Aspect Ratio Calculator

How Does the Formula Work?

The aspect ratio calculator works in four modes: simplify any pixel dimensions to their ratio (1920×1080 → 16:9), find a missing width from a ratio and height, find a missing height from a ratio and width, or scale dimensions proportionally by entering a new width or height. Aspect ratio is the proportional relationship between width and height; it determines the shape of screens, images, videos, and print layouts. Getting it wrong causes stretched, squashed, or cropped content. This calculator uses GCD (Greatest Common Divisor) for exact ratio simplification, ensuring mathematically precise results for any resolution. Ratio terms may have decimals, so 2.39:1 can be entered as written; pixel sizes are whole numbers, and a missing side is rounded to the nearest pixel.

Ratios of Familiar Resolutions

Resolutions of very different sizes can share one shape. 1280×720, 1920×1080, 2560×1440, 3840×2160 and 7680×4320 all simplify to 16:9, and 1920×1200 simplifies to 8:5, which is the same shape as 16:10. Some ratio names are rounded: 2560×1080 simplifies to 64:27, about 2.37:1, while 21:9 is about 2.33:1, and 1366×768 gives 683:384, close to 16:9 without being equal to it. The Decimal box makes such comparisons easy: 16:9 shows as 1.7778 and 683:384 as 1.7786.

Image Resizing Without Distortion

Changing only the width or only the height of an image stretches it. Scale mode keeps the shape: enter 1920×1080 as the original size and 800 as the new width, and the height comes out as 450. Fill in just one of the two new sides; if both are filled in, the calculator asks you to clear one, because the second side has to follow from the ratio. Fitting an image into a frame of another shape always costs something. A 4:3 image 1600 pixels wide is 1200 pixels tall, but a 16:9 frame of the same width is only 900 pixels tall, so a quarter of the image height has to be cropped.

Letterbox and Pillarbox

When a picture and a screen have different shapes and nothing is cropped, empty bands fill the gap. A 2.39:1 picture shown at full width on a 16:9 screen covers about 74% of the screen height (1.7778 ÷ 2.39 ≈ 0.744), so bands appear above and below it; this is called letterboxing. A 4:3 picture shown at full height on the same screen covers 75% of its width (1.3333 ÷ 1.7778 = 0.75), with bands at the sides; this is called pillarboxing. Find Height gives the exact picture height: 1920 pixels wide at 2.39:1 is 803 pixels tall.

Web Design and CSS

CSS has an aspect-ratio property that fixes the proportions of a box directly, for example aspect-ratio: 16 / 9. The older technique relies on percentage padding, which CSS measures against the width of the containing block: padding-bottom: 56.25% makes a 16:9 box because 9 ÷ 16 = 0.5625. The Decimal box shows width divided by height, and 100 divided by that number gives the same padding percentage (100 ÷ 1.7778 ≈ 56.25).

Photography and Print

A photo's pixel size tells you its shape: 6000×4000 simplifies to 3:2 and 5184×3888 to 4:3. Print sizes can be checked the same way by entering them as whole numbers: 8×10 inches and 20×25 cm both simplify to 4:5. A 3:2 photo does not fit 4:5 without cropping. Printed 8 inches on its short side, it would be 12 inches long, so 2 of those 12 inches, one sixth of the picture, fall outside an 8×10 print.

Scaling for Several Formats

When one picture has to be delivered in several shapes, pick a width and run Find Height once for each ratio. At 1080 pixels wide, 1:1 needs 1080 pixels of height, 4:5 needs 1350 and 9:16 needs 1920. Keep the main subject away from the edges that the shorter crops will cut off.