How to Measure Room
Windows

How to Measure Windows for Blinds

For an inside mount, measure the exact width and height inside the window frame and let the deduction happen at manufacturing — don't subtract anything yourself unless the product instructions say to. For an outside mount, add a few inches of overlap on every side.

Blinds are measured differently depending on whether they’ll mount inside the window frame’s recess or outside it on the surrounding wall — and getting that call right first matters more than fractional measuring precision.

Step 1: Decide inside mount or outside mount

  • Inside mount sits within the frame’s recess for a clean, built-in look. It needs at least 1.5-2 inches of frame depth to work (check with a tape measure held into the recess), and it won’t fully block light at the edges the way outside mount does.
  • Outside mount attaches to the wall or trim surrounding the window, covering the frame. It’s the only real option for shallow frames, and it blocks more light around the edges since the blind extends past the opening.

Step 2: Measure for inside mount

  1. Measure the width at the top, middle, and bottom of the frame’s inside recess — use the narrowest of the three, since that’s the tightest point the blind has to fit through.
  2. Measure the height at the left, middle, and right of the recess — use the longest of the three, so the blind reaches the sill everywhere.
  3. Note the frame’s depth, to confirm it’s enough for the blind’s headrail and any tilt mechanism.
  4. Order using your exact measurements. Most manufacturers apply a small deduction themselves (commonly 1/4-1/2”) so the blind clears the frame — check the specific product’s instructions before subtracting anything yourself, since manually deducting on top of a manufacturer’s own deduction leaves the blind too small.

Step 3: Measure for outside mount

  1. Measure the width you want covered — typically the outer edge of the trim, plus 2-3 inches of overlap on each side.
  2. Measure the height from where the headrail will be mounted (commonly a few inches above the frame) down to your chosen end point — the sill, or a few inches past it.
  3. Confirm the wall space above and beside the frame is clear enough for the headrail brackets and the added overlap.

Vertical blinds and sliding doors

Vertical blinds are usually mounted outside the frame by default, with the track positioned above the opening rather than recessed into it — measure the full width you want covered (with overlap) and the drop from the track to your chosen end point, the same way you would for any other outside-mount blind.

A quick double-check before ordering

Measure every window separately, even ones that look identical — older homes especially can have frame sizes that vary by half an inch or more from window to window, and a blind ordered to a “standard” size based on one window can end up a poor fit on its neighbor.

Frequently Asked Questions

Should you subtract anything from the measurement for inside-mount blinds?

Usually not yourself — most blind manufacturers build in a small deduction (often 1/4 to 1/2 inch off your stated width) at the factory so the blind clears the frame without you needing to do the math. Always check the specific product's ordering instructions before subtracting anything, since a small number of manufacturers do expect you to measure and order the exact frame size.

What's the difference between inside mount and outside mount for blinds?

Inside mount fits the blind within the window frame's recess, giving a clean, built-in look but requiring at least 1.5-2 inches of frame depth to work. Outside mount attaches to the wall or trim around the window, covering the frame entirely — it's the better choice for shallow frames, or when you want the window to look larger.

How much overlap should outside-mount blinds have?

2-3 inches of overlap on each side and at the top is typical, which blocks more light around the edges and makes the window appear larger. Bottom length is usually measured to the sill or a few inches below it, depending on the look you want.

How do you measure a window for vertical blinds?

The same inside/outside mount logic applies, but vertical blinds (common on sliding doors and large windows) are usually measured for outside mount by default, since the track typically sits above the frame rather than recessed into it. Measure the full width you want covered, plus a few inches of overlap on each side, and the height from the track position down to your chosen end point.

Do you measure width or height first for blinds?

It doesn't affect the outcome, but most retailers list width first, then height, when you enter dimensions or read a spec sheet — matching that order when you write down your measurements avoids mixing them up later.

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