How an aspect ratio is simplified

Divide width and height by their greatest common divisor. For 1,920 × 1,080 pixels, the greatest common divisor is 120, giving the familiar 16:9 ratio.

Proportional resizing

To keep the original proportions, multiply the new width by source height divided by source width. A 16:9 image resized to 1,280 pixels wide is 720 pixels high.

Rounding pixel dimensions

A proportional calculation can produce a fractional pixel. Raster output normally requires whole pixels, so choose an appropriate rounding direction and confirm the encoder, layout, or platform requirements. A one-pixel difference can slightly change the exact ratio.

Resize and crop answer different questions

Resizing a 4:3 image from 4,000 × 3,000 to 1,200 pixels wide gives 1,200 × 900 and keeps every part of the frame. Filling a 16:9 area at the same width would require 1,200 × 675, so some height must be cropped or empty space must be added. The calculator reports the proportional resize; it does not decide which content is safe to remove. Check faces, captions, logos, and platform safe areas before cropping.

Verify an export specification

Start with the actual pixel dimensions reported by the source file rather than a preview’s CSS size. Calculate the target height, round to a whole pixel only if necessary, and compare both ratios by division: width ÷ height. For video, confirm whether the delivery standard requires even dimensions, a fixed codec block size, or non-square pixels. For responsive web images, prepare enough source pixels for the largest rendered size and device density instead of repeatedly enlarging a small export.

This tool calculates geometry only. It does not crop, resize, compress, or upload an image, and it does not account for pixel aspect ratios other than square pixels.