Blinds Size Calculator
Blind order size for inside or outside mount.
What it does
Blinds order from the window, not the wall. Inside mounts subtract clearance to fit the jamb, while outside mounts add overlap to block light past trim. This page applies both rules in inches or cm.
The default is a 36 by 48 in window with inside mount. The blind orders 35.5 by 48 in. Flip to outside and the same window orders 39 by 51 in. Metric defaults convert the window to 91 by 122 cm and give 89.7 by 122 cm inside.
Tape discipline decides fit. Steel beats cloth, three measures beat one, and the smallest inside width wins since jambs lean. Note crank handles and tiles that steal depth before ordering.
Mount is a light and look choice. Inside shows trim and leaves small side gaps, outside darkens more and hides crooked openings but covers wall. Doors need clearance for handles either way.
How to use
- Pick imperial or metric first.
- Pick inside or outside mount.
- Enter window width and height.
- Read blind width first, then height.
- Measure thrice and order the smallest inside width.
Formula
- window W
- Window opening width, jamb to jamb.
- window H
- Window opening height.
- allowance
- Inside clearance or outside overlap.
Inside clearance keeps the headrail off the jamb; outside overlap blocks side light and covers trim. Measure each window in three spots and order the smallest inside width, since jambs run out of square. Metric converts 0.5 in to 1.3 cm and 3 in to 7.6 cm.
Worked example
Example: 36 by 48 in window
36 by 48 in window, inside mount (metric 91 by 122 cm).
- Inside width: 36 - 0.5 = 35.5 in.
- Inside height: stays 48 in.
- Outside width: 36 + 3 = 39 in.
- Outside height: 48 + 3 = 51 in.
- Metric inside: 91 - 1.3 = 89.7 cm by 122 cm.
- Tool shows 35.5 by 48 in for inside mount.
Reference table
| Window | Inside | Outside |
|---|---|---|
| 24 x 36 in | 23.5 x 36 in | 27 x 39 in |
| 30 x 48 in | 29.5 x 48 in | 33 x 51 in |
| 36 x 48 in | 35.5 x 48 in | 39 x 51 in |
| 36 x 60 in | 35.5 x 60 in | 39 x 63 in |
| 48 x 48 in | 47.5 x 48 in | 51 x 51 in |
Limits
- Width and height only. Depth and handles excluded.
- Assumes square openings. Shim or trim gaps remain.
- One window per pass. Bays need splitting.
- Light gap differs by product.
Common mistakes
- Ordering window size as blind size.
- Using cloth tape that stretches.
- Averaging three measures instead of smallest.
- Forgetting crank and tile depth.
- Mounting outside with no wall bite.
Try it yourself
Quick checks. Answers sit tucked under each question.
24 by 36 in window, inside?
23.5 by 36 in (24 - 0.5, height stays).
24 by 36 in window, outside?
27 by 39 in (24 + 3, 36 + 3).
Questions
Inside mount size?
Width minus half inch, height as measured. Here 35.5 by 48 in.
Outside mount size?
Width plus 3 in, height plus 3 in. Here 39 by 51 in.
What is that in metric?
89.7 by 122 cm for inside mount.
Which mount?
Inside sits in the jamb, outside covers trim. Light gap differs.
Steel tape needed?
Yes for planning. Out of square jambs need the smallest of three measures.
Sliding doors?
No. Headrail depth and door clearance sit outside width math.