Aspect Ratio Calculator
Resize an image, video, or frame without distorting it. Give the size you are starting from and one dimension of the size you want — the other follows, along with the ratio in lowest terms.
Resize in proportion
The width you are starting from. Any unit works — the ratio is what matters.
The height you are starting from, in the same unit as the width.
Which side of the new size you already know.
The dimension you are resizing to. The height follows from it.
1,920 × 1,080 is 16∶9, so a 1,280 px width gives a 720 px height.
- Aspect ratio
- 16∶9
- Decimal ratio
- 1.7778
- New width
- 1,280 px
- New height
- 720 px
How the proportion and the ratio are worked out
Keeping a shape means keeping one number constant: width divided by height. Everything on this page falls out of that.
r = W₁ ÷ H₁ • H₂ = W₂ ÷ r • W₂ = H₂ × r- W₁, H₁
- The width and height you are starting from
- r
- The decimal ratio — 1.7778 for a 16∶9 shape
- W₂, H₂
- The resized width and height
The named ratio is a separate step: take the original width and height, divide both by their greatest common divisor, and what is left is the ratio in lowest terms. For 1920 and 1080 the greatest common divisor is 120, which is how 1920∶1080 becomes 16∶9.
A 1920×1080 frame at 1280 wide
The decimal ratio is 1920 ÷ 1080 = 1.7778. The new height is 1280 ÷ 1.7778 = 720. So the resized frame is 1280×720 — and because both 1920÷120 = 16 and 1080÷120 = 9, the shape is named 16∶9. Had you instead known you needed a height of 720, the same ratio run the other way gives 720 × 1.7778 = 1280.
A less tidy case shows why the two answers are computed differently. A 1000×1080 source has a decimal ratio of 0.9259, and nothing divides 1000 and 1080 into small whole numbers — the greatest common divisor is 40, so the named ratio is 25∶27. The proportion still works perfectly; it just does not have a memorable name.
What this calculator assumes
- Width and height are in the same unit. The tool labels them pixels because that is the common case, but the units cancel — inches, centimetres and metres all work.
- The named ratio is computed from the inputs rounded to whole numbers, because a ratio is a pair of integers by definition. The decimal ratio uses the exact values you typed.
- Results are shown to two decimal places. A resized dimension is rarely a whole number, and rounding it to one is a decision only you can make — round, or crop the difference.
- No pixel-density or DPI conversion is applied. This is geometry, not print preparation.
Aspect ratio FAQ
What does an aspect ratio like 16∶9 actually mean?
It is the width divided by the height, reduced to the smallest whole numbers that give the same fraction. 1920×1080, 1280×720 and 3840×2160 are all 16∶9 because each divides down to 16 over 9. The ratio describes the shape; the pixel dimensions describe the size.
Why does my resized image still look stretched?
Almost always because one dimension was rounded to a whole pixel and the other was not, or because the software is filling a fixed frame rather than fitting inside it. If you need whole pixels in both directions, pick the dimension you care about, round the other one, and accept a fraction of a percent of distortion — or crop instead of stretch.
Is 4K the same aspect ratio as 1080p?
Yes. 3840×2160 reduces to 16∶9, exactly like 1920×1080. 4K is four times the pixel count — double the width and double the height — but the same shape, which is why 1080p content fills a 4K screen with no bars.
What ratio should I use for social media?
1∶1 for a square post, 4∶5 for a portrait feed post, 9∶16 for full-screen vertical video, and 16∶9 for landscape video. Enter your source dimensions above and set the new width or height to the target, and the tool gives you the matching side.
Does this work for things that are not images?
Yes — the arithmetic is a proportion, so any two pairs of measurements work. Print sizes, monitor dimensions, garden beds, model scales. The units cancel out, so long as the width and height are given in the same one.
Sources and review notes
- ITU-R BT.709 — the recommendation that fixes HDTV at a 16∶9 picture aspect ratio
- ITU-R BT.2020 — UHDTV, which keeps 16∶9 at 3840×2160 and 7680×4320
The arithmetic here is exact and has no data behind it to go stale: a proportion and a greatest common divisor. The standards above are cited for the conventional ratio names, not for the calculation.