Windows are rarely perfectly square, so measure the width at the top, middle and bottom and the height at the left, centre and right. For an inside mount (blind inside the recess) the narrowest width ensures it fits; for an outside mount the blind overlaps the frame to block light and gaps.
How to use the Blind Size Calculator
- Mount
- Width at top
- Width in middle
- Width at bottom
- Height on left
- Height in centre
- Height on right
- Overlap each side (outside mount)
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose the mount type and enter the three widths and three heights of the window opening.
Tips
- Use the figures from your own bills and product labels – prices and appliance ratings vary widely.
- Look at yearly totals: small daily costs add up and make it easier to compare options.
- Round quantities up to whole packs, rolls or bulbs when you shop.
- Re-check the numbers when prices change; most results scale directly with the unit price.
Common mistakes to avoid
- Confusing watts (power) with watt-hours or kilowatt-hours (energy).
- Comparing prices without bringing them to the same unit first.
- Ignoring one-off costs such as installation, delivery or replacement parts.
Inputs explained
| Field | What to enter |
|---|---|
| Mount | Choose one: Inside the recess, Outside, over the frame |
| Width at top | Units: in, cm, ft, m |
| Width in middle | Units: in, cm, ft, m |
| Width at bottom | Units: in, cm, ft, m |
| Height on left | Units: in, cm, ft, m |
| Height in centre | Units: in, cm, ft, m |
| Height on right | Units: in, cm, ft, m |
| Overlap each side (outside mount) | Units: in, cm, ft, m |
What each result means
- Width (fraction) = [Width to order]÷0.0254
- Drop (fraction) = [Drop to order]÷0.0254
Example values
How the main result changes when Width at top changes and everything else stays at the example values above:
| Width at top (in) | Width to order (in) |
|---|---|
| 17.63 | 17.625 |
| 26.44 | 26.438 |
| 35.25 | 35 |
| 44.06 | 35 |
| 52.88 | 35 |
| 70.5 | 35 |
Formula
Inside mount: width = narrowest of the three widths (factory deduction ≈ ¼ in); height = longest of the three heightsOutside mount: width = widest + 3 in (1½ in overlap each side); height = longest + 3 inWorked example
Inputs — Mount: Inside the recess; Width at top: 35.25 in; Width in middle: 35 in; Width at bottom: 35.125 in; Height on left: 60 in; Height in centre: 60.25 in; Height on right: 60.125 in; Overlap each side (outside mount): 1.5 in.
Results — Width to order = 35 in; Width (fraction) = 35; Drop to order = 60.25 in; Drop (fraction) = 60 1/4.
Frequently asked questions
Should I deduct for an inside-mount blind?
Give the exact narrowest width; most manufacturers deduct about ¼ in at the factory so the blind clears the recess.
How much should an outside-mount blind overlap?
About 1½–3 in on each side, and 2–3 in above and below.
What if my window is out of square by more than ½ in?
Consider an outside mount, which hides the difference.
What formula does the Blind Size Calculator use?
Inside mount: width = narrowest of the three widths (factory deduction ≈ ¼ in); height = longest of the three heights.