Keeping aspect ratio when you resize

An aspect ratio is height divided by width, or the pair written as 16:9. The Aspect Ratio Calculator stores the original pair, then applies this formula while the lock is on: new height = new width × (original height ÷ original width).

That is why 1920 × 1080 at a new width of 1280 becomes 720. 1080 ÷ 1920 is 0.5625, and 1280 × 0.5625 is 720. The picture stays 16:9. If you start from a new height instead, the calculator uses new width = new height × (original width ÷ original height).

Write the original size first. Then change one new side. If you edit both new fields while the lock is on, the field you last touched wins and the other side follows. Turn the lock off only when you want a different shape, such as cropping a 16:9 still into a square.

Zero is rejected. Dividing by a zero width does not produce a frame size, so the error sits on that field. Empty boxes are the same: enter the numbers you have, then resize.

Export dialogs in editors often ask for one side and offer “constrain proportions.” This page is that step without the rest of the editor. Copy the new pair, then type it where you export.