How to simplify a ratio, step by step
The greatest common divisor method, with examples for decimals, three-part ratios and mixed units.
3 min read →What aspect ratio means
The aspect ratio of an image or screen is the ratio of its width to its height, written as width : height. A 1920 × 1080 pixel screen has an aspect ratio of 16 : 9, which means that for every 16 units across there are 9 units up.
It describes shape, not size. A tiny 160 × 90 thumbnail and a huge 3840 × 2160 display both have a 16 : 9 shape.
How to calculate it
Treat the width and height as a ratio and simplify it by dividing both by their greatest common divisor.
Find the aspect ratio of 1920 × 1080.
The GCD of 1920 and 1080 is 120.
The GCD of 1920 and 1080 is 120.
1920 : 1080 = 1920120 : 1080120 = 16 : 9
Another check is to divide width by height. 1920 ÷ 1080 = 1.778, and 16 ÷ 9 is also 1.778. Same shape.
Common aspect ratios
| Ratio | Where you see it | Example size (pixels) |
|---|---|---|
| 16 : 9 | Widescreen video, most monitors and TVs, YouTube | 1920 × 1080 |
| 4 : 3 | Older TVs and monitors, many tablets | 1024 × 768 |
| 3 : 2 | Many cameras and 35 mm film | 3000 × 2000 |
| 1 : 1 | Square posts and profile pictures | 1080 × 1080 |
| 4 : 5 | Portrait posts on social apps | 1080 × 1350 |
| 9 : 16 | Vertical video such as Stories and Reels | 1080 × 1920 |
| 20 : 9 | Tall modern phone screens | 2400 × 1080 |
| 21 : 9 | Ultrawide monitors and cinema | 2560 × 1080 |
Platforms change their recommended sizes from time to time, so check the current guidance when you are exporting for a specific app. The ratios above are the usual ones.
How to resize without stretching
To keep the same shape, set up a proportion between the old and new sizes and solve for the missing side. If the new width is known:
new height = height × new widthwidth
Shrink a 1920 × 1080 video to 1280 wide.
H = 1080 × 12801920 = 720
So the new size is 1280 × 720. If you know the new height instead, swap the roles: new width = width × new height ÷ height.
Cropping versus stretching
If you need a different shape, you have two honest options. You can crop, which removes part of the picture, or you can add bars or background to fill the gap (letterboxing). Stretching to fit changes the proportions and distorts faces and circles, so avoid it.
A fun one: paper sizes
A4 paper measures 210 × 297 mm. Divide 297 by 210 and you get 1.414, which is the square root of 2. That is deliberate: cut an A4 sheet in half across its long side and you get two A5 sheets with exactly the same shape. Every A-series size keeps the same ratio.
Find your aspect ratio
Open the aspect ratio calculatorEnter a width and height, add a new width or height, and get the matching size with all the working.
Frequently asked questions
How do I find the aspect ratio of an image?
Divide the width and the height by their greatest common divisor. A 1500 × 1000 image becomes 3 : 2.
What is the aspect ratio of 1920 × 1080?
16 : 9. Both numbers divide by 120, giving 16 and 9.
How do I resize an image without distorting it?
Keep the aspect ratio. Set up the proportion old width : old height = new width : new height and solve for the missing value.