DesignFree
Free aspect ratio calculator. Simplify width×height ratios, use presets (16:9, 4:3, 1:1, 9:16), and scale dimensions for video or images.
Last updated 14 October 2026
Give it a width and a height and it reduces them by their greatest common divisor to give the simplified ratio, shows the decimal, names the orientation, and will scale the shape to a target width or height. All of that is exact arithmetic on whole numbers.
The scaling and the presets are where whole numbers stop cooperating, because a shape that divides cleanly at one size usually does not at another. Two of the six presets on this page do not actually produce their own ratio, so that comes first.
Choosing a preset keeps your current width and computes a height from it, rounded to the nearest whole pixel. When the rounding bites, the pair no longer shares the factor that would simplify it back to the preset. Measured, every preset selected with the width left at 1920:
The error in the shape is tiny, under a tenth of a percent, and perfectly usable for a layout. The reported ratio, though, is wrong in a way that looks authoritative, and if you were using this page to find out what ratio a size represents, 1920:823 is the answer it gives you.
It is not only those two presets. Starting from a width that does not divide, even 16:9 misbehaves: width 1000 gives 1000 × 563 reported as 1000:563, width 375 gives 375 × 211 reported as 375:211, and width 1366 gives 1366 × 768 reported as 683:384. None of those is 16:9, and the last one is the common laptop resolution that has never been exactly 16:9.
Read the simplified ratio the tool reports back, not the preset you chose. If it does not say what you expected, the rounding is why.
The two scaling boxes have the same problem from the other direction. From 1920 × 1080, measured:
That last pair is the clearest illustration: going out by width and back by height loses a pixel. Neither answer is wrong, since both round correctly, but the operation is not reversible and nothing says so. For video encoding, where many codecs want dimensions divisible by 2 or by 16, round the result yourself to a number that suits the encoder rather than taking what the tool gives.
No widely sold ultrawide monitor is 21:9. You can check that here, with the two resolutions those panels actually use. Measured on this page:
True 21:9 would be 2.3333. Both real panels are wider than that, and they are not the same shape as each other either. 64:27 has a pedigree, being 16:9 cubed and the ratio used for ultrawide video, while 43:18 is simply what 3440 by 1440 happens to reduce to.
So if you have been handed “21:9” as a requirement, ask which resolution is meant. The page's own preset, as shown above, produces a third answer again.
Width and height are read as whole numbers, and anything that is not a usable one is treated as 1 for the calculation while the box keeps what you typed. Measured:
0 and -5 both compute as 1, giving “1:1080” against a height of 1080, with the 0 or the −5 still on screen.1920.7 computes as 1920, so decimals are cut off rather than rounded.1e3 computes as 1, not 1000, because the reading stops at the letter.A reported ratio of 1:1080 is the tell that one of these has happened. This is the opposite convention to our Image Placeholder Generator, which corrects its boxes to the value it used, and the disagreement between the two is worth knowing if you move between them.
The rest is straightforward. The decimal is given to four places in the headline and six in the table, orientation is landscape, portrait or square, and the preview box scales the shape to fit 280 pixels on its longer side.
Once you have the dimensions, Image Placeholder Generator will draw a PNG at exactly that size.
1e3 is read as 1 and 1920.7 as 1920: the value is parsed as an integer and the reading stops at the first character that is not part of one. developer.mozilla.orgEvery behaviour and every value on this page was measured against this tool as it is published, in a browser, on 14 October 2026. The preset results come from selecting each preset in turn with the width left at 1920.
It is accurate about the numbers and misleading about your intent. The preset keeps your width and rounds the height, so 1920 gives 823 instead of the 822.857 that true 21:9 needs, and 1920 and 823 share no factor to simplify. The shape is within a tenth of a percent of 21:9; the reported ratio is not 21:9. The 9:16 preset behaves the same way, giving 1920:3413.
From a width of 1920, the 16:9, 4:3, 1:1 and 3:2 presets all report back the ratio you chose. From other widths even those can drift: width 1000 with the 16:9 preset reports 1000:563, and width 1366 reports 683:384. Always read the simplified ratio the tool gives back.
Because each direction rounds to a whole pixel. From 1920 by 1080, a target width of 1000 gives 1000 by 563, and feeding 563 back as a target height gives 1001 by 563. Both are correctly rounded and the operation is simply not reversible.
Almost certainly not. Put its resolution into this page and see: 2560 by 1080 reports 64:27, which is 2.3704, and 3440 by 1440 reports 43:18, which is 2.3889. True 21:9 is 2.3333, so the name is a rounded marketing label rather than a measurement.
Because the width was unusable and was treated as 1. A zero, a negative number or letters all do this, and the box keeps whatever you typed rather than correcting itself, so the ratio is the only place the substitution shows.
Not necessarily. Many encoders require dimensions divisible by 2, and some by 16, and this tool knows nothing about that. 1000 by 563 is a valid shape and an awkward encode. Round the result yourself to suit your encoder.
No. Both inputs are whole pixel counts, so ratios that are not a clean integer pair cannot be entered directly. Enter a resolution that approximates it instead and read the decimal.
No. The calculation runs in your browser and nothing is stored or transmitted.