Design & Tech

Aspect Ratio Calculator

Three things you need a ratio for: naming one, completing one, and scaling to fit. Presets for 16:9, 4:3, 1:1, 3:2, 21:9 and vertical video are one tap away.

This calculator needs JavaScript to run. The formula, the worked examples and the explanations below all work without it.

Width

Height

Presets fill the ratio boxes — you can still type any custom ratio.

What an aspect ratio is

An aspect ratio is the relationship between width and height, written as two numbers separated by a colon. 16:9 means the width is 16 units for every 9 units of height. It says nothing about size: 1280×720, 1920×1080 and 3840×2160 are all 16:9.

Formuladivide both dimensions by their greatest common divisor

1920 and 1080 share a greatest common divisor of 120, so dividing both gives 16:9. That is exactly what this calculator does, which is why it can name a ratio you have never seen before rather than guessing at the nearest common one.

The ratios worth knowing

Ratio	Where you see it	Common sizes
16:9	HD video, YouTube, most monitors and TVs	1920×1080, 3840×2160
9:16	Vertical video — Reels, TikTok, Shorts	1080×1920
4:3	Older monitors, iPad, classic cameras	1024×768, 2048×1536
1:1	Square social posts and profile pictures	1080×1080
3:2	DSLR and mirrorless photography, 6×4 prints	6000×4000
21:9	Ultrawide monitors and cinematic video	3440×1440, 2560×1080
2.39:1	Anamorphic widescreen cinema	4096×1716

A note on "21:9": almost no ultrawide monitor is exactly 21:9. A 3440×1440 panel is precisely 43:18, or about 2.389:1. The calculator reports the true ratio and tells you what the industry calls it, rather than quietly rounding.

Working out a missing dimension

This is the everyday use: you know you want 16:9 and you know one side. Multiply by the ratio the right way round.

Formulaheight = width × (ratio height ÷ ratio width)

For a 2560-pixel-wide 16:9 image: 2560 × 9 ÷ 16 = 1440. Going the other way, height 1080 at 16:9 gives 1080 × 16 ÷ 9 = 1920.

Ratios rarely divide into whole pixels. 1000 pixels wide at 16:9 is 562.5 high — the calculator rounds to 563 by default and tells you it did, or gives the exact decimal if you switch the option. For video, round to an even number: most codecs require even dimensions, and some require multiples of 4.

Resize to fit, without distortion

The scale tab answers "make this fit inside a box without squashing it". It works out how much each dimension would have to shrink and uses the smaller of the two factors, so the whole image fits and the ratio is preserved.

A 1920×1080 image into a 1280×1280 box

The width factor is 0.667 and the height factor is 1.185. The smaller wins, so the result is 1280×720 — it touches the sides and leaves space above and below.

Distortion happens when the two factors are applied independently — that is what CSS object-fit: fill does, and why stretched thumbnails look wrong. Keeping one factor for both dimensions is the whole trick.

Worked examples

What ratio is 1080×1350?

4:5 — the Instagram portrait format, and the largest area the feed will show without cropping.

A 16:9 video at 1440 pixels tall

1440 × 16 ÷ 9 = 2560 pixels wide.

A photo is 6000×4000. What ratio, and how many megapixels?

3:2, and 24 megapixels — the classic full-frame camera format, which prints at 6×4 inches with no cropping.

Common uses

Video: exporting at the right dimensions for each platform.
Web design: responsive image boxes and CSS aspect-ratio values.
Photography: cropping to a print size without surprises.
Displays: comparing monitors and working out real screen dimensions.
Print: scaling artwork to a frame or a page.

Frequently asked questions

How do I calculate an aspect ratio?

Divide the width and the height by their greatest common divisor. For 1920×1080 that divisor is 120, giving 16:9. If the numbers do not reduce to something small and tidy, the honest answer is the decimal ratio — 2.39:1, for example.

What size is 16:9?

16:9 is a shape, not a size. Common 16:9 resolutions are 1280×720, 1920×1080, 2560×1440 and 3840×2160. Enter either dimension in the second tab and the calculator gives you the other.

Why does my ultrawide monitor say 43:18 instead of 21:9?

Because 3440×1440 genuinely is 43:18 — about 2.389:1 rather than 2.333:1. The industry markets it as 21:9 for simplicity. The calculator gives you the exact ratio and names the marketing label alongside it.

How do I resize an image without stretching it?

Scale both dimensions by the same factor. The resize tab does this: it finds the factor for each side, uses the smaller one so the whole image fits, and gives you dimensions that keep the original ratio exactly.

What if the ratio does not give whole pixels?

Most do not. 1000 pixels wide at 16:9 is 562.5 high. Round to the nearest whole pixel for most purposes, and to an even number for video — many codecs reject odd dimensions, and some need multiples of 4.

Which ratio should I use for social video?

9:16 for full-screen vertical formats like Reels, TikTok and Shorts. 1:1 for square feed posts, 4:5 for portrait feed posts, and 16:9 for landscape YouTube. Platforms change their preferences, so check the current spec before a big export.