Aspect Ratio Calculator
Simplify pixel dimensions to their aspect ratio, or resize an image or video to a new width or height while keeping the proportions exact.
How to use this calculator
- 1Enter the original pixel dimensions to see the ratio in lowest terms.
- 2Choose whether to resize by a new width or a new height; the other dimension follows automatically.
- 3Check the note if rounding was needed — some targets cannot hold the ratio exactly.
How the calculation works
ratio = w : h ÷ gcd(w, h) new height = new width × h ÷ w new width = new height × w ÷ h- w, h
- Original width and height in pixels
- gcd
- Greatest common divisor — the largest number dividing both exactly
- ratio
- The proportions in lowest terms, such as 16:9
Reducing to lowest terms is a greatest-common-divisor problem. 1920 and 1080 share a divisor of 120, giving 16:9; 2560 and 1440 share 160, giving the same 16:9.
Resizing preserves the ratio when the new dimension is multiplied by the original proportion. Because pixels are whole numbers, the result is rounded — so not every target width can hold the ratio exactly.
A ratio that reduces to unfamiliar numbers usually means the dimensions were rounded somewhere. 1366×768 reduces to 683:384 rather than 16:9, because 768 × 16/9 is 1365.33 and panel makers rounded up.
Worked example
1920×1080 scaled to 1280 wide
- 1.The greatest common divisor of 1920 and 1080 is 120.
- 2.1920 ÷ 120 = 16 and 1080 ÷ 120 = 9, so the ratio is 16:9.
- 3.New height: 1280 × 1080 ÷ 1920 = 1280 × 0.5625 = 720 exactly.
- 4.No rounding was needed, so the proportions are preserved perfectly.
- 5.Scale factor: 1280 ÷ 1920 = 66.67%, reducing 2.07 MP to 0.92 MP.
Result: 16:9 — 1280 × 720
A ratio that is not quite 16:9
- 1.The greatest common divisor of 1366 and 768 is 2.
- 2.So the reduced ratio is 683:384, not the 16:9 this resolution is sold as.
- 3.As a decimal: 1366 ÷ 768 = 1.77865, against 16:9 which is 1.77778.
- 4.The discrepancy exists because 768 × 16/9 = 1365.33, and panel makers rounded up to an even 1366.
- 5.The difference is under 0.05% and invisible, but it is why the reduced ratio looks so strange.
Result: 683:384 — nearly, but not exactly, 16:9
Why some resizes cannot be exact
Preserving an aspect ratio when resizing is simple arithmetic until pixels get in the way. Pixels are indivisible, so the calculated dimension must be rounded to a whole number, and rounding changes the ratio very slightly.
Scaling 1920×1080 to 1280 wide works perfectly because 1280 × 9/16 is exactly 720. Scaling it to 1000 wide gives 562.5, which must become 562 or 563 — either way the ratio is no longer precisely 16:9. The error is under a tenth of a percent and nobody will see it, but it is real, and repeated resizes compound it.
The practical rule is to pick target widths divisible by the ratio's reduced width. For 16:9 content, widths that are multiples of 16 always land on whole pixels. This is also why video encoders prefer dimensions divisible by 8 or 16 — the underlying compression works on blocks of that size.
Ratio, resolution and quality are three different things
Aspect ratio describes shape and nothing else. 1920×1080 and 640×360 are both exactly 16:9, but one has nine times the pixels. Changing resolution while holding ratio changes how much detail fits, not the shape of the frame.
Neither figure describes quality. A heavily compressed 4K video can look considerably worse than a well-encoded 1080p one, because bitrate and codec govern how much of the detail those pixels actually carry. Resolution sets a ceiling on detail; it does not deliver it.
Enlarging is the case where this matters most. Scaling a 640×360 image up to 1920×1080 produces a file with nine times the pixels and no additional detail whatsoever — every new pixel is interpolated from neighbours that were already there. It is occasionally necessary to meet an output requirement, but it never improves an image.
What this assumes, and where it stops
Assumptions
- Dimensions are in square pixels, as used by all modern digital images and displays.
- Resized dimensions are rounded to the nearest whole pixel.
- The ratio is reduced using the greatest common divisor, giving lowest terms.
Limitations
- Assumes square pixels. Some legacy video formats use non-square pixels, where the storage ratio and the display ratio differ.
- Reports geometry only — it does not crop, letterbox or pad to fit a different target ratio.
- Rounding to whole pixels means some resizes cannot preserve the ratio exactly. The calculator says when that happens.
- Says nothing about image quality, which depends on codec, bitrate and the resampling algorithm, not dimensions.
Common questions
How do I calculate aspect ratio from pixel dimensions?
Divide both dimensions by their greatest common divisor. For 1920×1080 the GCD is 120, giving 16:9. For 2560×1440 the GCD is 160, which also gives 16:9 — different resolutions, same shape.
How do I resize an image without distorting it?
Multiply by the original proportion: new height = new width × original height ÷ original width. Changing one dimension without adjusting the other is exactly what stretches an image. Where the arithmetic does not land on a whole pixel, rounding introduces a negligible error.
Why is 1366×768 not exactly 16:9?
Because 768 × 16/9 is 1365.33, and panel manufacturers rounded up to 1366 for an even number. The true ratio is 683:384, or 1.77865:1, against 16:9's 1.77778:1. The difference is under 0.05% and invisible, but it is why the reduced ratio looks unusual.
Is 21:9 really 21:9?
No. Ultrawide monitors sold as 21:9 are almost always 64:27, about 2.370:1, where a true 21:9 would be 2.333:1. The standard comes from multiplying 16:9 by 4:3, and 21:9 was adopted as an easier marketing name.
Sources
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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