How to Measure Windows for Blinds: Inside vs Outside Mount

Accurate measurements are the difference between a custom blind that looks built-in and one that leaves gaps of light or scrapes the frame. The good news is that measuring is straightforward once you know whether you want an inside or outside mount and which numbers to record.

This guide walks you through both methods. If you would rather not measure yourself, we measure every window for free during an in-home consultation.

Inside mount or outside mount?

	Inside mount	Outside mount
Where it goes	Inside the window frame	On the wall or trim above the window
Look	Clean, built-in	Makes the window look larger
Light control	Small light gaps at the edges	Overlaps the frame for better darkness
Needs	Enough frame depth for the headrail	Flat wall or trim space above and beside the window

Inside mounts suit most windows with a deep enough frame. Outside mounts are the better choice for shallow frames, windows with handles or cranks that stick out, and bedrooms where you want as little light leakage as possible.

What you need

• A steel tape measure (fabric tapes stretch)
• A pencil and a sheet to record each window
• A step stool for tall windows

How to measure for an inside mount

1. Width: measure the inside of the frame from side to side at the top, middle and bottom. Record the narrowest of the three.
2. Height: measure from the top inside of the frame to the sill on the left, centre and right. Record the longest of the three.
3. Depth: measure how deep the frame is, from the glass or any obstruction to the front edge. This confirms the headrail or cassette will fit.
4. Check for obstacles: note handles, locks, alarm sensors or cranks that could get in the way.

How to measure for an outside mount

1. Width: measure the area you want to cover, then add at least 5 cm (2 in) on each side so the blind overlaps the opening.
2. Height: measure from where the headrail will sit, usually 5 to 8 cm (2 to 3 in) above the opening, down to where you want the blind to end, usually the sill or just below it.
3. Check the wall space: make sure there is flat surface above the frame to mount the brackets.

Common measuring mistakes

• Measuring only once instead of in three places. Few window frames are perfectly square.
• Mixing up width and height. Always write width first.
• Rounding measurements. Record them to the nearest 1/8 inch or millimetre.
• Assuming matching windows are identical. Measure every window separately.
• Forgetting the depth, then finding the headrail does not fit inside the frame.

Special windows

Bay windows, arched tops, very wide spans and patio doors need extra care. For bays, each window usually gets its own blind with a small gap or overlap at the corners. For patio doors, see our guide to vertical blinds for large windows and patio doors. These are good cases to leave to a professional measure.

Frequently asked questions

Should I measure in inches or centimetres?

Either works, as long as you are consistent and precise. Record to the nearest 1/8 inch or millimetre.

How much bigger than the window should an outside-mount blind be?

Add at least 5 cm (2 in) on each side. More overlap gives better privacy and light blocking.

Do you measure the windows for me?

Yes. Measuring is included free with every in-home consultation, and we take responsibility for the fit.