Picture Frame Mat Opening Calculator
A mat board is cut to the frame's inner size with a window slightly smaller than the photo cut out of it. Subtracting the border width twice (once per side) from the outer width and height gives the opening's width and height; multiplying them gives the opening area.
Your numbers
Window opening area
1,200cm²
Window opening width
30cm
Window opening height
40cm
Formula
Window opening area = (Mat outer width (cm) − 2×Mat border width (cm)) × (Mat outer height (cm) − 2×Mat border width (cm))
Borders are often uneven (a wider bottom border is common) — calculate width and height separately in that case.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Mat outer width (cm) (cm) | 40 | 5 ~ 200 |
| Mat outer height (cm) (cm) | 50 | 5 ~ 200 |
| Mat border width (cm) (cm) | 5 | 0.5 ~ 15 |
Step by step
Quick reference table
Results when only Mat outer width (cm) (cm) changes and everything else stays put.
| Mat outer width (cm) (cm) | Window opening area (cm²) | Window opening width (cm) | Window opening height (cm) |
|---|---|---|---|
| 20 | 400 | 10 | 40 |
| 30 | 800 | 20 | 40 |
| 40 | 1,200 | 30 | 40 |
| 60 | 2,000 | 50 | 40 |
| 80 | 2,800 | 70 | 40 |
What each result means
| Result | At default values |
|---|---|
| Window opening area (cm²) | 1,200 |
| Window opening width (cm) | 30 |
| Window opening height (cm) | 40 |
Common mistakes
Borders are often uneven (a wider bottom border is common) — calculate width and height separately in that case.
Frequently asked questions
QHow is Picture Frame Mat Opening Calculator calculated?
Window opening area = (Mat outer width (cm) − 2×Mat border width (cm)) × (Mat outer height (cm) − 2×Mat border width (cm)) — A mat board is cut to the frame's inner size with a window slightly smaller than the photo cut out of it. Subtracting the border width twice (once per side) from the outer width and height gives the opening's width and height; multiplying them gives the opening area.
QCan you walk through an example?
With Mat outer width (cm) 40cm, Mat outer height (cm) 50cm, Mat border width (cm) 5cm, the answer is Window opening area 1,200cm².
QWhat do I need to enter?
Enter Mat outer width (cm), Mat outer height (cm), Mat border width (cm). The result recalculates as you type, and an empty box counts as zero.
QHow much does the answer move if I change a number?
Changing only Mat outer width (cm) (cm) moves the answer to Mat outer width (cm) (cm) 20 → Window opening area (cm²) 400 and Mat outer width (cm) (cm) 80 → Window opening area (cm²) 2,800. The table below lays out five steps.
QHow are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
QAnything to watch out for?
Borders are often uneven (a wider bottom border is common) — calculate width and height separately in that case.
Related calculators
Keep your length units consistent across inputs — mixing centimetres and metres throws results far off.