How to Measure Your Windows for Shades, Blinds, and Drapery

What you'll need

Three things, none of them expensive. The metal tape is non-negotiable — soft tapes flex enough to throw a measurement off by 1/4 inch, which is the difference between a shade that fits and one that doesn't.

• A 25-foot metal tape measure (Stanley FatMax, Milwaukee, or similar — $15–$25 at any hardware store).
• A pencil and a notepad. Phone notes work too, but pencil is faster.
• A short stepladder for the upper measurement points on tall windows.

Inside mount vs. outside mount — pick this first

The very first decision changes which numbers you record and how you'll order. Inside mount means the treatment sits flush inside the window opening. Outside mount means it covers the opening and overlaps the surrounding trim.

Choose inside mount when: the window opening has at least 2 inches of depth (most modern construction does), you want the cleaner architectural look, and the opening is roughly square. This is the right pick for 70–80% of residential windows.

Choose outside mount when: the window depth is less than 2 inches, the opening is noticeably out-of-square (more than 1/2 inch variance), you want the window to look bigger, or you need true blackout. The overlap kills the light leak that even blackout shades suffer from on inside mount.

How to measure for inside mount

The number-one rule: measure each dimension at three points, never one. Windows aren't square, especially in older homes — a 36-inch opening might measure 36 1/8 at the top, 36 1/4 in the middle, and 36 1/8 at the bottom. The shade has to fit at the narrowest point, period.

1. Measure the width at three points. Measure the width of the window opening at the top, middle, and bottom. Record all three. Order the smallest of the three measurements — never the average.
2. Measure the height at three points. Measure from the top of the opening to the windowsill on the left, middle, and right. Record all three. Order the longest of the three measurements.
3. Measure the depth. Measure the depth of the window opening from the front of the trim to the glass. Most cellular and roller shades need at least 2 inches of depth for a flush mount.
4. Record measurements as exact. Write down measurements to the nearest 1/8 inch. Do not round. Do not subtract anything for clearance — the manufacturer will deduct the correct amount automatically based on the lift mechanism.

How to measure for outside mount

Outside mount is more forgiving — the treatment covers the window completely instead of fitting inside it, so small measurement variances don't matter. The decisions you have to make are about overlap.

1. Measure the trim casing width. Measure the full width of the trim casing — outside edge to outside edge. This is the reference number you'll add overlap to, not the glass or interior opening.
2. Add overlap on each side. Add 1–2 inches per side beyond the outside of the trim casing for normal windows. Bump that to 3 inches or more for very out-of-square windows or rooms where light leak matters (bedrooms, nurseries, blackout setups).
3. Measure the height with overlap. Measure from where you'll mount the headrail (at least 2 inches above the trim) down to where you want the shade to end — at the sill, below the sill, or at the floor.
4. Confirm wall clearance. Look for outlets, switches, vents, or radiators below the window. The shade or drapery needs unobstructed space to hang. If the wall is uneven (older homes, plaster), you may need a longer mounting bracket.

5 measurement mistakes that cause returns

Online retailers don't accept returns on measurement errors — the shade was made to your specs. After hundreds of in-home consultations, these are the five mistakes I see almost every time.

1. Measuring at one point only. A window that "looks square" almost never is. Three-point measuring takes 30 extra seconds and saves the entire order.
2. Subtracting for clearance. The manufacturer does it. Don't double-subtract.
3. Confusing width and height. Always record width first, then height. It's the universal convention — every order form, every spec sheet. Reversing them is the most common typo.
4. Using a soft tape measure. Cloth tapes can stretch up to 3/8 inch over a 60-inch measurement. Always metal.
5. Rounding to the nearest inch. Cellular and roller shades are made to 1/8 inch precision. Round 36 1/8 down to 36 and your shade comes back too narrow by an eighth — visible from across the room.

Treatment-specific notes

Most of the steps above apply universally, but a few treatment types have their own quirks worth knowing about.

Cellular and roller shades: straightforward. Inside-mount-friendly with 2 inches of depth. The default for the steps above.

Roman shades: measure exactly as for cellular, but require 3 inches of depth minimum for inside mount. Less than that, go outside mount.

Wood and faux-wood blinds: the headrail is deeper than other shades — confirm 2 1/2 inches of opening depth before ordering inside mount.

Drapery: measured very differently. Width is based on rod length plus fabric and stack (where the drapes sit when fully open). Height is from rod to floor with a 1/2 inch clearance for "kissing" the floor or a deliberate 3–6 inches for a "puddle." We cover the full process in our drapery measurement guide.