How this instrument works
An aspect ratio describes a rectangle's shape — its width compared to its height — independent of the actual size. A 400×300 rectangle and a 1200×900 rectangle look identical in shape even though one has nine times the area of the other, because both reduce to the same 4:3 ratio. Reducing to lowest terms is what lets you compare shapes directly, regardless of the units or scale behind them.
The reduction works by finding the greatest common divisor (GCD) of the two numbers — the largest whole number that divides both exactly — and dividing both dimensions by it. Keep dividing and you eventually reach a pair of numbers that share no common factor at all; that pair is the ratio in its simplest form, and it's the same method whether your original numbers came from a screen resolution in pixels, a print size in inches, or a canvas in a design tool.
Some reduced ratios have familiar names because they recur across so many formats: 16:9 (widescreen video), 4:3 (classic television and monitors), 3:2 (most camera sensors), 1:1 (square) and 21:9 (ultrawide) are the ones you'll see most often. Plenty of dimensions reduce to something less tidy, though, and that's just as valid an answer — the ratio is whatever the numbers actually are, not whichever named ratio is closest.
- Enter your first dimension into Width (px) — this can represent pixels, inches, centimetres or any consistent unit.
- Enter the matching second dimension into Height (px), using the same unit as Width.
- Read the simplified ratio directly beneath the inputs — the smallest whole-number pair with the same proportions.
- If the result doesn't match a name you recognise, that's fine — not every rectangle lands on a 'named' ratio like 16:9 or 4:3.
Worked example — a 1920×1080 screen
Enter 1920 into Width and 1080 into Height — dimensions that could describe a screen, a video frame or any rectangle you happen to be measuring. Their greatest common divisor is 120, so dividing both numbers by 120 leaves a ratio width of 16 and a ratio height of 9. Simplified ratio reads 16:9.
The same instrument works for any pair of numbers, not just screens: 1024×768 reduces to 4:3, the shape of a classic computer monitor, and 3000×2000 reduces to 3:2, a common photography sensor shape — three completely different physical objects, reduced by the identical gcd method.
Questions
What is an aspect ratio?
An aspect ratio is the proportional relationship between a rectangle's width and its height, written as two numbers separated by a colon, like 16:9 or 4:3. It describes shape only, not size — a tiny thumbnail and a huge cinema screen can share the exact same aspect ratio if their width-to-height proportions match.
How do I calculate an aspect ratio by hand?
Find the greatest common divisor (GCD) of your width and height — the largest number that divides both exactly, often found using the Euclidean algorithm — then divide both dimensions by it. For 1920 and 1080, the GCD is 120, giving 1920÷120=16 and 1080÷120=9, so the ratio is 16:9. This instrument does that division automatically.
Does aspect ratio depend on the unit I use (pixels, inches, cm)?
No, as long as you use the same unit for both width and height, the resulting ratio is identical regardless of which unit you picked. A photo that's 6 inches by 4 inches and one that's 3000 pixels by 2000 pixels both reduce to the same 3:2 ratio, because aspect ratio is a relationship between the two numbers, not a property tied to any particular unit.
What are the most common aspect ratios?
16:9 (widescreen video and most modern displays), 4:3 (classic television and older monitors), 3:2 (most camera sensors), 1:1 (square) and 21:9 (ultrawide monitors and cinemascope film) cover the large majority of screens, photos and video you'll encounter, though plenty of legitimate formats fall outside these named ratios.
Why does my ratio not match a 'named' ratio like 16:9?
Because not every rectangle is designed to a standard proportion — a custom banner, an arbitrary crop, or a print size chosen for other reasons can reduce to an unfamiliar ratio like 37:24, and that's a perfectly valid answer. Named ratios like 16:9 are simply the ones that recur often enough across industries to earn a common label.