Measure the window’s width at the top, middle, and bottom, and its height at the left, center, and right — then order using the narrowest width and the tallest height for an inside mount.
An out-of-square opening ruins a blind order faster than anything else. Windows are rarely perfectly square, so a single measurement taken across the middle can come in wider than the top of the frame — and a blind cut to that number simply will not fit inside the recess. Three measurements per direction solve the problem, and the whole job takes about five minutes with a steel tape.
Below is the exact process for inside and outside mounts, the minimum depth your window needs, and the two errors that send the most orders back.
What You Need Before You Start
A steel tape measure is the only tool that matters here, since cloth tapes stretch and skew the reading. Record every number to the nearest 1/8 inch, which is the standard increment blind manufacturers work in.
One more habit saves headaches later: write measurements down in width-by-height order and label them. Mixing the two up is one of the most common ordering mistakes, and it is entirely avoidable.
How Do You Measure for an Inside Mount?
An inside mount sits within the window frame, and it relies on three width readings and three height readings. Take width first, then height.
For width, run the tape across the inside of the frame at the top, the middle, and the bottom. Write down the narrowest of the three and use that as your ordering width. For height, measure inside the frame at the left edge, center, and right edge, and this time keep the tallest reading. Order at the exact opening size when the manufacturer states that deductions are made during production — Bali’s wood and faux wood guide specifies exactly that, so the blind arrives slightly smaller than your opening by design.
Also measure the window diagonally from corner to corner. If the two diagonal readings differ, the opening is out of square, and a deeper mount or a shim may be needed.
Does Your Window Have Enough Depth?
Not every window can take an inside mount, and depth decides it. Measure from the front of the opening back to the glass.
Check your specific product’s requirements before ordering, since these thresholds vary by line.
Inside Mount Measurements at a Glance
| Measurement | Where to Take It | Which One to Use |
|---|---|---|
| Width | Inside frame at top, middle, bottom | The narrowest reading |
| Height | Inside frame at left, center, right | The tallest reading |
| Depth | Front of opening back to the glass | Compare to the product minimum |
| Diagonals | Corner to corner, both ways | Equal readings mean square |
| Order size | Based on the manufacturer’s deduction policy | Often the exact opening size |
How Do You Measure for an Outside Mount?
An outside mount hangs on the wall or trim in front of the opening, so it needs overlap on every edge instead of a tight fit. Measure the area you want covered, then add extra so light does not leak around the slats.
If your window has no sill, measure down past the opening to the length you want the blind to reach. A cordless lift system is worth considering here, since there is no dangling cord to work around the added width — these tested cordless faux wood blind options run through the picks worth measuring for.
Round Down on Fractions
Tape measures rarely land on a clean eighth-inch mark.
Measure every window in the room separately, even if they look identical. Builders rarely install frames to the same tolerance, and two windows in the same wall can differ by an eighth of an inch.
The measurement you hand to the retailer matters more than any other decision in the process, and Lowe’s How to Measure and Install Blinds guide repeats the three-point method for exactly this reason.
What Are the Most Common Measuring Mistakes?
Single-point measuring causes the majority of bad orders, followed by swapped width and height numbers. Both are easy to prevent with a written record.
- Measuring only the center of the window and missing a wider top or narrower bottom.
- Writing the height as the width, which reverses the entire order.
- Skipping the depth check on an inside mount, then finding the brackets do not fit.
- Using a cloth tape or estimating instead of a steel tape.
- Assuming the factory adds its own allowance rather than checking the product’s deduction policy.
Order sizing conventions genuinely differ between manufacturers, so read the fine print of whichever product you choose rather than assuming one system applies everywhere.
FAQs
Do I order the exact window size or subtract something?
It depends on the manufacturer. Some instruct you to order at the exact opening size because deductions are made during production, while others want you to subtract according to their own system. Read the sizing notes on the product page before submitting, and never guess.
Can I mount faux wood blinds outside the frame without trim?
Yes. Outside mount works on a bare wall, and you simply add overlap on each side — commonly 1 1/2 to 2 inches per side — so the blind covers the opening and blocks light gaps. Measure the wall area you want covered rather than the window itself.
Why do my measurements differ from window to window?
Frames are installed by hand and settle over time, so even matching windows in one room can vary by an eighth of an inch. Measure each window separately and order for each one rather than assuming a single size covers the room.
References & Sources
- Bali Blinds. “Measuring Guide: Wood, Cornices, Wood & Faux Wood, Valances.” Source for the three-point method, rounding down on fractions, and factory deduction policy.
- Lowe’s. “Blinds Measuring Guide.” Supports minimum bracket depth of 3/4 inch and 3 1/2 inch flush-mount depth.
- Lowe’s. “How to Measure and Install Blinds.” Confirms the three-point method and diagonal check for 2-inch cordless faux wood blinds.
