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Aspect Ratio Scaler🔒 100% local
Calculate proportional dimensions. Privacy-first, no server uploads, no tracking.
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Works out proportional sizes from a width and a height. Give it a pair of dimensions and it reduces them to a ratio, lists the matching height for a set of common widths and writes the CSS aspect-ratio declaration for that shape. A third number in the input is read as a width to scale to, and the arithmetic runs in the page on numbers you type.
How to use it
- Enter the dimensions as width, height, for example 1280, 720. Add a third number to scale to that width.
- Press Process to run the calculation.
- Read the reduced ratio, the scaled row and the width table, then use Copy Output if you want the whole block.
Worked example
1920, 1080, 800becomesOriginal: 1920 x 1080 (16:9)
Ratio: 1.7778 (decimal)
Scaled to width 800: 800 x 450
Common targets (matching this ratio 16:9):
1920px wide: 1920 x 1080
1280px wide: 1280 x 720
1200px wide: 1200 x 675
1080px wide: 1080 x 608
800px wide: 800 x 450
640px wide: 640 x 360
480px wide: 480 x 270
320px wide: 320 x 180
Closest standard ratios:
16:9 (matches 0.0%)
16:10
4:3
21:9
3:2
1:1
9:16
2:3
CSS preserve-aspect:
aspect-ratio: 16 / 9;The complete output for that input. Heights are rounded to whole pixels, which is why 1080 px wide at 16:9 reads 608 instead of 607.5.
1600, 900becomesOriginal: 1600 x 900 (16:9)
Ratio: 1.7778 (decimal)
Common targets (matching this ratio 16:9):
1920px wide: 1920 x 1080
1280px wide: 1280 x 720
1200px wide: 1200 x 675
1080px wide: 1080 x 608
800px wide: 800 x 450
640px wide: 640 x 360
480px wide: 480 x 270
320px wide: 320 x 180
Closest standard ratios:
16:9 (matches 0.0%)
16:10
4:3
21:9
3:2
1:1
9:16
2:3
CSS preserve-aspect:
aspect-ratio: 16 / 9;With no third number there is no Scaled to width row, and the ratio still reduces to 16:9 because 1600 and 900 share a factor of 100.
When to use it
- Resizing a video or an image to a container width and needing the height without opening an editor.
- Checking that a thumbnail keeps the same shape as the file it was made from.
- Copying the exact aspect-ratio rule for a CSS box that has to reserve space for media.
What it does not do
- Only width is solved for. The third number is always treated as a target width, and there is no way to scale to a target height or to a target area.
- All widths are rounded to whole pixels, so the paired height can sit slightly off the exact ratio: 1080 at 16:9 gives 608, not 607.5.
- The match flag on the standard-ratio list uses a five percent tolerance and no explanation, so a ratio almost five percent away still shows as matching.
- Numbers are read as bare pixels. There are no units and no way to enter 80% or 16cm, and the tool does not open or resample an image file at all.