Aspect ratio calculator

Getting a clean resize instead of a squashed one

Every stretched-looking thumbnail on the internet is someone typing both dimensions of a resize by hand. The discipline this tool enforces is the right one: you only get to choose one new dimension, and the other is computed from the original's proportions. In practice, decide which dimension your platform constrains. Web layouts and video encodes are usually width-constrained, so type the new width and take the height you are given. Print and portrait phone screens are often height-constrained, so type the height instead. Whichever of the two new fields you typed last is the one in charge, and the calculator fills the other for you.

A tip that saves re-encodes: if the result is destined for video, nudge your chosen dimension so both numbers come out even. Common video encoders require even pixel dimensions, and some pipelines want multiples of 8 or 16. Scaling 1920x1080 to a width of 1286 gives a height of 723.38, which no encoder will accept; choosing 1280 instead lands on a clean 720. When the computed value has a decimal, treat it as a warning that your target width does not divide the ratio cleanly, and try a neighbor.

A worked example with a phone photo

A typical phone photo is 4032x3024 pixels. Enter that as the original, and the tool reports the simplified ratio 4:3, decimal 1.33:1, because 4032 and 3024 share a greatest common divisor of 1008. Now say a blog template wants images 1600 pixels wide. Type 1600 into the new width. The height comes back as 1600 x (3024 / 4032) = 1200, so the export target is 1600x1200 and nothing gets cropped or stretched. Had you needed a height of 900 instead, typing it into the height field would drive the calculation the other way and return a width of 1200.

The math and the ratio readouts

The scaling is cross-multiplication: new height equals new width times the original height over the original width, and symmetrically when height drives. Results round to two decimal places. The simplified ratio divides both original dimensions by their greatest common divisor, which is exact but only defined when both inputs are whole numbers; feed it fractional dimensions and that stat sits out while the decimal ratio still reports. The decimal figure, width divided by height, is the fastest way to compare two sizes that simplify to ugly ratios: two images are proportionally identical exactly when their decimals match. The presets load reference dimensions for the six ratios you will meet most, from 16:9 down to ultrawide 21:9.

What resizing math cannot fix

This calculator preserves proportions; it does not change them. If your source is 4:3 and the destination demands 16:9, no scaled size will fit without either cropping or letterboxing, and choosing between those is an editorial call, not arithmetic. It also knows nothing about resolution quality: scaling 640x480 up to 1600x1200 keeps the shape and produces a soft, upsampled image anyway. And fractional pixel outputs must ultimately be rounded by whatever software does the resize, which on tiny images can shift the true ratio by a hair.