measure across the recess in three places:

• Top, just under the head of the recess
• Middle
• Bottom, close to the sill or lower frame

Use a steel tape and keep it level. If the tape bows, the reading can drift.

Write down all three figures, then use the narrowest width for a recess blind. That is the one that matters. If you order to the widest point, the blind can catch lower down where the opening pinches in. I see this regularly on older reveals and on openings that look square until you put a tape on them.

Height

Now measure the drop inside the recess in three places:

• Left
• Centre
• Right

Measure from the top inside edge to the sill, or to the exact point where you want the blind to finish. Then use the longest height.

That gives enough drop across the full opening. On many British windows, one side of the sill sits a little lower, or the head is not perfectly level, so a single centre reading is not enough.

Depth

Depth catches out plenty of DIY jobs because the opening can be wide enough and high enough, yet still be wrong for the blind.

Measure from the front of the recess back towards the glass at the actual bracket position. Check the shallowest point, not the deepest one. On UPVC windows, the bead, trickle vent, handle, or tilt-and-turn hardware often steals space. On sash windows, staff beads and catches can do the same. If the blind hardware cannot sit clear and operate freely, the width and drop are irrelevant.

Do not make your own deductions

For a recess blind, supply the exact recess measurements unless the seller states otherwise on the order form. Do not trim a few millimetres off because it feels safer. Many manufacturers build in their own allowance, and a second deduction from you creates side gaps that you cannot fix afterwards.

That rule carries over to fly screens as well. Measure the opening accurately, then order to the supplier’s measuring instructions, not to what looks about right by eye.

If you want a second check on the basic method, this guide on how to measure windows for blinds covers the same fundamentals in plain language.

Measuring for an outside mount

With an outside mount, you are choosing the finished size and position of the blind. That gives you more freedom, but it still needs proper measuring.

Measure the area you want to cover. Include the overlap you want beyond the opening. Do not measure only the glass unless the blind is meant to sit that way, which is uncommon in practice.

Width for outside mount

Measure the overall span you want the blind to cover.

A balanced overlap on both sides usually gives the cleanest result, especially on a window that is slightly out of square. In real rooms, there are often limits. A wall return on one side, a cupboard, a radiator pipe, or architrave detail may force an uneven overlap. That is acceptable if it is planned. It only looks wrong when it happens by accident.

Height for outside mount

Measure from the intended top fixing point down to the finished bottom line of the blind.

That top fixing point may be:

• Above the recess
• On the trim
• On the face of the wall

The bottom line may be:

• To the sill
• Below the sill
• In line with nearby windows for a tidier overall look

Outside mounting often solves awkward openings in British homes because it ignores small twists in the recess and clears handles more easily. It is also a practical fix where the recess depth is marginal.

Checking the opening for squareness

Width, height, and depth are the core numbers. They still do not tell the full story if the opening is racked.

Take two diagonal measurements across the recess:

• Top left to bottom right
• Top right to bottom left

If those two figures differ, the opening is out of square. A small difference is common and often manageable. A bigger difference means you should be cautious with any recess-fitted product, especially rigid systems. The same principle applies whether you are ordering blinds or a made-to-measure insect screen. A neat fit depends on the actual shape of the opening, not the shape you hoped it had.

Recording the numbers properly

Keep the notes plain and consistent. Fancy systems are not needed. Reliable ones are.

A usable record looks like this:

• Dining room right window
• Inside mount
• Width top / middle / bottom
• Height left / centre / right
• Final order size W x H in mm
• Depth
• Obstructions noted

Those last notes save orders. “Handle projects 18mm”, “trickle vent at head”, or “sill falls away on left” is the sort of information that stops a bad fit later.

The mistakes that cause most trouble

The same measuring errors come up again and again:

• Taking one width and one drop only
• Writing height before width on some windows and width before height on others
• Using the wrong final recess figure
• Skipping depth
• Forgetting handles, vents, catches, and beads
• Assuming matching windows are the same size

A careful measure takes a few extra minutes. Reordering a custom blind, or a custom screen, takes far longer and costs far more.

Adapting Measurements for Different Blind Types

A window opening can measure correctly and still produce the wrong blind if the product itself has not been allowed for. That is the part many generic guides miss.

Different blind types take up space in different ways. Some need side clearance for controls. Some build up at the top when raised. Some sit neatly within a recess on paper but clash with a handle, trickle vent or sash movement once fitted. In British homes, especially with UPVC frames, older timber casements and slightly uneven reveals, those details matter as much as the width and drop.

The rule is simple. Measure the opening first, then measure for the blind you are buying. I use the same approach for made-to-measure fly screens. The opening gives you a starting point. The hardware, fixing position and operating clearance decide whether the finished job works.

Roller blinds

Roller blinds often catch people out because the quoted blind width and the fabric width are not always the same.

The brackets, pin end and control side usually add to the overall size, while the fabric sits narrower between them. For an outside fit, that can leave more light at the edges than expected. For a recess fit, it can mean the blind technically fits but gives poorer coverage than you wanted.

Check three points before ordering:

• Whether the stated width is overall bracket-to-bracket size or fabric size
• Which side the control sits on
• How far the roll, brackets and chain mechanism project into the recess

This matters on UPVC windows with chunky handles. A roller can clear the recess width but still rub the handle every time it goes up and down. If there is any doubt, an outside fit or a face-fixed cassette can be the cleaner answer.

Venetian blinds

Venetian blinds need more thought at the top of the window than many DIYers expect.

When raised, the slats stack under the headrail. On a shallow recess, that stack can block part of the glass, sit across a vent, or interfere with how the window opens. On a sash window, it can end up right where the meeting rail or fastener needs to move.

Ask a practical question before you order. Where does the blind sit when it is fully up, not just when it is down?

That answer often decides whether a Venetian suits the recess at all. If headroom is tight, a slimmer blind type may be easier to live with day to day.

Roman blinds

Roman blinds bring bulk. Much more than many people expect from the front view alone.

The fabric folds gather into a stack at the top, and that stack projects forward into the room. On a narrow landing, above a kitchen sink, near a cupboard door or over a radiator, that projection needs checking before you commit. The blind may look right on a measurement sheet and still feel awkward once fitted.

Roman blinds often suit an outside mount because the fabric hangs better with some breathing room around it. A recess fit can work, but only if the recess depth, handle clearance and folded stack have all been allowed for.

Vertical and other headrail-based blinds

Vertical blinds, panel blinds and similar systems rely heavily on the fixing line.

The opening width is only part of the job. The headrail wants a flat, solid surface, and the vanes or panels need room to hang clear of the frame, cill and any projections below. Older reveals are a common problem here. The sides may be serviceable, but the head can be bowed, crumbly or out of level.

In that situation, face fixing above the recess is often the more reliable option. It gives a straighter line, cleaner operation and fewer surprises during fitting.

Matching the measurement to the product

A good order is based on how the blind will sit, move and stack after installation.

That principle carries across to other bespoke window products as well. A fly screen frame, for example, also needs proper clearance, a sound fixing area and enough room to operate without fouling handles or vents. People who get the best results usually do one thing well. They measure for the fitted product, not just the empty opening.

Handling Special Windows and Common Obstructions

The awkward windows are where generic advice usually falls apart. This is also where a careful DIYer can still get professional results by slowing down and measuring the specific fixing conditions rather than forcing a standard method onto a non-standard opening.

Bay and corner windows

Treat each section as its own opening unless the blind system is specifically designed to span multiple faces.

With bays, the usual issue isn’t just size. It’s clearance at the returns and corners. If two blinds sit too close to each other at an angle, the brackets, controls, or lower portions can clash.

For bay windows:

• Measure each face separately. Don’t assume the left and right sides match.
• Check projection at the corners. A blind that fits one face can still foul the adjacent one.
• Note the angle and fixing surface. The bracket position often matters more than the glass size.

If the bay is tight, slimmer blind types tend to be easier to live with than bulky folded products.

Sash windows

Sash windows need a different mindset because the frame is part of a moving system.

The blind mustn’t block sash travel, catch on furniture, or fight the meeting rail. For some setups, measuring each sash zone separately gives a cleaner result than trying to cover the whole opening with one recessed product.

What to watch on sash windows:

• The meeting rail. This can interrupt where a blind naturally wants to sit.
• Pulleys, catches, and lifts. These projections affect inside mounting.
• Limited recess depth. Older timber sashes often don’t give much room.

If the opening is shallow or the inner frame is busy with hardware, outside mounting often saves trouble.

Handles, trickle vents, and stays

These are the details that turn a neat order into a frustrating install if they are ignored.

A projecting handle can stop a blind lowering cleanly. A trickle vent at the head can steal bracket space. Restrictor stays and catches can snag on the back of a blind even when the width and height are fine.

The right way to handle obstructions is to measure them as part of the opening, not as an afterthought.

A practical check

Stand side-on to the window and look at the furthest point any hardware projects into the path of the blind.

Then ask:

• Will the blind clear it when lowered?
• Will the blind clear it when raised?
• Will the window still open if the blind is in place?

If the answer is uncertain, don’t guess. Reconsider the mount choice.

A handle that “just misses” during measuring often becomes a handle that rubs every day.

Uneven plaster, trims, and nearby walls

Outside mounts solve many recess problems, but only if the fixing surface itself is usable.

Check that the wall or trim around the opening is flat enough for the brackets to sit square. Decorative mouldings, uneven plaster, and narrow returns can shift the blind off line if you measure the desired cover area and ignore the mounting surface.

For outside fits, look beyond the window and check:

• Is there enough flat area above the opening for the top brackets?
• Will one side hit a wall return before the other?
• Will the blind finish level against the room, even if the opening does not?

That last point matters. On visibly imperfect walls, the “mathematically correct” size isn’t always the best-looking size. You are aiming for a blind that looks right in the room and operates cleanly.

Older timber windows and visibly out-of-square openings

Some older openings tell you straight away that a recess fit will be a compromise. Others only reveal it when you take the second and third readings.

If one side bows, the sill is out, or the diagonals differ enough to make you uneasy, don’t force an inside mount because you prefer the look. Outside mount is often the more professional choice. It lets the blind read straight, gives better edge coverage, and avoids constant rubbing against an imperfect reveal.

The same principle applies to many custom frame-mounted products. When the opening is honest about being irregular, the best result often comes from measuring around that fact, not pretending it isn’t there.

From Measurement to Ordering Your Bespoke Screens

A bad order often starts with a rushed note. I see it all the time. The measurements may be right, but the job still goes wrong because the room is unnamed, the handing is unclear, or nobody wrote down the vent, handle, or shallow head that changes what will fit.

Keep your notes in one clear format and use millimetres throughout. That matters in British homes, where small differences in UPVC frames, timber beads, and slightly uneven reveals can decide whether a made-to-measure product fits first time or needs remaking.

A simple record sheet works well:

• Room: Lounge
• Window: Left
• Mount: Recess
• Width readings: top / middle / bottom
• Height readings: left / centre / right
• Final order size: W x H in mm
• Depth
• Notes: handle projection, vent, shallow head, uneven sill

The costly mistakes worth checking once more

Read the order back before you submit it. Slowly.

These are the errors that cost people money:

• Wrong mount type recorded. Recess and exact fit mean different things on many order forms.
• Width and height reversed. It sounds obvious, but it still happens.
• Missing notes about obstructions or depth. The size can be correct and the product can still foul a handle, vent, bead, or sash detail.
• Ordering from the wrong figure. If the supplier asks for recess size, give the measured opening, not a self-deducted version unless their instructions say otherwise.

This part matters just as much for bespoke fly screens as it does for blinds. The principles are the same. You need the true opening size, the usable fixing area, and an honest note of anything that projects into the path of the product.

That crossover catches many DIYers out. They measure blinds as if only width and drop matter, then order screens without recording bead shape, handle clearance, or where the opening window sits within the frame. On a typical UK casement or sash, those details decide whether a screen frame sits neatly or needs a different fixing method.

Good measuring is not just about numbers. It is about giving the manufacturer enough information to make the right thing the first time.

If your notes would let another person walk into the room and understand the opening without asking you a single question, you are ready to order.

Frequently Asked Questions About Window Measuring

My window is badly out of square. What should I do

Don’t force a recess blind just because you like the cleaner look. If the opening is obviously uneven or your measurements vary enough to make clearance doubtful, an outside mount is usually the safer route. It gives you a straighter visual line and more room to clear imperfections.

Can I use the measurements from my old blinds

It’s a poor shortcut. Old blinds don’t tell you the exact opening size, and they may have been made with different tolerances or deductions. Measure the actual window again.

What does recess or exact mean on an order form

Recess usually means the true internal opening size for an inside mount. Exact usually means the finished size you want supplied for an outside mount. Read the supplier’s wording carefully, then match your measuring method to it.

Do I really need to measure in three places

Yes. One reading only tells you one point on the frame. British windows often vary from top to bottom or side to side, especially in older properties.

Should I make my own deductions for an inside fit

Not unless the order instructions specifically tell you to. For many made-to-measure products, the manufacturer applies the fitting clearance. If you deduct as well, the finished blind can come up undersized.

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If you’re ordering more than blinds and want the same made-to-measure accuracy for insect control, ventilation, or pollen reduction, Premier Screens Ltd can help. Their bespoke fly screens are built for UK homes and commercial settings, with options for windows and doors, including retractable, sliding, magnetic, hinged, and pollen mesh systems. If you’ve taken careful measurements and want a product made to fit them properly, visit Premier Screens Ltd.