Expert Tips: Measuring for Window Blinds

You’re usually here after the same moment. You’ve offered up the frame, ordered something that looked right on paper, and the finished blind arrives just a touch wrong. It rubs on one side, leaves a sliver of light on the other, or sits proud because the recess wasn’t as deep as it looked. A few millimetres doesn’t sound like much until you try to live with the result every day.

That’s why measuring for window blinds isn’t a clerical step. It’s the part that decides whether the finished job feels neat, quiet and intentional, or like a compromise. In UK homes, that matters even more. Period properties, older sash frames and tired uPVC units rarely behave like the neat rectangular openings you see in generic guides.

Why Accurate Measurement is Non-Negotiable

You see the cost of a bad measurement the day the product goes up. On a blind, it might mean a narrow strip of light or a bottom rail that sits slightly off. On a fly screen, precision is more critical. A few millimetres too loose and you leave gaps for insects. A few millimetres too tight and the frame binds, twists, or refuses to sit cleanly in the opening.

UK windows make this harder than many guides admit. In older terraces and Victorian conversions, recesses are often out of square, plaster bellies into the opening, and timber has moved over decades. Even newer uPVC windows can catch people out because trim, gasket lines, trickle vents and handles eat into the space you thought you had. What looks straight by eye often proves otherwise once a steel tape goes across the top, centre and bottom.

The fitting style changes the job as well.

• Inside mount, or recess fit, depends on the tightest usable point and enough clearance for the product to operate properly.
• Outside mount, or face fix, depends on proper coverage, sound fixing points, and a neat visual line once the product is installed.

Those are different measuring exercises, not interchangeable versions of the same one. A good fitter reads the opening first, then decides what the numbers need to achieve.

For screens, fit is function. The frame has to sit where it can seal, clear hardware, and still look right against the window. If you are ordering fly screens for windows, the measurements form part of the product specification, not a rough guide for manufacture.

Another point that generic, often US-led measuring advice tends to miss is how varied British housing stock really is. Sash windows, shallow reveals, tiled internal sills, retrofitted uPVC and uneven masonry all change how a made-to-measure unit should be assessed before a single figure is written down. If you want a general comparison for soft window coverings, Lewis and Sheron Textiles offers a useful overview, but screens usually demand tighter tolerances and more attention to obstructions.

Good measuring saves more than time. It prevents remakes, avoids awkward fitting compromises, and gives you the crisp finish people expect from a properly made product.

Gather Your Tools and Learn the Lingo

A measuring job usually goes wrong before the first number is written down. It starts with the wrong tape, a rushed reach across the top of the opening, or notes that make sense at the time and look meaningless half an hour later. On British windows, especially older sash openings and retrofitted uPVC, small errors have a habit of turning into visible fitting problems.

Use proper tools and use them consistently. That matters more than having a long kit list.

What to use

Keep these to hand before you start:

• Steel tape measure. A rigid tape gives a straight, repeatable reading across a recess. Soft tapes and bowed tapes do not.
• Pencil and paper. Write each figure down as you take it. Widths and drops get mixed up more often than people expect.
• Spirit level. Useful for checking whether a face-fix line will look right on uneven plaster or out-of-true timber.
• Step stool if needed. Measuring from floor level while stretching upward is one of the quickest ways to lose a few millimetres.
• A simple labelling system. Mark each opening clearly, especially on bay windows or where several similar windows sit side by side.

One trade habit worth copying is this: label the window first, then record width and height under that label in the same format every time.

Terms worth knowing

A few trade terms make the process clearer and help avoid ordering errors.

Term	What it means
Recess	The space inside the window opening
Reveal	The inside side or top surface of that opening
Face fix	Fixing the blind or screen onto the wall, architrave, or frame face rather than inside the recess
Drop	The vertical measurement, or height
W x H	Width first, then height

If sizing language feels a bit trade-specific, this guide to what a linear metre means in made-to-measure products gives useful context.

The rule that stops a lot of measuring errors

Take more than one reading across the opening. On real UK windows, that is standard practice, not belt-and-braces caution. Plaster bellies, painted timber, bowed reveals, and slightly twisted frames all affect the usable size.

For width, fitters usually check top, middle, and bottom. For height, they check left, centre, and right. That gives a true picture of the opening instead of a single optimistic number.

Practical rule: If you measure once, you have measured one point on the window, not the window.

That matters even more for made-to-measure fly screens. A bespoke screen frame is built to fit the opening it is intended for, with enough accuracy to sit neatly, clear the hardware, and seal properly.

Before the tape comes out

Spend a minute looking at the window itself.

• Check for obstructions such as handles, vents, alarm sensors, stays, trims, or tile returns.
• Look at the frame material. Timber, aluminium, and uPVC all present different fixing and clearance issues.
• Check whether the opening is visibly out of square. In period properties, it often is.
• Decide the fitting method before recording sizes. Recess and face-fix measurements serve different purposes.
• Measure every window separately. Matching windows on the same elevation are often slightly different once you put a tape on them.

That short inspection saves a lot of trouble later, especially on older British housing stock where no two openings are quite alike.

How to Measure for an Inside Mount (Recess Fit)

You see this problem constantly in UK homes. The recess looks square from a step back, but once the tape goes on it, the top corners are tight with old paint, the sill is a touch high on one side, or a uPVC handle steals the space the blind or screen requires. Recess fit works well, but only if the measurements reflect the precise opening, not the one your eye assumes is there.

Start by checking whether the recess is actually usable

Open the window fully and look into the reveal, not just at it. On sash windows, check parting beads, staff beads, lifts and catches. On uPVC, look for trickle vents, hinge covers, bulky handles and any frame lip that narrows the opening. On older timber windows, paint build-up often reduces the top width more than homeowners expect.

The question is simple. Will the blind or fly screen sit inside the recess and still clear the moving parts?

If the answer is uncertain, stop and assess the obstructions properly before you record a size. That matters even more for bespoke screening. A made-to-measure frame is built to the dimensions you give it, so if a handle projects into the run of the screen, the problem gets built in. For doors and wider openings, our guide to retractable door fly screens shows the kind of clearance points worth checking before you commit to a recess fit.

Measure the width at three points

Take the width across the top, middle and bottom of the recess. Keep the tape level and measure from the actual fixing surface to the opposite fixing surface, not from decorative trim or rounded edges.

Use the narrowest width.

That is the figure the product has to pass into and sit within. Period properties regularly catch people out here because the plaster belly is in the middle, while many newer uPVC openings pinch nearer the top where the frame and vent details sit. Write down all three measurements, then clearly mark the smallest one so there is no doubt later.

Measure the drop the same way

Now measure the height on the left, centre and right. Run the tape from the top of the recess to the sill or bottom fixing point, depending on how the product will sit.

For a recess fit, use the shortest height if you are measuring to a physical opening and need the unit to clear the tightest point. That helps avoid binding on an uneven sill or a slightly dropped head. If you are ordering a product where the manufacturer applies its own recess deduction, do not invent one yourself. Supply the true recess measurements and follow the product-specific instructions.

Record everything in millimetres, and keep the format consistent as width x height.

Depth decides whether the fit is practical

Depth gets missed more than it should. It is often the difference between a neat install and one that fouls the glass, handle or sash movement.

Measure from the front edge of the recess back to the first obstruction. Do that where the brackets or screen frame will sit, not at the deepest point where there happens to be more room. On uPVC windows, the available depth can change across the reveal because of beads, vents and frame profiles. On older timber frames, the recess may be deep enough overall but still too tight at the head where paint and repair work have built up over the years.

If the depth is marginal, do not guess. Check the product requirement first.

A fitter’s checklist for recess measurements

• Measure width at top, middle and bottom
• Use the narrowest width
• Measure height at left, centre and right
• Use the shortest relevant height for the opening
• Measure depth to the first obstruction
• Record each window separately, even if they look identical
• Note anything that projects into the recess, especially handles, vents and alarm contacts

Good recess measurements are cautious and specific. That is how you get a blind or fly screen that sits square, clears the hardware and looks like it was made for the window, because it was.

How to Measure for an Outside Mount (Face Fix)

You see this most often on British windows that refuse to behave like the textbook example. A Victorian sash with shallow staff beads, a uPVC opening with bulky trickle vents, or a kitchen window where the handles sit proud of the frame. In those cases, a face fix usually gives a cleaner result because the screen or blind is sized for proper coverage, not forced into a space that does not suit it.

For fly screens, that extra coverage is what makes the job work. If the frame only matches the visible opening and ignores the fixing area around it, insects will find the gaps you have left.

Measure the area you want to cover

Start by deciding where the product will fix. That might be the wall above the reveal, the face of the timber architrave, or the front of a uPVC frame if there is enough flat surface. The right answer depends on what is square, solid, and clear of moving parts.

Then measure the finished cover size, not just the opening. Take the width of the opening and add enough overlap on each side to hide the gap properly and give the brackets or screen frame something sensible to land on. On many UK windows, especially older plastered reveals, I like to check the available fixing surface before settling on the overlap because one side can be plumb while the other wanders out by 10mm or more.

Height works the same way. Measure from the top fixing line down to the point where you want the blind or screen to finish, making sure it clears the sill, tile line, or any trim below.

Mark the top line before you trust the numbers

A face-fixed product can be made to the right size and still look wrong if the head line is off. I see this regularly on older houses where the lintel has dropped slightly or the top of the architrave has been scribed to uneven plaster.

Mark the intended top fixing line first. Check it with a spirit level. Then measure from that line.

Do not use the frame or trim as your datum unless you have checked it. Straight-looking joinery is often only straight enough from across the room.

A fitter’s method for outside-mount measuring

Use this order:

1. Choose the fixing surface
   Pick the flattest, strongest point, whether that is wall, architrave, or frame face.

2. Measure the opening width
   Take the visible opening size first, then add the overlap needed on both sides for proper coverage and fixing room.

3. Measure from the marked top line
   Drop the tape from your level fixing line to the desired finished bottom point.

4. Check clearance in front of the frame
   Handles, vents, alarm contacts, sash lifts, and deep beads can all push the product forward or foul its movement.

5. Write down any offset needed
   If brackets, packers, or spacers will be required, note that with the measurements so nothing is assumed later.

This matters even more on doors and wide openings. If you are planning insect protection across a patio or French doors, the fixing position becomes just as important as the opening size. The same principles are used on retractable door fly screens, where a few millimetres in the wrong place can affect how cleanly the system closes.

UK-specific points generic guides often miss

Sash windows need room for the meeting rails and travel path. A face fix can solve that, but only if the cassette or frame sits clear of anything that moves.

uPVC windows can be awkward in a different way. The outer face may look flat until you notice a projecting vent, a rounded frame profile, or a handle that sits exactly where the side channel wants to go. Measure to the true fixing plane, not the prettiest visible edge.

Dormers also catch people out because the wall line and the window line are often not the same. If your opening sits within a sloping or boxed-out area, advice on measuring dormer window curtains is useful for understanding how surrounding structure affects the finished cover size, even though the product is different.

Common face-fix mistakes

These are the ones that cause trouble:

• Ordering to the opening size only. The finished product looks undersized and leaves visible gaps.
• Too little overlap at the sides. Light, drafts, and insects still get around the edges.
• Ignoring projections. Handles, vents, and trims stop the blind or screen from sitting where it should.
• Measuring from an unlevel feature. The product is square, but the install looks crooked.
• Assuming both sides match. In period properties, one return often differs from the other.

A good face fix looks intentional. The secret is to decide the finished footprint first, then measure for that exact footprint on the surface you will use. That is usually the difference between a made-to-measure product that looks built in and one that always feels slightly off.

Tackling Awkward Windows and Obstructions

A Victorian sash in a slightly twisted reveal can fool even careful homeowners. The tape says one thing, but the moving sash, proud staff bead, handle position and uneven plaster say something else. That is why awkward windows need to be measured as working openings, not just neat rectangles on paper.

Sash windows need extra thought

UK sash windows catch people out because the frame moves inside the box. A blind or fly screen can fit the visible opening and still interfere with the sash travel, the lift furniture, or the parting bead. Generic measuring advice often misses that because it assumes a simpler window build-up than you find in period homes here.

Check the full path of the sash before settling on a fixing point. Open it, close it, and note where hardware would sit in relation to the moving frame. For fly screens in particular, that clearance decides whether the finished unit feels properly integrated or like an afterthought.

Online forum discussions and video analyses regularly highlight the same problem with sash windows. The measurement itself may be accurate, but it is taken from the wrong reference point.

Bay windows are rarely symmetrical

Treat each bay face as its own job. Measure left, centre and right sections separately, then check how the angles and returns affect handles, cords, cassettes or screen frames. In older bays, one side is often tighter than the other, and the plaster line can drift enough to make matching products look out of line if you assume the bay is perfectly balanced.

Dormers need the same mindset. The opening, side returns and sloping ceiling all affect where the product can sit cleanly, which is why guidance on measuring dormer window curtains can still be useful when you are planning around boxed-in roof geometry.

Handles, vents and alarm contacts

Obstructions do not always stop an installation. They do force better decisions.

Obstruction	What to check	Typical solution
Window handle	How far it projects into the blind or screen path	Shift the fixing position or allow for stand-off clearance
Trickle vent	Whether it interrupts the head fixing line	Drop the fixing line or move to a face-fix position
Alarm contact	Whether the frame will strike it in use	Reposition the product or allow clearance at the edge
Uneven sill	Whether the eye will read the finished line as straight	Measure from fixed datum points, not the sill alone

One trade tip here. Measure the projection, not just the width and height. A chunky espag handle on a uPVC casement can be the one detail that turns a neat recess idea into a poor fit.

If the terms are causing confusion, our guide to what a window reveal is helps clarify the difference between the visible frame and the true opening you should measure from.

uPVC and tilt-and-turn windows

uPVC frames bring a different set of problems. The frame may look square from the room side but still have rounded internal profiles, drainage caps, trickle vents or slight bowing on older units. Tilt-and-turn windows add another complication because the sash swings inward through the room.

Always test the full opening action before you commit to a recess measurement. I have seen plenty of openings where the size was fine, but the inward swing meant the blind rail or screen frame sat exactly where the sash needed to pass. On deep reveals, also check that you can still reach the controls comfortably once the product is installed.

What seasoned fitters do differently

Experienced fitters measure how the window behaves in daily use. They check movement, projections, fixing surfaces and whether the opening is square enough for a made-to-measure product to sit flush. That is especially important in UK housing stock, where period joinery, patched plaster and later uPVC replacements often meet in ways the original builder never intended.

With bespoke fly screens, those details matter even more because a small measuring error does not just look untidy. It leaves gaps, fouls the opening action, or forces the frame to sit off the surface it was designed for.

Submitting Your Measurements to Premier Screens

Once your numbers are right, the ordering side should be simple. The biggest mistake at this stage is trying to be too clever and making your own deductions without being asked. That’s where good measurements get spoiled.

Send measurements in a clear format

Always submit your sizes in width x height, in millimetres. Keep the labels plain and consistent if you’re ordering for several openings, such as kitchen left, kitchen right, bedroom rear, and so on.

For recess-fit products, send the tight opening size you measured. That means the unadjusted recess dimensions, not an adjusted figure you’ve trimmed down yourself. Factory deductions are there for a reason. If you take material off your numbers before ordering, you risk doubling the allowance and ending up with unwanted gaps.

Know when the factory deducts and when it doesn’t

The practical distinction is simple:

• Recess fit
  Provide the exact opening measurement you recorded.

• Face fix
  Provide the exact finished size you want the product to be.

That division removes guesswork. It also mirrors normal trade practice, where deductions for clearance are handled during manufacture rather than improvised by the customer. In the verified measurement guidance, a 2 to 3mm factory deduction is specifically noted for some Venetian and roller blind applications in recess fitting, which is exactly why homeowners shouldn’t make assumptions on the fly.

Before you press order

Use a final checklist:

• Check every label against the actual window
• Confirm W x H order for each entry
• Make sure all units are in millimetres
• Review mount type so recess and face-fix sizes don’t get mixed
• Flag anything unusual such as vents, handles or deep reveals

If a window is especially awkward, ask the question before ordering rather than after delivery. That isn’t hesitation. It’s sensible specification.

A bespoke product is at its best when the numbers reaching the workshop are clean, consistent and honest. Get that part right and the finished blind or screen has every chance of fitting exactly as intended.

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If you want a made-to-measure solution built around those measurements, Premier Screens Ltd manufactures bespoke fly screens for windows, doors and patios across the UK. You can order online for supply only, or speak to the team if you’d like help checking a tricky opening before production.