Measuring is the one step you can’t “eyeball” when you want blinds to look clean and operate smoothly. This guide walks you through a simple, repeatable way to measure windows for blinds—whether you’re planning an inside mount (in the frame) or an outside mount (on the trim/wall). If you’re unsure which mount you’ll choose, you can still measure both ways now and decide later.

If you’d prefer professional measuring and installation instead of DIY, see how Better Blinds Plus handles custom measuring for blinds:

What do you need to measure for blinds?

You only need a few core measurements: width, height, and (for many inside mounts) depth. Taking them consistently—window by window—is what prevents “almost fits” surprises.

Measuring checklist (before you start)

• Metal/steel tape measure (avoid cloth or flexible tapes)
• Pencil + notepad (or a notes app)
• Step stool/ladder (if needed)
• A simple labeling system (e.g., “Kitchen Window 1,” “Bedroom Left,” etc.)
• 5 extra minutes to measure each window independently (even if they look identical)

How do you measure for inside-mount blinds?

For an inside mount, you’re measuring inside the window opening where the blind will sit. The goal is a precise fit that still operates smoothly.

1. Measure width in 3 places.
   Measure across the opening at the top, middle, and bottom.
2. Use the smallest width.
   Windows are rarely perfectly square, so the smallest width is the safest ordering width.
3. Measure height in 3 places.
   Measure from the top of the opening down to the sill at the left, center, and right.
4. Use the largest height.
   Using the largest height helps ensure full coverage when the blind is fully lowered.
5. Measure depth at the top of the window frame.
   Depth is the space from the glass to the front edge of the inside frame where brackets/headrail will sit. If you’re tight on depth, check the product’s mounting requirements (depth needs vary by product and headrail size).

How do you measure for outside-mount blinds?

For an outside mount, you’re measuring the coverage area you want the blind to span. This method is forgiving for shallow windows and can reduce side light gaps.

1. Choose your coverage width.
   Measure the window opening (or trim) width, then decide how far you want the blind to extend beyond it on each side for coverage.
2. Choose your coverage height.
   Measure from where you want the headrail to start (often a bit above the opening) down to where you want the blind to end (often the sill or slightly below).
3. Keep the extension balanced.
   If you extend the blind beyond the opening, try to keep the left and right overhang roughly equal so it looks centered.

How do you check depth and obstacles before you order?

A quick obstacle check prevents the most common install-day problems. Look at the top inside corners and the path where the blind will travel.

• Cranks, handles, and locks: Make sure the blind won’t hit them when lowered or tilted.
• Trim shape: Rounded or decorative trim can affect how brackets sit.
• Window “squareness”: If you suspect the window is out of square, measure diagonally from corner to corner. Large differences often mean an outside mount will look and function better.

Reference for the diagonal “square check”:

How should you round, label, and record measurements?

Write measurements as Width × Height and label them clearly the moment you take them. Most manufacturers ask for measurements to the nearest fraction (often 1/8 inch) and provide their own guidance on rounding and deductions.

Practical recording tips:

• Use a consistent format: W × H
• Record the “three reads” (top/middle/bottom) in parentheses, then circle the value you’ll use
• Add notes like “inside mount,” “outside mount,” “shallow depth,” or “handle on right”

General measuring tips and rounding guidance example:

What’s the quick cheat sheet for inside vs outside mount measurements?

This table is a fast way to confirm you’re taking the right numbers for the mount you’re planning.

What you’re measuring	Inside mount (in the frame)	Outside mount (on trim/wall)	Why it matters
Width	Take 3 widths (top/middle/bottom) and use the smallest	Measure desired coverage width (opening/trim + planned overhang)	Prevents binding inside the frame; improves coverage outside
Height	Take 3 heights (left/center/right) and use the largest	Measure desired coverage height (mount point to endpoint)	Ensures full drop and clean finish
Depth	Measure depth at top of inside frame; check product requirements	Typically not required	Determines whether an inside mount is physically possible
“Out of square” check	Measure diagonals; big differences can cause tight spots	Still useful, but outside mount often hides uneven openings	Helps avoid uneven gaps and difficult installs

What does this look like in real homes?

Seeing the steps in context makes them much easier to replicate.

Mini-scenario 1: Older home window that isn’t perfectly square

A homeowner measures an inside mount and gets widths of 35 1/4″, 35 3/8″, and 35 5/16″. They record all three, then use 35 1/4″ as the ordering width (smallest). When they measure diagonally, the opening is slightly out of square, so they decide an outside mount may look cleaner—without remeasuring from scratch.

Mini-scenario 2: Kitchen window with a crank handle and shallow depth

A renter wants blinds but notices the crank handle sticks out and the inside depth is shallow. They measure for outside mount coverage instead—adding a little overhang past the opening so the blind clears hardware and looks centered. The result is fewer obstructions and better side coverage.

What are the most common measuring mistakes (and how to avoid them)?

Most “wrong size” problems come from a few repeatable errors. If you avoid these, your odds of a clean fit go way up.

• Measuring just one window and copying it to the next: Similar-looking windows can vary more than you’d think.
• Not writing down the three measurements: You need the “top/middle/bottom” reads to choose the right number.
• Mixing up width and height: Always record W × H.
• Skipping depth checks on inside mounts: Depth limitations can force a last-minute change to outside mount.
• Centering mistakes on outside mounts: Uneven overhangs can look “off” even when the blind technically fits.

Quick FAQs about measuring for blinds

Do I subtract anything for inside-mount blinds?

Usually, no—you provide your inside opening measurements and the manufacturer’s deductions (if any) are handled by the product specs. Always follow the product’s measuring instructions.

What if I’m not sure whether I’ll do inside or outside mount?

Measure both ways on the first window. Once you see depth and obstacle realities, your mount choice usually becomes obvious—and you can measure the rest consistently.

What if my window has trim that isn’t flat?

For outside mounts, pick a mounting surface that allows brackets to sit flush (often above the trim or on the wall). If the mounting surface isn’t stable, the blind can tilt or bind.

When is it worth getting a professional measure?

If you have very wide windows, hard-to-reach openings, or windows that seem noticeably out of square, a professional measure can prevent expensive re-orders. It’s also helpful when you’re coordinating multiple rooms and want consistent alignment across a wall of windows.

When you’re ready to move from measurements to custom blinds that are measured and installed to fit, start here:

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