Aspect-ratio planner

The same shape, with rounding explained.

Divide the source width and height by their greatest common divisor to get the reduced ratio. For a known target width, ideal height = target width × original height / original width. For a known target height, reverse that relationship.

For 1600 × 900, the reduced ratio is 16:9. A width of 1000 gives an ideal height of 562.5. Whole-pixel rounding gives 1000 × 563, which is only approximately the same shape. Exact half pixels round up. Integer multiples of 16 × 9 keep the ratio exactly.

When the ideal dimension is less than half a pixel, rounding produces zero. The tool marks that result unusable and asks for a larger target dimension. Very large computed dimensions are arithmetic results, not a claim that a browser or image editor can create them. The preview shows the original ratio; it is not a file-size, print-quality, pixel-density or cropping estimate.