How to use this calculator
Enter source width and height to simplify their ratio, or apply a common preset such as 16:9, 9:16, 4:3 or 1:1. Source values can be pixel dimensions or whole-number ratio values. Swap the source dimensions to change orientation. In Resize dimensions, edit either new width or new height and the other follows automatically. The preview shows the resulting proportions, including any whole-pixel rounding.
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. Source 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); the original and rounded output ratios are both shown. A calculated dimension that rounds to zero or exceeds the limit is rejected. Decimal ratios display up to 6 places. The preview is scaled to fit, not actual size. Nominal ratios such as 21:9 may differ from a display’s exact pixel ratio; use its resolution when precision matters. No image processing, cropping or upload is performed.
Common questions
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.
How do I calculate 16:9 dimensions?
Select the 16:9 preset and switch to Resize dimensions. Enter your new width or height; the other dimension updates automatically. For example, 1280 pixels wide gives 720 pixels high.
Does changing the ratio crop an image?
This tool only calculates dimensions. To put an image into a different ratio without stretching it, an image editor must crop part of the image or add borders. Swapping width and height here changes the target orientation; it does not rotate a file.
Why is 3440 × 1440 not exactly 21:9?
3440 × 1440 simplifies to 43:18. The label 21:9 is a nominal ultrawide format, not the exact ratio of every screen sold under that description. Enter the actual resolution when matching dimensions or FOV.