How to use this calculator
Enter the original width and height to get a simplified ratio. To resize, choose the new dimension you know and enter its size. The other dimension is calculated automatically.
The math behind it
Ratio = width ÷ GCD(width, height) : height ÷ GCD(width, height) New height = new width × original height ÷ original width New width = new height × original width ÷ original height
A worked example
1920 × 1080 simplifies to 16:9. At a width of 1280 pixels, that ratio gives a height of 720 pixels.
Assumptions & precision
This tool uses square pixels. Original and requested dimensions must be whole numbers from 1 to 1,000,000,000. Resizing uses exact integer arithmetic and rounds to the nearest whole pixel (halves round up), so the resized image can have a slightly different exact ratio. A calculated dimension that rounds to zero or exceeds the limit is rejected.
A common question
Will resizing keep the exact ratio?
When both new dimensions are integers in the original ratio, yes. Otherwise, rounding to a whole pixel introduces a small difference. This calculator shows the rounded pixel dimensions.