Crop to aspect ratio.

Worked example

A 4000 × 3000 photo cropped to 16:9 is taller than 16:9, so the full width is kept and the height becomes 4000 × 9 ÷ 16 = 2250 px. That removes 750 px of height; centred, the crop starts 375 px from the top. A 1920 × 1080 frame cropped to a 1:1 square keeps the full height and becomes 1080 × 1080, starting 420 px from the left.

How it works

If the image is wider than the target ratio, the height is kept and the width is height × ratio, rounded down to a whole pixel. If it is taller, the width is kept and the height is width ÷ ratio, rounded down. The offset is the leftover space times your position choice (0 for left or top, 0.5 for centre, 1 for right or bottom), rounded to the nearest pixel.

Rounding down means the crop can be one pixel short of the exact ratio on odd sizes. Cropping only removes pixels; to hit an exact output size, scale after cropping.

FAQ

How do I crop an image to 16:9?

If the image is taller than 16:9 (for example 4:3), keep the full width and set the height to width × 9 / 16. If it is wider than 16:9, keep the full height and set the width to height × 16 / 9.

How do I crop to a square?

Use the shorter side as the square. A 1920 × 1080 image crops to 1080 × 1080.

Which part of the image should I keep?

Keep the subject. Use the position choices to slide the crop box toward the top, bottom, left or right.

Should I crop first or resize first?

Crop first, then scale the cropped result to the output size you need.