How to Measure for Custom Blinds | Get It Right the First Time

One wrong number turns a perfect window treatment into a return label. The good news is that measuring for custom blinds takes about ten minutes and a steel tape measure. The process comes down to one decision first—inside mount or outside mount—then a few careful measurements that follow the same rhythm: measure three spots, use the right one, and round by the retailer’s rules.

Inside Mount or Outside Mount: Decide First

This choice changes every number you write down, so make it before you touch the tape measure.

An inside mount sits within the window frame, giving a built-in, clean look. It needs enough depth for the headrail and hardware. The Home Depot’s measuring guide says to check the window depth so you know how deep the blinds can be. An outside mount covers the full opening and mounts on the wall or trim above, which works when depth is tight or you want to make a small window look larger.

Here’s the catch: check for obstructions before choosing inside mount. Protruding handles, locks, levers, sockets, and window tracks can block the blinds from hanging properly. Outside mounts need enough flat wall or molding above the window for the brackets and headrail.

How to Measure an Inside Mount

For inside mounts, measure the width at the top, middle, and bottom, then use the narrowest measurement. Measure the height at the left, center, and right, then use the longest measurement.

Follow this order with a metal tape measure, never cloth, and work in inches:

1. Measure the inside width at the top, middle, and bottom of the frame. Record all three.
2. Use the narrowest width—that’s the number that guarantees the blinds will fit without scraping.
3. Measure the inside height at the left, center, and right. Record all three.
4. Use the longest height so the blinds fully cover the window, not just most of it.

Home Depot’s guide says to measure inside-frame width in those three spots and use the narrowest, then measure height at left, middle, and right and use the longest. Lowe’s agrees and notes the three-point height check matters because window construction varies. If your window depth is under the product’s minimum, the blinds won’t mount properly—check the spec sheet before ordering.

How to Measure an Outside Mount

Outside mounts cover the entire opening plus extra wall space, so you choose the dimensions rather than discover them. Measure the full area you want covered, then record width and height at three points and use the widest and longest measurements.

Outside mounts start with a decision: how much overlap you want. Most guides recommend a few inches of extra width and height so light doesn’t leak around the edges. Then measure the width at the top, middle, and bottom of that planned area, and the height at the left, center, and right. The widest and longest numbers are the safe picks, since they guarantee full coverage even if the window isn’t perfectly square.

Retailers like Lowe’s remind buyers that the mounting surface matters. You need enough flat, solid area above the window for the headrail and brackets—drywall alone may not hold heavier blinds without anchors or a mounting board.

Rounding, Deductions, and Common Mistakes

The most common measuring error is averaging the three width readings instead of using the narrowest. Window frames are rarely perfectly square, and an average width guarantees the blinds will bind or scrape. For height, the opposite rule applies—averages leave a gap at the bottom.

Other mistakes that cause returns:

• Using a cloth tape that stretches, or measuring in feet instead of inches.
• Skipping the depth check for inside mounts, then discovering the headrail won’t fit.
• Rounding inconsistently—many retailers require the nearest 1/8 inch, and some have specific round-up or round-down rules for each product.
• Adding your own deductions when the retailer asks for exact measurements. Their ordering system handles the fit adjustments; your job is to report what you measured.

Product-specific rules differ, so check the product page before placing the order. A cellular shade and a wood blind may have different minimum depths and different deduction instructions. When you’re ready to compare options that fit your measurements, our roundup of the best custom blinds on the market can point you toward the right style.

Write every measurement down before you order. When the numbers are on paper, you can double-check them against the retailer’s rounding guide and the product’s minimum depth requirement. That ten minutes with a tape measure is the difference between blinds that slide into place and a box that goes straight back to the store.