How to Use
- Select or enter a ratio
Choose a preset (16:9, 4:3, etc.) or type a custom ratio.
- Enter width or height
Input the known dimension in pixels.
- View result
Click Calculate to see the missing dimension and pixel resolution.
What is an aspect ratio?
Aspect ratio expresses the ratio of width to height of a screen or image as a pair of whole numbers. Even when the absolute pixel counts differ (like 1920×1080), shapes with the same ratio look identical, so it is essentially a unit that describes 'shape' independently of resolution.
Why it matters
- Compatibility: Monitors, TVs, and projectors have fixed ratios; when the ratio does not match, you get black bars on the sides (pillarboxing) or top and bottom (letterboxing).
- No distortion: Forcing 16:9 footage into a 4:3 frame stretches people so they look too wide or too tall.
- Platform specs: YouTube favors 16:9, Instagram feed 1:1 or 4:5, Reels and Shorts 9:16. Each channel recommends a different ratio, so you should match it before uploading to avoid cropping.
Formula
Divide width and height by their greatest common divisor (GCD) to reduce them to the simplest whole-number ratio.
ratio = width ÷ gcd(width, height) : height ÷ gcd(width, height)
decimal ratio = width ÷ height (rounded to 4 decimal places)
Example: 1920 × 1080
- gcd(1920, 1080) = 120
- width: 1920 ÷ 120 = 16
- height: 1080 ÷ 120 = 9 → result 16:9
- decimal ratio: 1920 ÷ 1080 = 1.7778
Conversely, if you know the ratio and just one side, you can find the other. For example, in 16:9 with a width of 2560 → height = 2560 × (9 ÷ 16) = 1440.