Aspect ratio calculator

How the ratio is reduced

A ratio is reduced by dividing both sides by their greatest common divisor, the largest whole number that divides both. For 1920 × 1080, Euclid's method finds it in four steps:

• 1920 ÷ 1080 leaves a remainder of 840
• 1080 ÷ 840 leaves 240
• 840 ÷ 240 leaves 120
• 240 ÷ 120 leaves 0, so the divisor is 120

1920 ÷ 120 = 16 and 1080 ÷ 120 = 9, so the ratio is 16:9. The height for a new width of 1280 is 1280 × 1080 ÷ 1920 = 720, the familiar 720p.

The decimal form is often more useful than the pair of numbers, because it lets you compare shapes at a glance: 16:9 is 1.7778, 16:10 is 1.6 and 4:3 is 1.3333. Wider shapes have bigger numbers, and anything below 1 is taller than it is wide.

Common ratios and where they are used

Social apps change their recommended pixel sizes from time to time, so check their current guidance before a big upload. The ratio is what decides whether a picture fits the frame or gets cropped; the pixel count only decides how sharp it looks.

When the ratio comes out strange

Some sizes are sold under a ratio they only roughly match, and exact reduction shows it:

• 1366 × 768, a common laptop screen, reduces to 683:384. As a number it is 1.7786, close to 16:9's 1.7778 but not equal.
• 2560 × 1080 reduces to 64:27 (2.3704) and 3440 × 1440 to 43:18 (2.3889). Both are sold as “21:9”, which would be 2.3333.
• A4 paper, 210 × 297 mm, reduces to 70:99, because A sizes are built on the square root of 2, which no pair of whole numbers matches exactly.

When the reduced numbers look odd, compare the decimals instead. A difference in the third decimal place means a few pixels of bars or cropping that nobody will notice; a difference in the first decimal means the picture will be visibly letterboxed or cut.

The calculator rounds width and height to whole numbers before reducing, so give it whole numbers: enter US Letter paper, 8.5 × 11 inches, as 85 and 110, which gives 17:22.

Changing shape means cropping or bars

Keeping the ratio is what stops a picture from looking stretched or squashed. If the target has a different shape, no resize can fix it: a 16:9 video shown in a 9:16 frame either gets bars above and below, or loses its sides. Decide which before you export.

A 1920 × 1080 video cropped to 9:16 at full height keeps a strip 607.5 pixels wide, under a third of the frame, so frame your subject in the middle when you film. A 4032 × 3024 phone photo turned into a 4:5 post keeps its full height of 3024 and a width of 2419.2, which you would round to 2419 or 2420.

Octoolo's crop tool and image resizer in the Photo Editor, and the video resizer in the Video Editor, do the cutting and scaling once you know the numbers.