Do I measure inside or outside the recess?

Inside if the recess is deep enough and reasonably square — it looks neater and leaves the sill clear. Outside if the recess is shallow, out of true, or you want to block more light, since a face-fitted blind overlaps the opening on every side.

Do blinds come exactly the size I order?

No, and they are not meant to. For an inside fit you give the exact recess size and the maker deducts around 10mm so the blind clears the brickwork. Deduct it yourself too and the blind arrives twice as narrow as it should.

How do I measure for a roller blind?

Measure the width in three places and take the narrowest, then the drop in three places and take the longest. Give both figures exactly as measured and let the maker apply the deduction.

Why is there a light gap around my blind?

The fabric has to be narrower than the brackets holding it — on a roller, roughly 15mm in at each end. A thin line of light down each side is normal on an inside fit, and only an outside fit removes it.

How much should an outside-fit blind overlap the window?

7 to 10cm each side, 10 to 15cm above so the mechanism clears the opening, and 5 to 10cm below the sill. More overlap means less light leak and a window that reads larger.

How deep does a recess need to be for a blind?

About 50mm for a Venetian, 70mm for a roller, 90mm for a Roman or vertical. Below that the blind has to be face-fitted instead — the calculator flags it as you type.

How much does a blind stack when it is raised?

A roller rolls to 6 to 10cm depending on the drop. A Roman stacks far deeper, usually 20 to 30cm of folded fabric, which is why it eats into the top of the glass. Verticals stack sideways rather than up.

A roller is cheaper, tidier, and disappears when raised. A Roman is softer and reads as a piece of furnishing, but it uses real fabric with hems and lining, stacks much deeper, and costs two to three times as much.

What size blind for a bay window?

One blind per straight section, each measured on its own rather than as a set. Bays suit blinds better than curtains, since there is no fabric to drag round the corners.

Can I have blinds and curtains at the same window?

Yes, and for warmth and light control it is the best combination there is. Fit the blind inside the recess and hang the curtains outside it on a pole, so the two never foul each other.

How much fabric do I need for a Roman blind?

Usually one width of 137cm fabric, plus about 25cm on top of the finished drop for hems and the heading. Anything wider than roughly 125cm needs a second width joined, and the pattern has to match across the seam.

What width are vertical louvres?

Two widths are standard: 89mm and 127mm. The wider louvre suits big windows and patio doors, while the narrower one looks better on a small window and stacks tighter.

Should a blind finish above or below the sill?

An inside-fit blind stops at the sill. An outside-fit blind should carry 5 to 10cm below it — any less and light spills underneath while the blind reads as short.

How much does a made-to-measure blind cost?

Rollers start around £40 for a small window and rise with width and fabric. Romans are typically £120 to £300 made up. Fitting is usually charged per blind on top.

Can I cut a blind down to size?

Some ready-made rollers can be trimmed and come with a kit for it. Romans, verticals and wooden Venetians cannot — the mechanism is built to the ordered width.

Blind Size Calculator

What size blinds do you need?

1Your window

Units

How is it fitted?

Give the recess size exactly as measured. The maker takes the deduction — do not take it yourself.

The window

Recess widthcmRecess dropcm

Recess depthcm

Deep enough for a roller blind, which needs about 7 cm.

2Your blind

The plainest and cheapest. Rolls away to almost nothing and suits a shallow recess.

Price per m²£Fitting per blind£

Estimates only — every maker deducts a little differently, so check their measuring guide before you order.

Measuring

How to measure for a blind

Six measurements, not two. Openings are rarely square, so you measure the width three times and the drop three times, then check the recess is deep enough to swallow the mechanism.

Measure the width three times

Top, middle and bottom of the recess. The narrowest wins.

Measure the drop three times

Left, centre and right. The longest wins, so the blind covers everywhere.

Check the recess depth

The mechanism must clear the front edge, or the blind face-fits instead.

The method

How to size a blind: the method

The first decision is whether your blind will sit inside or outside the window recess. An inside fit creates a neat, streamlined finish, while an outside fit offers greater coverage and can make a window look larger.

Decide whether you want an inside or outside recess fit before measuring.
For an inside fit, measure the width at the top, middle and bottom, using the narrowest measurement.
Measure the drop at the left, centre and right, using the longest measurement.
For an outside fit, allow approximately 7–10cm beyond each side and 10–15cm above the window, with enough overlap below for good coverage.
When ordering made-to-measure blinds, provide your measurements exactly as requested by the retailer rather than making your own deductions.

Always check the brand’s measuring instructions before ordering. Allowances vary between manufacturers, and making an additional deduction yourself can leave an inside-fit blind unnecessarily narrow.

The deduction is theirs

For an inside fit you give the recess size exactly and the maker takes around 10mm off so the blind clears the brickwork. Take it off yourself as well and the blind arrives twice as narrow as it should.
Depth decides the mechanism

About 50mm for a Venetian, 70mm for a roller, 90mm for a Roman or vertical. Below that the blind has to be face-fitted instead.
Overlap is what stops light

7 to 10cm each side, 10 to 15cm above so the mechanism clears the opening, and 5 to 10cm below the sill. An inside fit always leaves a thin line of light down each edge.

Blind types

Every blind type, sized for your window

The same opening, sized and checked against each mechanism. Hover a row to see how it would hang.

Type	Needs	Notes	Stack / fit
RollerYour blind	7 cm deep	The plainest and cheapest. Rolls away to almost nothing and suits a shallow recess.	7.8 cmFits your recess
Roman	9 cm deep	Real fabric, hemmed and lined. Soft and furnished, but it stacks deep and costs the most.	30 cmFits your recess
Vertical	9 cm deep	Louvres that draw to one side. Made for wide windows and patio doors, not small ones.	12.7 cmFits your recess
Venetian 25mm	5 cm deep	Aluminium slats, tilt for light control. The one that fits the shallowest reveal.	15.5 cmFits your recess
Wooden 50mm	7 cm deep	Warmer than aluminium and considerably heavier. Wide slats stack tall when raised.	21.4 cmFits your recess
Pleated	5 cm deep	Concertina fabric on a slim frame. The usual answer for a conservatory or a skylight.	6.1 cmFits your recess

A roman blind lowered over a bright window — "Give the size of the opening exactly as you found it. The deduction is the maker's job, not yours."

Frequently Asked Questions

Once you know the size, choose the blind

Made-to-measure and ready-made blinds from the brands we love — filter by type, fabric and light control.

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Brands

The blind brands we love

The brands we love

House of Hackney

Statement blinds with real decorative presence, bringing richly drawn botanicals, bold colour and unmistakable maximalism to the window.
Shop House of Hackney
Julian Charles

A window-dressing specialist with impressive breadth, from understated neutrals to decorative patterns and practical blackout designs.
Shop Julian Charles
Dunelm

A go-to for beautifully considered blinds, spanning relaxed Roman styles, tailored rollers and an extensive made-to-measure collection.
Shop Dunelm