How to calculate a proportional size
- Enter the original width and height as positive integers.
- Enter a new width or height and choose which dimension it represents.
- Read the reduced ratio, exact missing dimension, and nearest whole-pixel size.
Exact ratio and pixel rounding
A 1920 by 1080 image reduces to 16:9. At a new width of 1280, its height is 720. The missing dimension equals the new dimension multiplied by the original opposite dimension and divided by the original matching dimension. If this is fractional, the tool shows the exact fraction and rounds half upward for the whole-pixel suggestion. Rounding can slightly change the ratio; a result below one pixel needs a larger target.
Supported dimensions
Use positive whole numbers with at most 12 digits. Calculations use exact integer arithmetic. This tool calculates dimensions; it does not resize an image file.
Where is my input processed?
All conversion runs locally in your browser. Your input is not submitted for processing.