Aspect Ratio Calculator
Work out aspect ratios and resize dimensions — one click for 16:9, 4:3, 1:1, 9:16, 3:2, 21:9 or A4, or find the missing width or height, with the shape drawn.
🔒 Calculated in your browser. Editing width or height keeps the ratio; edit the ratio to recompute.
Keep the right proportions
Set a ratio (like 16:9) and enter a width to get the matching height, or a height to get the width — the other value updates automatically. You can also paste any size to simplify it to its smallest ratio and load it back in. Great for video, images, screens and responsive layouts.
How it works
A size is reduced to its simplest ratio using the greatest common divisor, so 1920×1080 becomes 16:9. Resizing then scales one dimension from the other while keeping that ratio exact.
Ratio is not resolution
1920×1080, 1280×720 and 3840×2160 are three resolutions and one ratio. The ratio says what shape the frame is; the resolution says how many pixels are in it. That is why 16:9 footage fits a 16:9 screen of any size without bars, and why a 4:3 photo on a 16:9 screen gets pillarboxed no matter how large either is. When you are resizing for the web, the ratio is what you must keep and the resolution is what you are free to change.
Sizes that refuse to simplify, and A-series paper
Simplifying divides both numbers by their greatest common divisor, so it only produces a familiar ratio when one exists. 1000×667 reduces to 1000:667 — the numbers share no factor — even though it is within a pixel of 3:2. That is the arithmetic being honest rather than the tool failing. A-series paper is the clearest case: A4 is 210×297 mm, which simplifies to 70:99 and looks like nothing. The real ratio is 1:√2, chosen so that folding a sheet in half gives the same shape again — and √2 is irrational, so no whole-number ratio can express it exactly. Every A size is that same shape at a different scale.
Frequently asked questions
How is the ratio simplified?
The width and height are divided by their greatest common divisor, giving the smallest whole-number ratio (e.g. 1280×720 → 16:9).
Can I resize keeping the ratio?
Yes. Set the ratio, then type a new width or height and the other dimension is calculated to match.
Is anything uploaded?
No. All the maths runs in your browser.
Why did my size not simplify to a nice ratio?
Because it is not one. Simplifying divides by the greatest common divisor, and 1000×667 share no common factor, so 1000:667 is the smallest form. It is close to 3:2 but not equal to it.
Is A4 a ratio like 16:9?
Not a whole-number one. A-series paper is 1:√2, the only shape that keeps its proportions when folded in half, and √2 cannot be written as a fraction. 210×297 mm simplifies to 70:99, which is the nearest whole-number approximation.
What is 9:16 for?
Vertical video — phone screens held upright, and every short-form video platform. It is 16:9 turned on its side, which is why the preset is there next to it.
Can I use a decimal ratio like 2.35:1?
Yes. Type 2.35 in the first ratio box and 1 in the second, and the width and height below resize to it. The cinema ratios (2.35:1, 1.85:1) are not whole numbers, so both ratio boxes take decimals.