How this calculator works
A percentage resize multiplies both sides by the same factor, so the shape and ratio do not change. If the new image was made at 120 percent, each side grew by a factor of 1.2, and dividing the new width and height by 1.2 recovers the old size. The error is sub-pixel because the resize rounds to whole pixels, so a recovered size is usually within one or two pixels of the original. In ratio mode the arithmetic runs the other way: the height is the width times the ratio height divided by the ratio width, which is the same as the missing side of a similar rectangle.
Formula
scale factor = 1 + percentage change (as a fraction). original width = new width / scale factor. original height = new height / scale factor. In ratio mode, height = width x ratio height / ratio width.
Worked example
A photograph arrives at 1920 by 1080 pixels and you are told it was enlarged by 20 percent. The scale factor is 1.2, so the original was 1920 / 1.2 = 1600 pixels wide and 1080 / 1.2 = 900 pixels high. The reduced ratio is 16:9, unchanged by the resize. Going the other way, a 3840 pixel wide image at 16:9 has a height of 3840 x 9 / 16 = 2160 pixels.
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Frequently asked questions
Why is my recovered size a pixel off?
Resizing rounds each side to a whole number of pixels, so the stored dimensions lose a fraction. Dividing back amplifies that tiny loss, and one or two pixels is normal.
Does a percentage resize keep the aspect ratio?
Yes, provided both sides use the same percentage. If you resize only the width, or use different percentages for each side, the image stretches and the ratio changes.
How do I compare two images with different ratios?
Reduce both to their simplest whole-number form and compare. A 3:2 image and a 16:9 image are different shapes, so fitting one into the other needs either cropping or letterboxing.
Is this the same as resolution scale?
They are related. The resolution scale calculator works from a percentage of pixel dimensions, while this page also relates a percentage change back to a known starting image and helps match a chosen ratio.