How to use it
- Choose Calculate: Ratio of a width and height, or Missing side for a ratio.
- In ratio mode, type the Width (px) and the Height (px) of the picture.
- In missing-side mode, type the Ratio: width part and Ratio: height part, then one side in Width (px) or Height (px), leaving the other empty.
- Read the Ratio, the Decimal and the Nearest common ratio, or the Missing side (px).
- Use the table Common sizes at this ratio to pick a size of the same shape.
How the ratio is worked out
The aspect ratio is the width of a picture divided by its height, written as width:height. Both sides are divided by their greatest common divisor, so 1920 and 1080 both divide by 120 and become 16:9. The decimal value is the width divided by the height, 1.778 for 16:9. A portrait picture reads the other way round: 1080 by 1920 is 9:16.
The missing side follows from the same rule. Height = width × height part ÷ width part, so a 16:9 picture that is 1280 pixels wide is 720 pixels high.
ratio = width : height, both sides divided by gcd(width, height)
missing height = width × 9 ÷ 16Nearest common ratio and sizes
Many pictures do not reduce to a short ratio. 1366 by 768 becomes 683:384, which says little. The tool therefore compares the decimal value with the common ratios named in the Wikipedia article on aspect ratio: 1:1, 5:4, 4:3, 3:2, 5:3, 16:10, 16:9, 21:9 and 32:9. It names the nearest one when the picture is within 3 % of it. 1366 by 768 is less than 0.1 % away from 16:9.
The sizes table keeps the same ratio at the common widths of 640, 1280, 1920, 2560 and 3840 pixels, the widths that appear in the list of common display resolutions. A side that is not a whole number of pixels is rounded, and the result says so.
What the ratio does not change
A ratio describes the shape of the pixels, not the size of a print. To print a picture at a physical size, use the print size and dpi calculator. To cut a picture to a ratio, use crop image.
Example: a 1080p video frame
Type 1920 for the width and 1080 for the height. The greatest common divisor is 120, so the ratio is 16:9. The decimal value is 1.778, and the nearest common ratio is 16:9 itself, 0.0 % off.
In missing-side mode, a 16:9 ratio with a width of 1280 pixels gives a height of 720 pixels.
| Measure | Value |
|---|---|
| Ratio | 16:9 |
| Decimal | 1.778 |
| Nearest common ratio | 16:9, 0.0 % off |
| Missing side for 16:9 and a width of 1280 | 720 px |
Frequently asked questions
What is the difference between a ratio and a resolution?
A resolution is the number of pixels, such as 1920 by 1080. The aspect ratio is only the shape of that rectangle, 16:9 here. Many resolutions share one ratio: 1280 by 720 and 3840 by 2160 are both 16:9.
Why is 16:9 the usual ratio for video?
The Wikipedia article on aspect ratio describes 16:9 as universal for high-definition television and European digital television, while 4:3 was the universal video format of the twentieth century.
Why does the table round some sizes?
When the width times the ratio does not give a whole number of pixels, the table rounds to the nearest pixel and says so. For example, 21:9 at 640 pixels wide is 274.29 pixels high. The ratio itself stays exact.
What happens to a picture placed in a frame of another ratio?
It is zoomed and cropped, given bars at the top and bottom (letterboxing) or at the sides (pillarboxing), or stretched. The Wikipedia article on aspect ratio describes these options for converting between unequal ratios.
What is the ratio of a portrait photo?
The tool gives the nearest ratio in the same orientation, so a portrait photo of 3000 by 4000 pixels is 3:4, while the same photo turned to landscape is 4:3.
Is my picture uploaded when I calculate?
No. The sides are read and the ratio is worked out in your browser. Nothing is sent to us or stored.
Sources
Figures and text last checked on Tuesday, November 10, 2026.