Aspect Ratio Calculator
Work with aspect ratios two ways: simplify a pair of dimensions to their lowest whole-number ratio, or scale one dimension and get the other while keeping the proportions exact. Enter 1920 × 1080 and it reduces to 16:9; give it a new width and it returns the matching height. The calculator also names common formats — 16:9 for HD video, 4:3 for classic displays, 21:9 for ultrawide — so you can tell at a glance what a set of dimensions corresponds to.
How to use this tool
- Choose whether to simplify a ratio or scale dimensions.
- Enter the original width and height.
- To scale, add a new width (or leave it blank and give a new height).
- Read the simplified ratio, the decimal ratio and any matching common format.
- Use the result to resize images or video without distorting them.
The formula
An aspect ratio expresses the relationship between width and height independently of size. Reducing the two dimensions by their greatest common divisor gives the simplest whole-number form, so 1920×1080 and 1280×720 both reduce to 16:9.
divisor = gcd(width, height)
ratio = (width ÷ divisor) : (height ÷ divisor)
scaled_height = width × ratio_h ÷ ratio_w- gcd
- Greatest common divisor of the two dimensions
- ratio_w : ratio_h
- The reduced ratio in lowest terms
Some resolutions are advertised under a ratio they do not quite match. 3440×1440 is sold as 21:9 but reduces to 43:18, which is 2.389 rather than 2.333. This calculator reports the true ratio and notes the marketing name separately.
Worked examples
Simplify HD
- Given
- 1920 × 1080
- Result
- 16:9
The greatest common divisor of 1920 and 1080 is 120; dividing both gives 16:9, the standard HD ratio.
Scale keeping 16:9
- Given
- 16:9, new width 1280
- Result
- 1280 × 720
At 16:9, a width of 1280 gives a height of 1280 × 9 ÷ 16 = 720.
Classic display
- Given
- 1024 × 768
- Result
- 4:3
Dividing by their GCD of 256 gives 4:3, the traditional monitor and TV ratio.
Frequently asked questions
Aspect ratio is the proportional relationship between width and height, written as two numbers like 16:9. It describes shape, not size — 1280×720 and 1920×1080 are both 16:9. Keeping the ratio constant when resizing is what prevents images and video from looking stretched or squashed.
Divide both the width and height by their greatest common divisor. For 1920×1080 the GCD is 120, so both reduce to 16:9. This calculator does that automatically and, when the numbers do not reduce to a tidy pair, gives the closest common ratio as well as the exact decimal.
16:9 is the standard widescreen ratio for HD and most modern displays. 21:9 (actually closer to 64:27) is “ultrawide”, noticeably wider relative to its height, used for cinematic video and some monitors. The calculator identifies both from their dimensions.
Because pixels are whole numbers. When a scaled height or width does not come out to an exact integer, it is rounded to the nearest pixel, which can shift the ratio by a tiny fraction. For most work this is invisible; for pixel-perfect needs, choose dimensions that divide evenly.
Aspect ratio is unit-agnostic, so it applies equally to print, screens and video — any rectangle has a ratio. For turning a ratio into physical print dimensions at a given DPI, pair this with the print resolution calculator.
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