Plan an image size
Find the proportions of an existing image, then calculate the height that matches a new width. Use the result in Image resizer with proportional scaling rather than guessing a height that might stretch the picture.
Aspect ratio calculator
Find an image ratio and calculate proportional dimensions.
- ✓ Find a simplified pixel ratio
- ✓ Calculate a matching height
- ✓ Show the decimal proportion
Processed on your device
Keep a rectangle in proportion when its size changes
Work out the simplified ratio of a screen, image, or video. Enter a new width to calculate the matching height without stretching the content.
Compare a screen or video shape
Reduce a pair of whole-pixel dimensions to their simplest ratio to see whether a rectangle matches a familiar shape. The result describes geometry; it does not account for cropping, borders, or non-square display pixels.
Get the best result
The target height is rounded to the nearest whole pixel. A one-pixel rounding difference can slightly change the exact ratio.
A practical example
Starting point
Original dimensions: 1920 × 1080. Target width: 1280.
Expected result
Aspect ratio: 16:9. Target dimensions: 1280 × 720.
The greatest common divisor simplifies the original dimensions. A target height is calculated proportionally and rounded to whole pixels.
Troubleshooting and useful checks
The dimensions are rejected
Use positive whole numbers for pixel width and height. Decimal, zero, negative, or numerically unsafe integer dimensions are not valid inputs.
The rounded target differs slightly from the exact ratio
Pixel dimensions must be whole numbers. When the proportional height is fractional, rounding can introduce a small difference; choose a target width that fits the original ratio's multiples if exact integer proportions matter.
Frequently asked questions
A little clarity before you get started.
Can I use this for video dimensions?
Yes. It calculates proportions for any rectangular pixel dimensions.
Does this resize the actual image file?
No. It calculates proportions and suggested dimensions. Use Image resizer to create a new image with those pixel dimensions.
Why do unusual dimensions produce a large ratio?
The ratio is reduced using the greatest common divisor. Dimensions that share no substantial factor can produce a ratio with large numbers even when they look close to a common screen shape.
Can I calculate from a target height instead?
The current target field accepts width and returns the matching height. For a height-led calculation, swap the original width and height and interpret the result accordingly.
Does this account for non-square video pixels?
No. It treats the supplied dimensions as an ordinary rectangle of square pixels. Some video formats have separate pixel-aspect information that this geometry calculator does not read.
Do I need an account to use this tool?
No. This tool is free to use without signing in. An account lets you keep a list of favorite tools.
Can I use this on a phone?
Yes, with a current browser. File support, download behavior, and available memory vary by device. For large documents and images, a computer is often more comfortable.