The Aspect Ratio Calculator does the three jobs aspect ratios always demand: simplify a width and height to their cleanest ratio, find the missing dimension when you know the ratio and one side, and give the decimal form. Enter dimensions for the ratio, or a width (or height) plus a target ratio to get the partner dimension.
How It Works
Simplification divides both dimensions by their greatest common divisor: 1920 and 1080 share a GCD of 120, giving 16 and 9, the famous 16:9. Finding a missing dimension is a proportion: height = width x (ratio height / ratio width), and width = height x (ratio width / ratio height). The decimal form is width divided by height. Results for the missing dimension are rounded to whole pixels, as real displays require.
Modes
| Mode | You enter | You get | |---|---|---| | Ratio | Width and height | Simplified ratio and decimal | | Find height | Width plus ratio | The matching height | | Find width | Height plus ratio | The matching width |
Worked examples
1920 x 1080 simplifies to 16:9 (decimal 1.778), and so does 1280 x 720, which is why both are called 720p-class and 1080p-class HD. A 1200-pixel-wide image at 16:9 needs a height of 1200 x 9/16 = 675 pixels. A height of 500 at 4:3 needs a width of 500 x 4/3 = about 667 pixels. A phone photo at 4032 x 3024 reduces by a GCD of 1008 to 4:3, and a cinema frame at 2048 x 858 reduces to roughly 2.39:1, the anamorphic widescreen standard that 16:9 television letterboxes.
The ratios you will actually meet
16:9 dominates video and displays; 4:3 is legacy television and many camera sensors; 3:2 is the classic photo format of 35 mm film and many DSLRs; 1:1 is the social square; 4:5 and 9:16 are portrait social formats (feed portraits and vertical video respectively); 21:9 is ultrawide monitors and some cinema; 2.39:1 is theatrical widescreen. Knowing the family an image belongs to tells you instantly whether a crop will be gentle or brutal: converting 4:3 content to 16:9 means cropping roughly a quarter of the height or accepting pillarbox bars.
Cropping without distortion
Aspect ratio errors are visible as stretched faces and oval wheels, and they come from forcing content into a frame of a different ratio by scaling the axes independently. The two honest options are crop (lose the overflow) or pad (add bars). This calculator's find-dimension modes give you the frame size that needs neither: set the export dimensions it returns, and the image scales uniformly into them. When planning a responsive image set, compute each breakpoint's crop from the master ratio once, here, rather than letting each platform's auto-crop choose its own centre of interest.
Limitations
Dimensions are treated as exact pixel (or unit) counts and missing dimensions round to whole numbers, so a computed height can be off by one pixel from a strict proportion on odd sizes. The calculator reports the mathematical ratio; it does not warn that some platforms re-crop uploads to their own ratios regardless of what you supply. Non-integer dimension inputs are rounded before the GCD simplification, which matters only for exotic fractional inputs.
Frequently Asked Questions
Is 1280 x 720 really the same ratio as 1920 x 1080?
Yes: both divide by their GCD to 16:9. Resolution (how many pixels) and aspect ratio (their shape) are independent properties, which is why a 720p and a 1080p video fill the same screen without bars.
What ratio is a phone screenshot?
Most modern phones are about 19.5:9 (for example 1170 x 2532). Enter the exact pixel dimensions and the calculator will reduce them to the precise ratio your device uses.
Why is my computed height off by one pixel?
Because pixel counts must be whole numbers, the exact proportion (often a fraction) is rounded. A one-pixel difference is invisible in practice and is the standard behaviour of every image editor.