Aspect ratio calculator
The exact ratio, the standard shape it is nearest, and how much of the picture cropping would throw away — before you do it rather than after. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
Not every ratio reduces to something useful
1366 × 768 is the resolution of a great many cheap laptop screens, and it reduces to 683 : 384. That is arithmetically correct and no use to anybody. It is nearly 16:9 — 0.049% off — which is why everybody calls it 16:9 and why a full-screen 16:9 video on one of those screens is very slightly letterboxed or very slightly cropped, depending on the player.
This page gives the exact reduction, says whether it is tidy, and names the standard ratio it is nearest with the error attached. Most aspect ratio calculators give you the reduction and stop.
Cropping always throws something away
You cannot change the shape of a picture without losing part of it, adding bars, or distorting it. Going from 4:3 to 16:9 costs 25% of the image, off the top and bottom. Going back the other way costs the same 25%, off the sides. The tool above says how much and from which edge before you do it rather than after.
The alternatives are honest and ugly: letterboxing keeps everything and adds black bars, and stretching keeps everything and makes people the wrong shape. There is no fourth option.
The ratios worth knowing
| 4:3 | Old televisions, old monitors, most compact cameras, and the iPad. |
|---|---|
| 3:2 | 35 mm film, and therefore most system cameras. |
| √2 : 1 | ISO 216 paper — 1:1.414. Between 3:2 and 16:9, and the reason a photograph never quite fits an A4 sheet. |
| 16:9 | Television since about 2005, and almost every screen made now. |
| 1.85:1 and 2.39:1 | The two standard cinema shapes. A 2.39:1 film on a 16:9 television has bars taking up a quarter of the screen, and that is correct. |
Pixel aspect ratio is a separate thing
Almost everything now uses square pixels, but standard-definition video did not: PAL and NTSC pixels are rectangular, which is why old footage looks stretched when software treats them as square. If a video's stored resolution does not match its stated shape, that is usually why rather than a mistake.
What it will not do
- It will not crop anything. It tells you what cropping would cost.
- It will not know which part of the picture matters. The arithmetic says 25% goes; only you know whether it is 25% of sky or 25% of somebody's head.
- It will not handle non-square pixels. See above.