What this tool does
Two modes: reduce a width and height to their simplest ratio, or lock a ratio and work out the missing dimension.
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Both directions
Ratio mode takes a width and a height and reduces them to lowest terms — 1920 by 1080 comes back as 16:9.
Resize mode does the reverse. Fix a ratio, give one dimension, and it computes the other.
What the reduction actually does
It divides both numbers by their highest common factor, which is the same operation as simplifying a fraction.
1920 and 1080 share a factor of 120, giving 16 and 9. That is worth knowing because it explains the answers that look wrong: 1366 by 768 reduces to 683:384, not to 16:9, because 1366 is not divisible by the right number. It is close to 16:9 and it is not 16:9, and that discrepancy is exactly why images on those screens are very slightly stretched.
The ratios worth recognising
16:9 — video, television, most monitors. 4:3 — older screens, and still the default for many projectors and tablets. 3:2 — most digital cameras, and 6×4 prints. 1:1 — square, for social platforms. 2.39:1 — cinema widescreen.
The mismatch between 3:2 (what your camera captures) and 16:9 (what your screen shows) is the reason so many photographs arrive with the top and bottom trimmed off.
Why you cannot just scale one number
Changing width without changing height by the same proportion distorts the image, and the distortion is more visible than people expect — a 5% error is obvious on a face and invisible on a landscape, which is how it survives to print.
Resize mode exists to prevent exactly that. Enter the ratio, enter the width you need, and take the height it gives you rather than the height that looks about right.
Where it comes up
Sizing images for a website or a print layout. Working out whether a photograph will fill a frame or need cropping. Checking that a video export matches its target. Scaling a drawing up for a poster.
Also enlarging on a photocopier: the percentage you need is the ratio of the two dimensions, and getting it from a calculator beats guessing at 150%.
Related
For the paper dimensions themselves, the paper size calculator. For whether an image has the pixels to print at a given size, the resolution checker.
FAQs
Quick answers
What does an aspect ratio like 16:9 mean?
It means for every 16 units of width there are 9 units of height. The numbers describe the shape, not the size, so 16:9 is the same shape whether the image is 1600x900 or 1920x1080 pixels.
How do I resize an image without distortion?
Keep the aspect ratio fixed and change only one dimension; let the other follow. In resize mode, lock the ratio, type the new width, and use the height the calculator returns.
Which aspect ratio matches A4 paper?
The A-series uses a ratio of 1:1.414 (the square root of 2). That is why folding an A4 sheet in half gives two A5 sheets with the same proportions.
Why does my photo get cropped when I print it?
Because the photo's ratio does not match the print size. A 4:3 photo printed at 3:2 (6x4) must lose some height or width. Check the ratio first to see how much would be cropped.
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Further reading
