Aspect Ratio Calculator: Resize Resolution and Pixel Sizes
Enter a ratio and a width or height to get the other side without distortion, reduce a pixel size to its simplest ratio, or start from common formats (16:9, 9:16, 4:3, 1:1, 4:5, 21:9).
Video resolutions and 16:9, 9:16, 4:5, 21:9 sizes in pixels
To resize a video without distortion, the other side is height = width × H ÷ W. A vertical 9:16 frame 1,080 px wide is 1,920 px tall (2.07 megapixels), and 4K UHD at 3,840 × 2,160 simplifies to 16:9 (8.29 megapixels).
Method
- Pick a preset ratio (16:9, 9:16, 4:3, 1:1, 4:5, 21:9, 3:2) or enter your own, then a target width or height: the other side is calculated and rounded to the pixel.
- The resolution buttons (4K UHD, 1440p, 1080p, 720p, 1080 × 1920, 1080 square) fill in the dimensions and derive the ratio, simplified by the greatest common divisor.
- The tool also shows the decimal ratio, total pixel count and megapixels, with a proportion preview.
Video examples
| Format | Width entered | Calculated height |
|---|---|---|
| 16:9 | 1,920 px | 1,080 px |
| 9:16 (vertical) | 1,080 px | 1,920 px |
| 4:5 (portrait) | 1,080 px | 1,350 px |
| 21:9 | 2,560 px | 1,097 px |
The last row shows a subtlety: "21:9" monitors at 2,560 × 1,080 actually have a 64:27 ratio (2.370), not 21:9 (2.333). Enter 2560 and 1080 in the ratio fields to get the exact figure. Going from 4K to 1080p halves the width but divides the pixel count by four (8.29 vs 2.07 megapixels).
Limits
- The calculation assumes square pixels; anamorphic formats are not handled.
- Changing the ratio does not crop anything: choosing between cropping and letterboxing is up to your editing software.
- Decimal ratios are accepted (2.39:1 for scope): they are rounded to the thousandth before simplification, which shows 239:100.
The aspect ratio calculator page uses the same engine with examples for photos and web images.
Frequently Asked Questions
What resolution should a vertical 9:16 video be?
In 9:16, height = width × 16 ÷ 9: a 1,080 px width gives 1,920 px of height, or 2.07 megapixels. It is the same area as horizontal 1080p, rotated a quarter turn.
What pixel size is a 4:5 frame?
At 1,080 px wide, a 4:5 frame is 1,080 × 5 ÷ 4 = 1,350 px tall. This portrait shape takes up more screen height than a 1,080 × 1,080 square.
Is a 2560 × 1080 monitor really 21:9?
Not exactly: 2,560 ÷ 1,080 = 2.370, a 64:27 ratio, whereas 21:9 is 2.333. In strict 21:9, a 2,560 px width would give a 1,097 px height.
How many pixels does 4K have compared with 1080p?
4K UHD (3,840 × 2,160) has 8.29 megapixels, four times as many as 1080p (1,920 × 1,080, 2.07 megapixels). Both are 16:9.
How do I calculate an image's height from its width?
Multiply the width by the second term of the ratio, then divide by the first. In 16:9, a 1,280 px width gives 1,280 × 9 ÷ 16 = 720 px of height.
How do I find the aspect ratio of a 1920 × 1080 image?
Divide both dimensions by their greatest common divisor, here 120: you get 16:9, or 1.778 as a decimal. In the tool, enter 1920 and 1080 in the ratio fields.
What aspect ratio do camera photos use?
Full-frame DSLRs and mirrorless cameras shoot 3:2 (1.5), while compacts and smartphones often use 4:3 (1.333). A 3:2 photo 6,000 px wide is 4,000 px tall.
Why does the calculated side land on half a pixel?
When the width is not a multiple of the ratio's first term, the result is not a whole number. The tool rounds to the nearest pixel: 1,000 px in 16:9 gives 562.5, rounded to 563 px.
Is this tool really free?
Yes, all tools on Kristools are 100% free and accessible without registration.
Is my data protected?
Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.