Measuring for blinds starts with one decision, not one number: inside mount (the blind lives inside the window recess) or outside mount (it hangs on the wall over the opening). Decide that first, because the two mounts are measured in opposite spirits — inside is measured exact and tight, outside is measured generous. Then it is three width measurements, three heights, a steel tape, and no do-it-yourself deductions.
Our workroom has measured windows for over sixty years, and blinds are less forgiving than curtains: a curtain gathers past its mistakes, a blind displays them. Here is the exact routine we use, in the order we use it.
Inside or Outside Mount: Decide This First
| Inside mount | Outside mount | |
|---|---|---|
| Looks | Built-in, clean, shows off the window trim | Covers the frame; makes small windows read larger |
| Needs | A square-ish recess with enough depth | Flat wall or trim above the window |
| Light control | Small gaps at the sides — always | Better — the blind overlaps the opening |
| Choose it when | The recess is deep and true, and you keep the sill | The frame is shallow, crooked, or you want maximum darkness |
The depth check settles most cases. Measure from the glass (or the innermost obstruction — a crank handle counts) to the front edge of the recess. Most 2-inch blinds want about 3 inches of depth to sit fully recessed and roughly 1.5 inches minimum to mount at all; cellular and roller shades manage with less. Shallow recess, tilted frame, or an alarm sensor in the way — go outside mount and stop fighting the window.
Measuring for an Inside Mount
Measure the width inside the recess in three places: top, middle, bottom. Keep the narrowest number. Then the height in three places: left, center, right — keep the longest. Record to the nearest 1/8 inch, always as width × height, and label it “inside mount” on the order.
The rule people find hardest to trust: give the factory the exact opening size and make no deductions yourself. The manufacturer subtracts their own clearance — usually 1/4 to 1/2 inch on width — so the blind operates without scraping. If you shave the number “to be safe” and the factory shaves it again, the blind arrives narrow and daylight runs down both edges. One deduction, theirs, never yours.
Windows are not square — a house settles, and a recess that is 36 inches at the top is often 35¾ at the sill. That is exactly why the three-measurement routine exists, and it is the part most skipped instructions skip.
Measuring for an Outside Mount
Outside mount is measured generous, because its whole job is coverage. Take the opening width — glass plus frame — and add 3 to 4 inches on each side so the blind overlaps the wall and closes down the light gaps. For height, start about 3 inches above the opening to give the headrail and brackets room, and run to the sill — or past it, if there is no sill and you want the fullest blackout effect. For a bedroom that truly needs darkness, be generous on every side; overlap is what blocks the halo.
Here the number you send is the number you get: outside-mount orders are made exactly to your measurement, no factory deduction. Opposite spirit, opposite rule.
The Mistakes That Cost a Reorder
Four of them account for nearly every remake we see. Swapping width and height — the industry writes width first, every time, and a 36×60 ordered as 60×36 is an expensive lesson. Measuring the old blind instead of the window — the old blind already contains someone’s deductions. Deducting on an inside mount, as above. And using a cloth tape or a phone app — a steel tape that holds a straight line across the recess is the only tool for the job; a sagging tape reads long.
Notes by Product Type
Horizontal blinds (wood, faux wood, aluminum) need the most depth — the slats tilt inside the recess. Cellular and pleated shades need the least, which makes them the inside-mount answer for shallow modern frames. Roller shades mount shallow but remember the light gap: the fabric is narrower than the fitting, so bedrooms often do better with an outside mount or side channels. Vertical blinds and panel tracks flip the priority — height is the critical number, with about 1/2 inch of floor clearance so the vanes swing free. If you are still choosing between these families, our guides to the types of blinds and the types of window shades compare them construction by construction. Cell construction and insulation numbers live in our cellular shades guide.
How This Differs From Measuring Curtains
Blinds are measured to the window; curtains are measured to the rod, which usually sits well above and beyond the window. Same tape, opposite direction of thinking — and mixing up the two systems is how a “measured” order still comes out wrong. The fabric-side method is in how to measure curtains, and if you are still deciding between slats and fabric altogether, start with blinds vs shades.
How to Measure for Blinds FAQ
What size should I order for inside mount blinds?
Order the exact opening size — the narrowest width and longest height you measured — and let the factory make the deductions. They subtract about 1/4 to 1/2 inch for operating clearance; if you deduct as well, the blind arrives too narrow and leaks light at both sides.
How much wider should outside mount blinds be than the window?
Add 3 to 4 inches of overlap on each side of the opening and about 3 inches above it for the headrail. More overlap means less light leak, so bedrooms benefit from the generous end of the range.
Do you measure width or height first for blinds?
Width first, always — blind sizes are written and manufactured as width × height. Recording 36×60 as 60×36 produces a blind for a completely different window.
What if my window is not square?
Most are not. For an inside mount, measure width at top, middle and bottom and use the narrowest; measure height at left, center and right and use the longest. If the recess is badly out of true, switch to an outside mount, which covers the crookedness entirely.
How much depth do I need for an inside mount?
Roughly 1.5 inches of usable depth to mount most blinds at all, and about 3 inches for a 2-inch horizontal blind to sit fully flush inside the recess. Cellular, pleated and roller shades tolerate the shallowest frames. Measure from the glass or the innermost handle — whatever sticks out furthest is your real depth.
Exact and tight inside, generous outside, width before height, one deduction and it is the factory’s — that is the whole craft of it. Turn the numbers into the right product with the types of blinds and types of window shades, or settle the slats-versus-fabric question first in blinds vs shades. Every guide in this wing of the library is collected in our blinds & shades guides.