Hong Life CalculatorsUnderstanding the blind size calculator — concept and principle
This calculator turns your window measurements into the width and drop (height) to order for a blind, for either an inside-the-recess fit or an outside fit over the frame. It uses three width and three height measurements because window openings are rarely perfectly square.
How it is calculated
Inside mount: order width = narrowest of the three widths; order drop = longest of the three heights
Outside mount: order width = widest width + overlap on each side × 2; order drop = longest height + extra above + extra below
Inches = cm ÷ 2.54
The calculator also shows how much the widths and heights differ.
The principle
A blind inside a recess must fit at the narrowest point or it will jam, and must reach the bottom at the deepest point. An outside blind covers the opening and the frame, so overlaps on each side and above and below block light and look balanced. Most makers take a small deduction off inside-mount widths themselves so the blind clears the recess; do not deduct it yourself unless the order form says so.
Worked example
Default inputs: inside mount; widths 120.5, 120.2, 120.8 cm; heights 150.0, 150.4, 149.8 cm.
- Width to order = narrowest = 120.2 cm (47.32 in).
- Drop to order = longest = 150.4 cm (59.21 in).
- Widths differ by 120.8 − 120.2 = 0.6 cm; heights by 150.4 − 149.8 = 0.6 cm.
| Width in the middle (cm) | Width to order (cm) |
|---|---|
| 30.1 | 30.1 |
| 60.1 | 60.1 |
| 90.2 | 90.2 |
| 120 | 120.2 |
| 150 | 120.5 |
| 180 | 120.5 |
| 240 | 120.5 |
How to read the result
Differences under about 1 cm are normal and suit an inside mount. Larger differences mean the recess is out of square, and an outside mount often looks better. Always follow the measuring guide of the company you order from, since deductions and conventions differ.
Common mistakes
- Measuring only once in the middle.
- Taking the deduction yourself and again at the factory, so the blind is too narrow.
- Forgetting handles, tiles or window locks that stick into the recess.
Frequently asked questions
Should I measure in centimetres or millimetres?
Use the unit the supplier asks for; millimetres avoid rounding errors.
How much overlap for an outside blind?
Often 5–10 cm each side and a few centimetres above and below.
Why the narrowest width for inside mounts?
So the blind fits at every point of the recess.
Sources: Common window-measuring practice (three widths and three heights; narrowest width for a recess fit); 1 in = 2.54 cm exactly (NIST Special Publication 811)