Blind Size Calculator
Enter your window measurements and choose inside or outside mount. The calculator finds the correct blind width and height, applies the standard industry deductions or overlaps, and shows you the worked steps. Switch between inches and centimetres at any time.
Inside mount vs outside mount: which should you choose?
An inside mount blind sits within the window frame recess, creating a clean, built-in look that shows off attractive woodwork and does not block wall space. It is the preferred choice when the window frame is deep enough to accommodate the mounting hardware (typically at least 1.5 inches of flat depth). An outside mount blind installs on the wall or trim above and around the frame. It is the better choice when the frame is too shallow, when the window has poor depth, or when you want to make the window appear larger. Outside mounts also provide better light blockage because the blind covers the frame edges where light can leak through.
How to measure for inside mount blinds
Always measure in width-by-height order. For width, measure across the inside of the window frame in three places: at the top, the middle, and the bottom. Record all three, then use the narrowest measurement - this is the one that guarantees the blind will fit even if the frame is slightly out of square. For height, measure from the top of the frame to the sill in three places: left side, center, and right side. Use the longest measurement. If you want the blind to clear the sill rather than rest on it, subtract 1/4 inch (6 mm) from the height. Submit these exact figures to your manufacturer - most will apply their own small deduction to ensure a smooth fit.
How to measure for outside mount blinds
For outside mounts, your measurements define exactly how large the blind will be. Width: measure the span you want the blind to cover. A minimum of 1.5 inches of overlap on each side beyond the frame is recommended to block light; 2.5 inches per side is better for rooms where light control matters. Add the total overlap (both sides combined) to your measured width. Height: decide where the top of the blind will sit - at least 3 inches above the top of the frame is standard to hide the mounting hardware and prevent a light gap at the top. Measure from that chosen starting point down to wherever you want the blind to end - the sill, 2 inches below the sill, or the floor. The result is your order height.
Measuring tips for accurate results
Use a steel tape measure rather than a cloth or retractable plastic one - steel measures do not stretch or sag, and even a 1/4-inch error can mean a blind that does not fit. Always measure to the nearest 1/8 inch or the nearest millimetre. Round down for width measurements and round up for height measurements when a reading falls on an odd fraction. If you are replacing an existing blind, do not use its dimensions - measure the window itself, because previous blinds may have been cut incorrectly. Measure each window individually even if they look the same size; old houses especially have windows that differ by noticeable amounts.
Standard window blind sizing guide
| Window type | Typical width (in) | Typical height (in) | Notes |
|---|---|---|---|
| Small window | 18 - 24 | 24 - 36 | Bathroom, small bedroom |
| Standard single window | 24 - 36 | 36 - 60 | Most common residential size |
| Large single window | 36 - 48 | 48 - 72 | Living room, dining room |
| Sliding glass door | 60 - 72 | 80 - 84 | Use vertical blinds or panel track |
| French door | 14 - 16 | 62 - 66 | Mount on door, not frame |
| Picture window | 48 - 96 | 36 - 72 | May need split or wide product |
Common window sizes and typical blind dimensions. Always measure your own window - standard sizes can vary.
Frequently asked questions
Do I measure the window or the frame for blinds?
It depends on the mount type. For inside mounts, measure the opening of the window frame from inside edge to inside edge, not the glass or the outer frame. For outside mounts, measure the outer dimensions of the frame plus any overlap you want, because the blind will be installed on the wall above and around the frame.
Why do I need to measure in three places?
Almost no window is perfectly square. Walls settle, frames warp, and older construction was often imprecise. Measuring at three points across both the width and the height reveals how much the window varies. Using the narrowest width ensures an inside-mount blind will fit; using the longest height ensures full coverage without a gap at the top or bottom.
Should I deduct anything from my inside-mount measurements?
Most manufacturers automatically deduct a small amount (typically 1/4 to 3/8 inch) from the width you submit, to ensure the blind slides freely inside the frame. You should submit your exact narrowest measurement and check with your supplier whether they apply this deduction. The only adjustment you may want to make yourself is subtracting 1/4 inch from the height if you do not want the blind touching the window sill.
How much overlap do I need for an outside mount?
A minimum of 1.5 inches of overlap on each side (3 inches total added to the width) is standard. If light blockage is important - for a bedroom or media room - increase this to 2.5 inches per side (5 inches total). At the top, mount at least 3 inches above the frame and include that in your height measurement. At the bottom, extend at least to the sill or 2 inches below it.
What if my window measurements differ by more than 1/2 inch across the three points?
A difference of more than 1/2 inch indicates a significantly out-of-square window. For an inside mount, consider whether a blind will operate smoothly given that variance - you may get binding or gaps. An outside mount is often the better solution for seriously out-of-square windows because it covers the frame entirely and the size is less critical.
Can I use these calculations for roller shades and cellular shades?
Yes. The same inside-mount and outside-mount rules apply to most types of window coverings including roller shades, honeycomb (cellular) shades, Roman shades, and horizontal blinds. Vertical blinds and panel-track systems use the same width formula but height is measured from the mounting point to the floor rather than to the sill.