Written by Sarah Mitchell · Reviewed by Jane Smith · July 15, 2026
Calculate the missing dimension from any aspect ratio. Choose a preset or enter a custom ratio.
This tool solves two related problems. Given one pixel dimension and a target ratio, it derives the other dimension; given a width × height pair, it reduces the pair to its lowest-terms ratio. Both directions update live as you type, and a small preview box redraws to match the current proportions.
The eight preset buttons cover the ratios you meet most often — 16:9 and 21:9 for widescreen video, 4:3 and 5:4 for older monitors and photography, 3:2 for classic camera sensors, 16:10 for laptop displays, 1:1 for square social posts, and 9:16 for vertical phone video. Anything not in the list goes into the two ratio fields directly, from a 2.35:1 cinema scope frame to a 4:5 Instagram portrait.
The width and height fields accept whole-number pixels, while the two ratio fields accept decimals, so a 2.35:1 cinema frame is entered as 2.35 and 1. Clicking any preset fills the ratio fields and recalculates on the spot, keeping the preview in step with the fields.
Both directions rest on the same idea: a ratio pairs a width and a height that stay in a fixed proportion. In the direction from dimensions to ratio, the tool finds the greatest common divisor of the two pixel counts and divides both sides by it. Euclid's algorithm does the work. For width 1920 and height 1080, 1920 mod 1080 = 840, then 1080 mod 840 = 240, then 840 mod 240 = 120, and 240 mod 120 = 0, so the GCD is 120. Dividing both sides by 120 gives 16:9. The algorithm is exact for any pair of positive whole numbers, so the simplified ratio never drifts from the pixel counts you typed.
In the other direction the missing dimension is one multiplication and one division, rounded to a whole pixel: height = round(width × ratio height ÷ ratio width), or width = round(height × ratio width ÷ ratio height). A 1,280-pixel-wide frame at 16:9 works out to 1,280 × 9 ÷ 16 = 720 pixels exactly. Entering 2.35 in the first ratio field and 1 in the second treats the ratio as a single division: the missing dimension is width divided by 2.35, or height multiplied by 2.35.
The two modes are easiest to see back to back. Start with a 1920 × 1080 frame — the standard for Full HD video. Entering both dimensions shows the simplified ratio 16:9, because the GCD of 1920 and 1080 is 120 and both sides divide cleanly by it.
Now keep that ratio and ask for the height of a 1,280-pixel-wide export. The tool computes 1,280 × 9 ÷ 16 = 720, so a 1280 × 720 frame comes out — the familiar 720p resolution. The preview box redraws at exactly 16:9 proportions.
A third case shows rounding in action. A 1,000-pixel-wide frame at 16:9 yields height 1,000 × 9 ÷ 16 = 562.5, which rounds to 563. The result line then reads 1000 × 563 → 1000:563, because the simplified pair no longer reduces back to 16:9.
Because simplification only divides out shared factors, and 1280 and 854 share just the factor 2. A true 3:2 frame at that width would be 1280 × 853.33, which cannot exist in whole pixels; the nearest integer pair, 1280 × 853, reduces to 1280:853. Lock the 3:2 preset instead and let the tool choose the dimension.
The derived dimension rounds to the nearest whole pixel, because a bitmap has no half pixels. If you need the exact figure for a calculation, do the division yourself before rounding. The rounding goes half up, so a computed 562.5 becomes 563 while a computed 562.4 stays 562.
Yes. Put the first number in the Ratio Width field and the second in Ratio Height. The derived dimension is reliable; the simplified-pair line is only meaningful when both numbers are whole.
9:16. For a square crop use 1:1, and for a typical portrait feed post use 4:5 — that one is not a preset, so enter it manually in the ratio fields.
No. The numbers stay in this page; nothing is uploaded, and the tool keeps working offline once it has loaded.
No. A ratio carries no physical size. For print you also need a resolution in dots per inch; this page only shapes the proportions.
Reach for it when a platform or a camera asks for a specific frame shape: sizing a YouTube thumbnail to 16:9, checking that an exported photo keeps its sensor's native 3:2, cropping a social post to 1:1, or planning a 21:9 ultrawide wallpaper. It is equally handy for CSS and layout work, where a container's aspect-ratio property expects the same pair of numbers. It also pays off when a designer hands you a crop request such as "make it 16:9" and you need the exact pixel pair that matches your source file.
Put it away when the question is physical. Paper sizes, print resolution, and screen pixel density involve absolute measurements, not proportions. And when an image merely looks about right, a ratio check is overkill — this tool answers exact questions.