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Guide1 min readUpdated July 1, 2026

How to calculate aspect ratio

Short answer

Divide width and height by their greatest common divisor to express the ratio in simplest form. For 1920x1080, both divide by 120 to give 16:9. To resize proportionally, multiply both sides by the same factor.

What aspect ratio means

Aspect ratio is the proportional relationship between width and height, written like 16:9. It describes shape, not size — a 1280x720 and a 1920x1080 image share the same 16:9 ratio. See common aspect ratios explained.

Reduce a resolution to its ratio

Divide the width and height by their greatest common divisor (GCD). For 1920x1080, the GCD is 120, so 1920/120 : 1080/120 = 16:9.

ratio = width/GCD : height/GCD
1920 x 1080  ->  GCD 120  ->  16:9
1600 x 1200  ->  GCD 400  ->  4:3
The method

Resize while keeping the ratio

To find a missing dimension, keep the ratio constant: new height = new width x (original height / original width). For 16:9 at 1280 wide: 1280 x 9/16 = 720.

Try it: Aspect Ratio CalculatorReduce any resolution to its ratio and solve for a missing dimension.Open tool
Fitting a different ratioForcing an image into a different ratio either stretches it or crops it. To change shape without distortion, crop rather than stretch. See how to resize an image.

References

Questions

What aspect ratio is 1920x1080?

16:9. Both numbers divide by 120, giving 16:9 — the standard widescreen ratio used by most monitors, TVs, and video.

How do I keep the aspect ratio when resizing?

Scale width and height by the same factor. If you set a new width, compute the height as new width times the original height divided by the original width.

Does the calculator send my numbers anywhere?

No. Aspect ratio calculations run entirely in your browser tab. Nothing is uploaded or stored.

Keep reading

Explainer1 min readCommon aspect ratios explainedAn aspect ratio is the proportion of width to height. Here is what 16:9, 4:3, 1:1, and 9:16 mean and where each is used.Read
Guide1 min readHow to resize an image without losing qualityResize a photo to specific pixel dimensions in the browser. Learn the difference between fit and exact resizing, and how to avoid a blurry result.Read
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