How to Measure a Window for Bespoke Fly Screens

You're probably standing by an open window for a reason. You want fresh air in, but not flies in the kitchen, midges in the bedroom, or pollen drifting across the sill. That's usually when people decide to measure a window for a bespoke fly screen, and it's also the point where small mistakes start costing money.

A screen only works properly when it fits the opening it's meant for. Too tight, and it won't seat cleanly. Too loose, and you'll get gaps, rattling, or a frame that never quite sits flush. The job isn't difficult, but it does need a method.

Prepare for a Perfect Fit Before You Measure

You can lose the fit before the tape measure even opens. I see it happen when someone measures the glass, ignores a handle, or assumes every window in the house was made the same. Bespoke fly screens do not forgive that kind of shortcut.

Start by looking at the window as a working opening, not just a rectangle. The screen has to sit somewhere, clear the hardware, and still let the window do its job. On a casement, that means checking hinges, handles, and how far the sash projects. On a sash window, it means noting where the screen will sit in relation to the moving panels. On a tilt-and-turn, it means checking clearance for inward movement before you even think about final sizes.

Get the basics right first

Have these to hand:

• Steel tape measure. A rigid tape gives a truer reading across a reveal.
• Notebook or phone notes. Write each size down straight away.
• Pencil or masking tape labels. Useful if you are measuring several rooms.
• Step stool. Only if needed, and only if it lets you read the tape squarely and safely.

If you are measuring more than one opening, name each one clearly. “Back bedroom left” is useful. “Small one upstairs” is how orders get mixed up.

Check the opening before you measure it

A fly screen is only as good as the surface it has to fit against. Look for loose trim, blown plaster, chunky sealant, bowed timber, proud screw heads, and anything else that will stop the frame sitting flat. For this type of installation, guidance also notes the opening must be in sound condition. That matters more with made-to-measure screens than with off-the-shelf products, because there is less spare tolerance built in.

Then check for the details people miss on first pass:

• Handle projection
• Vent trickle covers
• Alarm contacts
• Deep beads or trims
• Uneven reveals
• Tiles or sill noses near the opening

Any one of those can change the fitting method, even if the width and height look straightforward.

Measure the space the screen must live in

Do not measure an old blind recess, the visible glass, or the outer frame unless that is the place the screen will mount. Measure the exact area the new screen frame or cassette will occupy.

That is the essential preparation work. Before taking final dimensions, confirm three things:

1. Where the screen will mount
2. Whether the surrounding surface is flat enough
3. Whether the window can still open and close without fouling the screen

That last point is where bespoke fly screens differ from generic window measuring guides. The size alone is not enough. A tidy inside fit can fail if a cockspur handle catches the frame. An outside-mounted screen can be the better job if a tilt-and-turn sash needs room to swing inward.

Good measuring starts with that decision-making. Get the mounting position and clearance checks right first, and the numbers you take will mean something.

Choosing Between an Inside and Outside Mount

The cleanest-looking fly screen isn't always the right one. Some openings suit an inside mount beautifully. Others need an outside mount because of handles, hinges, shallow reveals, or the way the sash or casement moves.

An inside mount sits within the reveal. An outside mount sits over the opening and overlaps it. Both can work well. They just solve different problems.

What an inside mount does well

Inside mounts are usually the first choice when the reveal is reasonably true and there's enough depth to house the screen frame or cassette without fouling the window's movement. They look tidy because the screen reads as part of the window rather than an added layer.

They also suit homeowners who want a minimal finish from the room side. On modern uPVC windows, that can be a very neat result.

The downside is tolerance. Inside-mounted screens have less forgiveness. If the reveal narrows, twists, or contains a protruding handle, you'll feel it quickly during fitting.

When an outside mount is the smarter option

Outside mounts are often the safer answer when the inside opening is awkward. If the reveal isn't square enough for a clean internal fit, or the window opens inward, mounting over the opening can avoid conflict with hardware and movement.

They also help where you need more coverage around the edges. That matters if your priority is sealing the opening cleanly rather than keeping the screen visually recessed.

If the opening is irregular, an outside mount can be the difference between a practical installation and a frame that never sits properly.

Inside mount vs. outside mount at a glance

Consideration	Inside Mount	Outside Mount
Appearance	Clean, built-in look within the reveal	More visible, sits over the opening
Tolerance for uneven openings	Less forgiving	More forgiving
Clearance needs	Needs enough internal depth and hardware clearance	Needs surrounding flat fixing area
Best for	Straight reveals and outward-opening windows with room inside	Inward-opening windows, awkward reveals, limited internal space
Common risk	Tight fit against handles or tapered jambs	Insufficient overlap or poor fixing surface

A simple way to choose

Use this quick filter before you measure:

• Choose inside mount if the reveal is deep enough, the screen won't clash with handles or hinges, and you want the neatest finish.
• Choose outside mount if the opening is uneven, the window opens inward, or there isn't enough room inside the reveal.
• Pause and check again if plaster, trim, sensors, trickle vents, or projecting hardware might interrupt the screen frame.

If you're torn between the two, don't choose on looks alone. The better option is the one that leaves the screen operating freely and sealing cleanly. A slightly more visible frame beats a neat-looking order that can't be fitted.

How to Take Accurate Window Measurements

The order usually goes wrong in one of two ways. The opening is measured once in the middle, or the tape is put to the wrong point and misses the tight spot that controls the fit. For a bespoke fly screen, the tight spot is what matters.

Start with the frame or reveal the screen will sit in, not the visible glass and not the outer face of the window. Hook the tape firmly, keep it level, and measure the clear opening.

Measure width first

For an inside fit, take three width readings across the opening:

1. Top
2. Middle
3. Bottom

Write down each figure exactly as it comes off the tape.

Use the smallest width. That is the governing measurement. If the reveal narrows by only a few millimetres at one point, the screen still has to pass that point and sit square.

Then measure height

Take three height readings:

• Left
• Centre
• Right

Again, record all three and work from the smallest height.

For fly screens, that approach matters more than it does on many standard DIY jobs. Screen frames do not forgive a tapered opening, and a cassette or side channel that is even slightly oversized will bind, sit proud, or refuse to seat fully.

Where a supplier asks for rounded figures, round down rather than up. Keep your units consistent across the whole order. If you are also comparing proportions on casement openings, this guide on how to choose casement window sizes is useful for understanding how window layout can affect the usable measuring area around the frame.

Check whether the opening is square

Measure diagonally from top left to bottom right, then from top right to bottom left. If the two diagonal measurements are close, the opening is reasonably square. If they differ noticeably, do not ignore it.

A screen can still be made for an out-of-square opening, but the measurement needs more judgement. In practice, I treat that as a warning to recheck the corners, look for bowed plaster or twisted timber, and confirm where the frame will make contact. The smallest clear opening still controls the size.

Record clearly and consistently

Bad notes cause expensive reorders. Label each window clearly and keep every set of measurements in the same format:

• Window name. Kitchen left, bedroom rear, office side
• Mount type. Inside or outside
• Widths. Top, middle, bottom
• Heights. Left, centre, right
• Diagonal check. Both measurements
• Obstacles. Handles, vents, alarms, catches, tile returns

If you are measuring for a replacement-style screen rather than a face-fixed one, check the condition of the opening and the available depth as well. As noted in the earlier measuring guidance already referenced, the opening itself needs to be sound, and enough jamb depth is needed for the screen system you intend to fit. That point catches people out on older timber windows and on reveals that look deep enough until the handles, trim or beads are taken into account.

Adapting Measurements for Your Window Style

Generic advice often falls apart. Measuring a plain fixed opening is straightforward. Measuring a real UK window, with handles, sash movement, tilt functions and shallow reveals, takes a bit more judgement.

Casement windows

A standard outward-opening casement is usually the easiest type for an internal screen, but only if the hardware leaves enough clearance. The opening itself may measure cleanly, yet the handle can still become the primary limiting point.

A common example is the bedroom casement with a chunky espag handle projecting into the reveal. On paper, the width and height look fine. In practice, the screen frame or cassette may hit the handle before it sits home. In that case, you either need a different mounting strategy or a screen style that works around the obstruction.

If you're also planning broader replacement or renovation work, this guide on how to choose casement window sizes is useful for understanding how opening proportions and window style affect the usable space around the frame.

Sash windows

Sash windows need a different mindset. You're not usually screening the whole external frame in the same way you would with a simple casement. You're measuring the clear opening created by the moving sash and paying attention to the meeting rail.

One common scenario is a traditional timber sash where the lower sash opens but the side linings aren't perfectly parallel. The visible frame may suggest one size. The true usable passage for a screen insert can be tighter near the meeting rail or lower channel.

That's why old sash windows reward slow measuring. Check where the screen will sit, not where it looks like it ought to.

On sash windows, the meeting rail and travel path matter as much as the opening size itself.

Tilt and turn windows

Tilt-and-turn units often push people toward an outside mount. In tilt mode, the sash comes inward at the top. In turn mode, the whole sash swings into the room. That movement can conflict with an internal screen very quickly.

A typical modern flat has a tidy uPVC tilt-and-turn in the lounge. The reveal may be deep enough for a screen, but once the handle rotates and the sash starts to move inward, the practical space disappears. That's why these windows often need the screen mounted where the sash won't interfere with it.

Sliding and other awkward openings

Sliding styles are simpler in one sense because there's no inward arc from a sash. But you still need to note catches, track details and any unevenness across the head or sill.

For awkward openings, ask one question before finalising the dimensions. Where will the frame sit, and what might stop it sitting there cleanly? That's the question that turns a basic measurement into a usable one.

Common Measuring Mistakes to Avoid

A screen can be out by 3 mm and still turn into a nuisance. It rattles, binds, leaves a gap, or will not sit square in the place you meant it to go. That usually starts with measuring the wrong reference point rather than reading the tape badly.

The pattern is predictable. A homeowner measures the visible glass, the neat front edge of the reveal, or an old screen frame on the floor. None of those tells you the true size a bespoke fly screen needs, especially if you are choosing between an inside fit and an outside fit or working around handles and opening sashes.

The mistakes that cause the grief

• Measuring the wrong part of the window. For an inside-mounted screen, the number that matters is the actual space where the frame will sit. Not the trim. Not the glass. Measure between the contact points of the opening.
• Taking one width and one height only. Timber moves. Plaster bellies. uPVC reveals can still be slightly out. Measure width and height in at least three places and work from the smallest usable figure.
• Copying an old screen. I only use an old screen as a rough clue. Previous screens are often bent, remade, or sized around an earlier problem that no longer applies.
• Forgetting the operating hardware. Handles, trickle vents, alarm contacts, tile lips and proud seals can all steal clearance from a screen frame. This catches people out on casements and tilt-and-turn windows more than any other style.
• Adding or subtracting tolerance by guesswork. A screen frame is not a generic replacement window. Some products need a deduction. Some are made to the exact opening size. Some outside-mounted screens need overlap instead. Follow the supplier's measuring basis exactly.
• Recording the opening size when the order form asks for finished size, or the other way round. That error is expensive because the screen may be manufactured correctly to the wrong instruction.

What works instead

Measure where the screen will sit. Then prove the number.

That means checking width, height, squareness and clearance, then writing down whether the figure is the opening size or the final screen size. For awkward windows, I also note the obstruction and its position, for example a handle projection on the right side or a vent at the head. That small habit prevents a lot of remake requests.

If you want a second reference point on careful measuring practice, this guide to measuring windows for a flawless custom look is a useful companion.

The costly mistakes are usually small. A few millimetres missed at the tape can be the difference between a bespoke fly screen that drops in neatly and one that never fits with confidence.

Placing Your Order and Fitting Your Screen

Once the dimensions are confirmed, ordering should be boring. That's a good sign. Clear width, clear height, chosen mount, known obstacles, done.

If you're ordering a made-to-measure product, enter the final numbers exactly as required by the manufacturer's form. Don't “improve” the dimensions afterward. Don't swap from millimetres to inches halfway through. Don't add a bit for luck. If the supplier asks for opening size, give opening size. If the supplier asks for finished size, give finished size.

Mesh choice matters at this stage too. A standard insect mesh suits many homes. Fine midge mesh and pollen mesh can be the better choice where biting insects or allergies are the main issue, though finer meshes can affect airflow compared with a more open screen. That's a practical trade-off, not a flaw. You choose based on the room and the problem you're solving.

The need for well-fitted screens is becoming more obvious as warmer weather makes open-window ventilation more important. The UK Health Security Agency has reported a marked increase in days with mean temperatures above 28°C, which reinforces the value of solutions that allow airflow while helping exclude insects and allergens, as noted in this UKHSA climate and heat discussion.

If you want another useful reference on getting dimensions right for a neat made-to-measure result, measuring windows for a flawless custom look is a sensible companion read. For fly screens specifically, suppliers such as Premier Screens Ltd use measurement-based ordering for bespoke window and door screens, so the quality of the final fit starts with the numbers you submit.

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If you want a made-to-measure fly screen built for your actual opening, Premier Screens Ltd supplies bespoke window and door screens across the UK, with options for standard insect control, fine midge mesh, pet mesh, and pollen mesh. Measure carefully, order to the right dimensions, and you'll end up with a screen that lets the air in while keeping the nuisance out.