Aspect Ratio Calculator
Keep a ratio like 16:9 and solve the missing width or height, or reduce any pixel size down to its simplest aspect ratio — all in your browser.
Pick a ratio, then enter the width or height you want. We'll work out the other side so the ratio stays exact.
Everything is calculated in your browser — nothing is uploaded. Dimensions are rounded to whole pixels.
Frequently Asked Questions
What is 16:9 of a 1920px width?
It is 1080px tall. Pick the 16:9 ratio, choose "Width", and enter 1920 — the calculator solves the missing height so the ratio stays exact.
Can it reduce 1920×1080 to a ratio?
Yes. Switch to "Reduce to a ratio", enter 1920 and 1080, and it divides both by their greatest common divisor to show 16:9, plus the decimal ratio 1.778 : 1.
Does it support custom aspect ratios?
Yes. Choose "Custom…" in the ratio menu and type any width and height, such as 5:4 or 2.39:1, then enter a known side to solve the other.
Understanding the Aspect Ratio Calculator
The Aspect Ratio Calculator does two jobs in one tool. First, it keeps a fixed ratio — like 16:9, 4:3, 21:9, 1:1, 3:2, 9:16, or any custom W:H you type — and solves the dimension you're missing: enter a width and it returns the matching height, or vice versa. Second, it reduces any pair of pixel dimensions down to its simplest aspect ratio and shows the decimal equivalent. It's perfect for sizing videos, images, thumbnails, screens, and social posts without distortion. Everything runs entirely in your browser, with results rounded to whole pixels and validated so bad input never breaks the page.
How it works
In "Solve a dimension" mode, choose a preset ratio or pick Custom and type a width:height pair. Then tell the tool whether you know the width or the height and enter that value; it multiplies by the ratio to find the other side. For 16:9 with a 1920px width it computes 1920 × 9 ÷ 16 = 1080px tall. In "Reduce to a ratio" mode, enter a pixel width and height; the tool divides both by their greatest common divisor (GCD) to produce the simplest whole-number ratio, then divides width by height for the decimal form. Inputs that are empty, zero, or negative show a friendly message instead of an error.
Worked example
Say you have a 1920×1080 banner and want a vertical 9:16 version at 1080px wide. Pick 9:16, choose Width, enter 1080 — the calculator returns 1080 × 1920 px, so the height becomes 1920px. Now switch to "Reduce to a ratio" and enter 1920 and 1080: the GCD is 120, so 1920÷120 : 1080÷120 gives 16 : 9, with a decimal ratio of 1.778 : 1. Both results include copy, CSV, and print buttons.
Tips & common mistakes
- Use "Solve a dimension" when you already know one side and want the other without stretching the image.
- For odd source sizes, the "Reduce to a ratio" mode reveals the true ratio — useful before cropping to a standard like 16:9.
- Custom ratios accept decimals too, so cinematic 2.39:1 or photo 3:2 both work.
- Use the built-in social presets for quick sizing: 1:1 for an Instagram post, 4:5 for an Instagram portrait, 9:16 for Reels/TikTok/Stories, and 1.91:1 for Facebook and LinkedIn link images.
- Remember 1920×1080, 1280×720, and 3840×2160 are all 16:9 — scaling within a ratio keeps everything sharp.
- Results round to whole pixels; if a computed side looks one pixel off, that rounding is why — pick the nearest even number for video encoders.
- Swap width/height inputs to flip between landscape and portrait versions of the same ratio (16:9 ↔ 9:16).