Rule of Three Calculator guide
How to Resize an Image and Keep the Aspect Ratio
To resize without distortion, set the original width against the original height, then the new width against x. For a 1920×1080 image resized to 1280 wide, x = (1080 × 1280) ÷ 1920 = 720. The ratio between width and height stays the same, which is exactly what keeps the picture from stretching.
What aspect ratio means
An aspect ratio is the relationship between something's width and its height. A 1920×1080 image has a 16:9 ratio, which is why it fits a widescreen display exactly.
Keeping that ratio when you resize is what stops faces looking stretched or squashed. Changing width and height by different amounts is precisely what causes that distortion.
And keeping the ratio is a direct proportion, which means the rule of three does the job.
Finding the new height
You have a 1920×1080 image and you need it 1280 pixels wide. What height goes with that?
- 1920 wide → 1080 tall
- 1280 wide → x tall
The calculation
x = (1080 × 1280) ÷ 1920 = 1382400 ÷ 1920 = 720.
So 1280×720 has the same proportions as 1920×1080. Both are 16:9, which is why they are both standard video sizes.
The calculator on this page is loaded with exactly this, so you can put your own dimensions in and read the matching height straight off.
Common ratios worth recognising
| Ratio | Typical sizes | Where you see it |
|---|---|---|
| 16:9 | 1920×1080, 1280×720, 3840×2160 | Video, TVs, most monitors |
| 4:3 | 1024×768, 1600×1200 | Older screens, some cameras |
| 1:1 | 1080×1080 | Social media squares |
| 3:2 | 6000×4000 | Most DSLR photography |
| 9:16 | 1080×1920 | Phone video, stories, reels |
Reducing a ratio to its simplest form
To find the ratio of a size you already have, divide both numbers by their greatest common divisor.
For 1920×1080, the greatest common divisor is 120. Dividing both gives 16:9.
This is the same simplification you do with fractions, because a ratio and a fraction are the same object written differently. 16:9 and 16/9 carry identical information.
Fitting inside a box
A related problem: you need an image to fit inside a 800×600 space without distortion and without cropping.
Work out the height that goes with a width of 800. If that height is 600 or less, use it. If it comes out taller than 600, do the calculation the other way round instead — find the width that goes with a height of 600, and use that.
In short, scale by whichever dimension is more constrained. Doing it by the other one always overflows the box.
