Aspect Ratio Calculator
Free aspect ratio calculator — get the reduced ratio of a width × height, or resize a width or height while keeping a target ratio. Works in your browser.
Pick a mode. Either type a width × height to see the reduced ratio, or type one dimension and a target ratio to find the other.
Reducing a ratio
GCD of width and height = how much both share.
Divide both sides by it; what’s left is the reduced ratio. 1920 × 1080 has GCD 120; 1920 ÷ 120 = 16, 1080 ÷ 120 = 9 — hence 16:9.
Common ratios
| Ratio | Where you’ll see it |
|---|---|
| 16:9 | Modern TV, monitor, phone (landscape) |
| 9:16 | Phone (portrait), vertical video |
| 4:3 | Old TV, classic monitor, iPad |
| 21:9 | Ultrawide monitor, cinema 2.39:1 ≈ |
| 1:1 | Square — Instagram feed |
| 3:2 | DSLR sensors, 35mm film |
| 1.85:1 | American widescreen cinema |
Resizing while preserving the shape
To shrink or grow an image without distortion:
new height = new width × (ratio height ÷ ratio width)
So if your target ratio is 16:9 and the new width is 1280, the height is 1280 × 9 ÷ 16 = 720. The other direction is symmetric.
Worked examples
-
1920 × 1080 → 16:9
1920 × 1080 → aspect ratio 16:9 (≈ 1.78)
-
Width 800 at 4:3 → height 600
Width 800 at 4:3 → height 600
-
Height 720 at 16:9 → width 1280
Height 720 at 16:9 → width 1,280
Frequently asked questions
How is the reduced ratio found?
By dividing both width and height by their GCD. So 1920 × 1080 (GCD 120) collapses to 16 × 9. 1280 × 720 (GCD 80) is also 16:9. The same screen aspect at any pixel size reduces to the same simple ratio.
What are the common ratios?
16:9 (modern TV / monitor / phone landscape). 9:16 (phone portrait, vertical video). 4:3 (old TV, classic monitors, iPad). 21:9 (ultrawide). 1:1 (square — Instagram feed default). 3:2 (most DSLR cameras, 35mm film). 2.39:1 (anamorphic cinematic).
When are non-integer ratios used?
Cinema: 1.85:1 (American widescreen), 2.39:1 (anamorphic scope). 16:9 is itself an irrational fit for many sensors. This calculator reduces using integer math, so it picks the closest small-integer pair — for cinematic uses keep the original decimal.
How do I resize an image while preserving its ratio?
Pick mode "Height (given width)" or "Width (given height)", enter your target dimension, and the calculator returns the matching other dimension. The math is just height = width × ratioHeight ÷ ratioWidth (or vice versa).
Does pixel density (DPI) matter here?
No — aspect ratio is purely the shape of the rectangle. A 1080p image and a 4K image of the same scene have different pixel counts but the same 16:9 shape. DPI is about print or physical size, not ratio.
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