How to work out an aspect ratio
- Type a width and a height, then choose Simplify. The page divides both by their greatest common divisor and shows the ratio, plus the decimal to two places.
- Or type a ratio and one side, then choose Other side. The missing side is rounded to the nearest whole pixel, and the page says when it rounded.
- Choose Crop an image to this ratio to open the crop tool with that ratio already selected.
Common ratios
| Ratio | Decimal | Use |
|---|---|---|
| 1:1 | 1.00 | A square. YouTube thumbnail help, read 21 October 2026, says podcast playlists use 1:1. |
| 4:5 | 0.80 | Four across for every five down. YouTube thumbnail help, read 21 October 2026, says vertical videos with a 16:9 custom thumbnail are replaced by an auto-generated 4:5 thumbnail on the home, explore and subscription pages. |
| 3:2 | 1.50 | Three across for every two down. The width is one and a half times the height. |
| 4:3 | 1.33 | Four across for every three down. The width is one and a third times the height. |
| 16:9 | 1.78 | Sixteen across for every nine down. YouTube thumbnail help, read 21 October 2026, says to try 16:9 for videos. YouTube banner help, read the same day, sets a minimum of 2048 x 1152, which is 16:9. |
| 9:16 | 0.56 | Nine across for every sixteen down. YouTube thumbnail help, read 21 October 2026, says to try 9:16 for Shorts. Instagram reel help, read the same day, allows an aspect ratio up to 9:16. Stories ads help, read the same day, recommends 1080 x 1920, which is 9:16. |
| 21:9 | 2.33 | Twenty-one across for every nine down. The width is two and a third times the height. |
What an aspect ratio is
The aspect ratio is the width divided by the height, with the greatest common divisor taken out. 1920 by 1080 simplifies to 16:9, and 1920 divided by 1080 is 1.78. A square is 1:1 and the decimal is 1.00. 4096 by 2160 simplifies to 256:135.
3:2 at a width of 6000 pixels is 4000 pixels tall, exactly. When the other side is not a whole number, the page rounds to the nearest whole pixel and shows the value before rounding, to up to three decimal places. 16:9 at a width of 1000 pixels is 562.5, which rounds to 563.
A zero, a negative number, text that is not a whole number, or a number too large for exact integer maths shows a plain message.
Questions
Is it private?
Yes. The calculator uses only the numbers you type. Nothing is uploaded. The crop tool keeps the photo on this device.
Which formats does it take?
The calculator takes numbers, not files. The crop tool it opens accepts JPG, JPEG, JFIF, PNG, WebP, AVIF and HEIC, one photo at a time.
Is there a limit?
Type whole numbers greater than zero. A number that is too large for exact integer maths shows a message. The crop tool still limits each photo to 50 MB.
Which browsers does it work in?
Current versions of Chrome and Safari. Nothing to install. The maths runs on this page.