Aspect Ratio Calculator
Simplify pixel dimensions to a ratio, or resize keeping the ratio.
Decimal ratio 1.78:1 · gcd(1920, 1080) = 120.
An aspect ratio compares width to height. To simplify, divide both by their greatest common divisor: 1920 and 1080 share a gcd of 120, so 1920×1080 becomes 16:9 — a decimal ratio of 1.78:1. To resize, multiply the known side by the ratio of the other two terms.
What an aspect ratio is
An aspect ratio is the proportional relationship between the width and height of a rectangle, written as two numbers separated by a colon, such as 16:9 or 4:3. It describes shape, not size: a 1280×720 image and a 1920×1080 image are both 16:9, so they look identical when scaled. Because the ratio is proportional, any two resolutions with the same ratio crop and fit the same way.
Ratios matter wherever a rectangle has to fit a frame. Screens and video use 16:9 as the modern standard; ultrawide monitors use 21:9; classic photography uses 3:2; older TVs and many documents use 4:3; and social platforms favour 1:1 squares and 9:16 vertical video. Choosing the wrong ratio forces the software to letterbox (add bars) or crop, so matching the ratio to the destination keeps the whole image visible without distortion.
Divide width and height by their greatest common divisor to get the simplest whole-number ratio.
- 1 Write down the width and height in pixels. For example a Full HD screen is 1920 pixels wide by 1080 pixels tall, written 1920×1080.
- 2 Find the greatest common divisor (gcd). Use Euclid’s algorithm: gcd(1920, 1080) → 1920 mod 1080 = 840, gcd(1080, 840) → 240, gcd(840, 240) → 120, gcd(240, 120) → 0, so the gcd is 120.
- 3 Divide both dimensions by the gcd. 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9, giving the simplified ratio 16:9.
- 4 Express it as a decimal if you need one number. Divide width by height: 1920 ÷ 1080 = 1.78, written 1.78:1.
- 5 To resize, keep the ratio fixed. For a 16:9 image with a known width of 1280, height = 1280 × (9 ÷ 16) = 720, so the full size is 1280×720.
Common aspect ratios and example resolutions
Decimal is width ÷ height. The same ratio covers many resolutions.
| Ratio | Decimal | Example resolutions | Typical use |
|---|---|---|---|
| 16:9 | 1.78:1 | 1280×720 · 1920×1080 · 3840×2160 | HD/4K screens, video |
| 4:3 | 1.33:1 | 640×480 · 1024×768 · 1600×1200 | Older monitors, slides |
| 21:9 | 2.33:1 | 2560×1080 · 3440×1440 | Ultrawide monitors |
| 3:2 | 1.5:1 | 1080×720 · 3000×2000 | Digital photography |
| 1:1 | 1.0:1 | 1080×1080 · 2048×2048 | Square social posts |
| 9:16 | 0.56:1 | 1080×1920 · 720×1280 | Vertical phone video |
Pitfalls to watch for
Non-integer results. Resizing can produce a fractional pixel, for example a 16:9 width of 1000 gives a height of 562.5. Screens use whole pixels, so round to 562 or 563 — the tiny change means the result is no longer a perfect 16:9, but it is visually indistinguishable.
Cropping vs stretching. Fitting a 4:3 photo into a 16:9 frame means either cropping the top and bottom or adding side bars. Never stretch to fill: changing the ratio distorts faces and circles. Match the source ratio to the target, or crop deliberately.
Portrait ratios. A ratio like 9:16 is just 16:9 turned on its side — its decimal is below 1 because the height is larger than the width. When you need clean whole pixels or want to check screen sharpness afterwards, pair this with a pixel density (PPI) calculator.