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. “Reset all fields” puts every box, the unit and the rounding back where the page opened.
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. 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.
Why did my size not simplify to a nice ratio?
Because it is not one. Simplifying divides by the greatest common divisor, and 1000 and 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 short-form video platforms. 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.
Can it read the ratio off an image, or work in centimetres?
Yes. Pick or drop an image and its size is read for you. Sizes can be in pixels, inches, centimetres or millimetres, each ratio is also given as a decimal so cinema ratios can be expressed too, and the common ratios are named on the page.
Does it say how many pixels a size has, or start from a named resolution?
Yes. A line under the width and height reads the size back — Landscape, Portrait or Square, its pixel count with the megapixels, and its diagonal; in inches, centimetres or millimetres there is no pixel count, only the diagonal in that unit. “Common sizes”, beside Units, sets the ratio, the width and the height at once from a named size — 4K UHD, Full HD, an Open Graph link image, a story and more.
Can I clear everything and start again?
Yes. “Reset all fields” puts every size box in all three panels back to the values the page opens on, sets the unit back to pixels with whole-pixel rounding on, and forgets the image; a picture still being read is dropped, so it cannot refill the boxes afterwards.