Made-to-measure blinds are cut to the numbers you supply and cannot be sent back if those numbers are wrong. That makes measuring a window one of the few household jobs where a five-minute mistake costs the full price of the product, and it is why almost every manufacturer’s guide opens with the same instruction: measure in three places. This window blind size calculator takes those measurements, applies the rule that governs each dimension, and returns the width and drop to put on the order.
Arb Digital built it around the two things people get wrong rather than around the arithmetic, which is trivial. The first is that width and drop follow opposite rules — narrowest for one, longest for the other — and it is easy to apply the same rule to both. The second is the deduction, which some suppliers take for you and some expect you to take yourself, and getting that backwards produces a blind either three-quarters of an inch too narrow or three-quarters of an inch too wide, both of which are useless.
What This Calculator Does
For an inside mount it takes the narrowest of your three width readings, applies whatever deduction your supplier asks for, and returns that as the order width. It takes the longest of your three drop readings by default and returns that as the order drop. It compares the two diagonals to tell you how far out of square the opening is, and it checks the recess depth you measured against the headrail depth the product needs.
For an outside mount it works from the same measurements but adds your overlap on each side and your chosen extra above and below, and it reports the total area the finished blind will cover. The overlap figures are yours to set; the commonly suggested starting point for light control is about an inch and a half each side.
How to Use It
- Use a steel tape, not a fabric one. A cloth tape stretches, and a quarter of an inch is enough to matter on a recess fit.
- Measure the width at the top, the middle and the bottom of the recess, and the drop at the left, the centre and the right. Enter all six.
- Take both diagonals, corner to corner each way. This is the step everyone skips and the one that predicts whether a flush fit will look right.
- Measure the depth at the shallowest point, allowing for handles, catches and beading, and put the product’s required headrail depth in the next box.
- Read your supplier’s measuring guide before you order and match the deduction convention to theirs. If they say to give the exact recess size, leave the deduction at zero.
The Rules and Why They Differ
The width rule exists because a blind is a rigid object travelling down a possibly tapering slot. If the recess is 36.5 inches at the top and 36.25 at the bottom, a blind made to 36.5 will jam partway down. Take the narrowest reading and the blind clears the whole drop; the cost is a slightly larger light gap where the recess is wider. Manufacturers such as Bali Blinds in their measuring and installation guidance ask for the recess measured in three places for exactly this reason, and most of them then take the running clearance themselves rather than asking the customer to.
The drop rule is the opposite because nothing binds vertically. A blind that is slightly too long simply rests a little further down or stacks slightly deeper; a blind that is too short leaves bare glass at the bottom of the shortest corner. Taking the longest of the three readings covers the whole opening. The exception is where the bottom rail must stay clear of something — a tiled sill, a window handle, a radiator shelf — and in that case the shortest reading is the safe one, which is why the tool offers both.
Worked example on the defaults. The three widths are 36.5, 36.25 and 36.375 inches, so the narrowest is 36.25 and with a zero deduction the order width is 36.25 inches. The three drops are 48, 48.125 and 47.875, so the longest is 48.125 inches. The diagonals are 60.25 and 60.75, a difference of 0.5 inches, which is a noticeably out-of-square opening. The recess is 3.5 inches deep against a required 0.75, leaving 2.75 inches of margin, so an inside mount has plenty of room. On an outside mount the same window with 1.5 inches of overlap each side and 3 inches above and below would be ordered at 39.25 by 54.125 inches.
What the Diagonals Tell You
A rectangle has equal diagonals. Any difference between them means the opening is a parallelogram or worse, and older houses are full of them. Half an inch of difference across a three-foot window is not unusual, and it has a direct visual consequence: a blind cut square and hung level will show a wedge of daylight that widens toward one corner, and no amount of adjustment removes it because the blind is straight and the window is not.
Knowing that before you order is the point. On a noticeably out-of-square opening the usual answer is to switch to an outside mount, which covers the offending edges entirely, or to accept the gap. It is also worth checking whether the recess is out of square or simply out of plumb, because a blind hung level in a recess whose sides lean will look wrong even when the diagonals match.
Depth, Which Rules Out More Inside Mounts Than Width
The width usually works out. The depth frequently does not, and it is the second most common reason an inside mount fails. Every product has a minimum flat depth it needs for the headrail brackets, and that depth is measured from the wall face back to the first obstruction — not to the glass. Tilt-and-turn handles, casement catches, secondary glazing, thick beading and trickle vents all eat into it.
Partial recess mounting is possible on many products, where the headrail sits proud of the recess and the blind hangs within it, but that is a product-specific option rather than a general one. Where the depth is genuinely short, an outside mount solves it at the cost of the flush look. Enter your depth and the required headrail depth and the tool reports the margin, including telling you plainly when it is negative.
Cords, Chains and Children
One thing about blinds has nothing to do with measurement and matters more than any of it. Looped cords and chains on window coverings are a recognised strangulation hazard for young children, and the industry standard that addresses it — ANSI/WCMA A100.1, developed by the Window Covering Manufacturers Association — has been revised repeatedly to move the market toward cordless and inaccessible-cord products. Requirements differ between jurisdictions and have tightened over time.
If the blind is going anywhere a child sleeps, plays or can reach, cordless or motorised operation removes the hazard rather than managing it. Where a corded product is used, the supplied cleats, tensioners and breakaway devices are part of the product’s safety design and are meant to be installed, not left in the bag. None of this affects the width and drop, and all of it affects whether the blind is a good choice for that room.
Arb Digital builds calculators that name the rule, cite the source and tell you where suppliers differ. Browse the library, or tell us what your customers keep getting wrong.
Browse Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Measuring once, in the middle. Three readings each way take a minute and are the whole reason manufacturers ask for them.
- Taking the deduction when the supplier already has. Deducting three-quarters of an inch from a size the factory will also deduct from produces a blind that is an inch and a half too narrow.
- Writing the dimensions the wrong way round. Width first, then drop, is close to universal, and a 48 by 36 order for a 36 by 48 window is a total loss.
- Measuring an existing blind instead of the window. The old blind may have been the wrong size, which is often why it is being replaced.
- Ignoring the depth until the brackets arrive. Handles, catches and beading steal depth, and the shallowest point is the one that decides whether the headrail fits.
Related Free Tools From Arb Digital
If curtains are the alternative, the curtain fabric calculator works out widths, cut lengths and total yardage. For the rest of the room, the wall area calculator handles surfaces with openings deducted and the paint calculator turns that into tins. Convert between inches and millimetres with the length converter or across several dimensions at once with the unit converter, and get glazing or panel areas from the rectangle area calculator. The full library sits in the free online tools hub.
Frequently Asked Questions
Measure the width in three places across the recess and the drop in three places down it, using a steel tape. For an inside mount take the narrowest width and, in most cases, the longest drop. Take both diagonals as well, and measure the depth at its shallowest point to check the headrail will fit.
Usually not. Most manufacturers ask for the exact recess measurement and take the running clearance themselves, which is why the deduction here defaults to zero. Deduct only if your supplier’s own measuring guide explicitly tells you to, because doubling the deduction leaves a blind well short of the opening.
Because the two dimensions fail differently. A blind wider than the narrowest point of the recess will jam as it travels down, so the narrowest reading governs. Nothing binds vertically, so a drop that is slightly long simply rests lower, while one that is too short leaves bare glass at the shortest corner.
Around an inch and a half each side is the figure manufacturers commonly suggest for light control, with a few inches above for the headrail and below the sill line. Those are starting points rather than rules, and this tool takes all three as inputs so you can set them to suit the window.
The two diagonals will differ, and a blind cut square will show a widening wedge of daylight at one side that cannot be adjusted out. On a noticeably out-of-square opening an outside mount is the usual answer, because it covers the offending edges rather than following them.
It depends entirely on the product, and the figure comes from the specification for the exact blind you are buying. Measure the available depth at its shallowest point, allowing for handles, catches and beading, and compare the two. Where the depth is short, some products allow a partial recess fit and an outside mount always solves it.
The window, always. The blind being replaced may have been ordered incorrectly in the first place, and copying its size copies the mistake. It also tells you nothing about the recess depth or how square the opening is.
Looped cords and chains are a recognised strangulation hazard for young children, and the industry standard covering them has been tightened repeatedly. Cordless or motorised products remove the hazard rather than managing it, and where a corded product is used the supplied cleats, tensioners and breakaway devices are meant to be fitted.
This page is a measuring aid only. Suppliers differ in whether they take the running clearance themselves, in the order they expect dimensions, and in the depth their products require, and their own measuring guide governs an order rather than this page. Made-to-measure blinds are generally non-returnable. Corded window coverings are a recognised strangulation hazard to young children; consult the manufacturer’s safety instructions and your local product safety authority before fitting one where a child can reach it.
