About the Aspect Ratio Calculator
This aspect ratio calculator reduces any pair of dimensions to its simplest ratio — 1920 × 1080 becomes 16:9, 2560 × 1080 becomes 64:27 (marketed as 21:9) — and shows the decimal ratio and the closest standard format. Enter a new width or height and it calculates the matching dimension so your image, video or embed scales without stretching or letterboxing.
Web developers use it to size responsive images and iframes, video editors to pick export resolutions, photographers to plan crops and prints, and anyone buying a monitor or TV to compare screen shapes. The table lists common resolutions at your ratio, from small thumbnails to 8K.
Dimensions can be pixels, inches, centimetres or any other unit — a ratio has no units. Decimal dimensions are rounded to whole numbers before the ratio is simplified, and scaled results are shown to two decimals so you can decide how to round.
How to use the aspect ratio calculator
- 1Enter the original width and height (pixels, inches or any unit).
- 2Read the simplified ratio, decimal ratio and closest standard format.
- 3Choose whether you know the new width or the new height.
- 4Enter it to get the other dimension without distortion.
- 5Copy the CSS aspect-ratio line or pick a size from the table.
Formula and method
An aspect ratio is the width divided by the height. To show it as two whole numbers, the calculator divides both dimensions by their greatest common divisor (found with Euclid’s algorithm): gcd(1920, 1080) = 120, so 1920 × 1080 becomes 16:9.
Resizing keeps the ratio constant: the new height is the new width multiplied by the original height and divided by the original width, and vice versa. The closest standard ratio is chosen by the smallest proportional difference, treating portrait sizes as the flipped landscape ratio.
- W, H
- Original width and height
- W₂, H₂
- New width and height
- gcd
- Greatest common divisor of the two dimensions
Worked examples
Full HD down to 1280 wide
gcd(1920, 1080) = 120, so the ratio is 1920/120 : 1080/120 = 16:9 (1.7778). Scaling to 1280 wide gives 1280 × 1080 ÷ 1920 = 720 px high — the standard 720p size.
Ultrawide monitor 2560 × 1080
gcd(2560, 1080) = 40, giving 64:27 ≈ 2.37:1, which is sold as “21:9”. A 3440-wide screen at the same shape would be 3440 × 27 ÷ 64 = 1451.25 px tall — real 3440 ultrawides are 1440 px, so they are slightly wider than 64:27.
Portrait phone photo to 1080 px tall
gcd(3024, 4032) = 1008, so the photo is 3:4 portrait. At 1080 px tall the width becomes 1080 × 3024 ÷ 4032 = 810 px.
Frequently asked questions
How do I calculate an aspect ratio?+
Divide the width and height by their greatest common divisor. For 1920 × 1080 the GCD is 120, so the ratio is 16:9. The decimal form is simply width ÷ height (1.78).
What resolution is 16:9?+
Common 16:9 sizes are 1280 × 720 (720p), 1920 × 1080 (1080p), 2560 × 1440 (1440p), 3840 × 2160 (4K UHD) and 7680 × 4320 (8K).
Is 21:9 really 21:9?+
Not exactly. Most “21:9” ultrawide monitors are 2560 × 1080 (64:27 ≈ 2.37:1) or 3440 × 1440 (43:18 ≈ 2.39:1). 21:9 itself is 2.33:1; the name is marketing shorthand.
What aspect ratio should I use for Instagram or TikTok?+
Vertical video for TikTok, Reels and Stories is 9:16 (1080 × 1920). Instagram feed posts support 1:1 (1080 × 1080), 4:5 portrait (1080 × 1350) and 3:4 portrait (1080 × 1440), which matches the profile grid.
How do I keep an aspect ratio in CSS?+
Use the aspect-ratio property, e.g. aspect-ratio: 16 / 9, on the element and set only its width. The browser then calculates the height automatically; the calculator gives you the exact line to copy.