Nothing is more frustrating than ordering new blinds, waiting for delivery, and then discovering they don't fit. Too narrow and light leaks in from the sides. Too wide and they won't mount properly. Getting your measurements right before you order is the single most important step — and it's easier than most people think.
This guide walks you through exactly how to measure windows for blinds, whether you're going with an inside mount (fitted within the window frame) or an outside mount (covering the frame and wall around it). By the end, you'll be ready to order with confidence.
Inside Mount vs. Outside Mount: Know This First
Before you grab a tape measure, decide which mounting style you want. This changes everything about how you measure.
Inside mount means the blind sits inside the window frame, flush with the wall. It looks clean and modern, and shows off your window trim. For this to work, your window frame needs to be at least 2–3 inches deep to hold the brackets.
Outside mount means the blind is installed above or outside the frame, covering it entirely. This is great for blocking more light, making windows look larger, or hiding windows that aren't perfectly square. It's also the only option if your frame is too shallow for an inside mount.
Not sure which to choose? Inside mount is the most popular option for a polished look. Outside mount is the go-to for bedrooms where light control matters most.
How to Measure for Inside Mount Blinds
For inside mount, you're measuring the interior width and height of the window opening — not the trim or the wall.
Width
- Measure the inside of the frame at three points: top, middle, and bottom.
- Record the narrowest measurement of the three.
- Most blind manufacturers will automatically deduct a small amount (called a deduction) so the blind fits and moves freely. Check with your retailer — at My Home Blinds, deductions are built into every custom order.
Height
- Measure from the top of the inside frame down to the window sill at three points: left side, centre, and right side.
- Record the longest measurement.
- If there's no sill, measure to where you want the blind to end.
Pro tip: Always measure in inches, and measure twice. A quarter-inch mistake can mean a poor fit.
How to Measure for Outside Mount Blinds
For outside mount, you have more flexibility — you're choosing how much of the wall and frame to cover.
Width
- Start with the width of the window frame (outside edge to outside edge).
- Add 3 inches on each side (6 inches total) for best light blockage and a proportional look.
- You can go wider if the wall space allows and you want a more dramatic appearance.
Height
- Decide how high above the frame you want to mount the blind. Adding 2–4 inches above the frame makes ceilings look taller and maximizes light control.
- Measure from your intended mounting point down to where you want the blind to end — typically the window sill or 2–4 inches below it.
Common Measuring Mistakes to Avoid
Even careful homeowners make these errors. Watch out for them:
- Using a cloth tape measure: Always use a steel measuring tape. Cloth stretches and gives inaccurate readings.
- Measuring to the nearest inch: Blinds are ordered to the nearest ⅛ inch. Measure precisely.
- Assuming all windows are the same size: Even windows that look identical often aren't. Measure every window individually.
- Forgetting to check for obstructions: Window cranks, handles, and deep sills can interfere with inside mounts. Check clearance before ordering.
- Not accounting for window depth: For inside mount, verify your frame is deep enough for the bracket type. Minimum depth is usually 1.5–2.5 inches depending on the blind style.
Frequently Asked Questions
Q: Do I measure the glass or the whole window frame?
A: For inside mount, measure the inside of the frame — not the glass. For outside mount, you can measure from the outer edge of the trim and then add extra width on each side as desired.
Q: What if my window isn't perfectly square?
A: Older homes especially can have windows that are slightly out of square. Always use the narrowest width and tallest height measurements to ensure the blind fits without binding. An outside mount is often the easiest solution for irregular windows.
Q: Should I measure before or after removing old blinds?
A: Always measure after removing old blinds if possible. Old hardware can distort your measurements, and you'll want a clear view of the actual frame.
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