What Is Aspect Ratio? 16:9, 4:3 and How to Compute It

What the two numbers really mean

An aspect ratio like 16:9 is not a measurement in centimeters or pixels; it is a proportion. It says nothing about actual size — a phone screen and a cinema wall can both be 16:9. What it fixes is the shape: how wide the rectangle is relative to how tall.

By convention the width comes first and the height second. A ratio greater than 1 (like 16:9 ≈ 1.78) is landscape, wider than it is tall; a ratio less than 1 is portrait; and exactly 1:1 is a square.

How to compute a ratio from dimensions

Start with the raw width and height in the same units, then divide both by their greatest common divisor (GCD) to reach the smallest whole-number form. For 1920 and 1080, the GCD is 120: 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9, giving 16:9. For 1024×768, the GCD is 256, which reduces to 4:3.

If you only need a quick sense of the shape, divide width by height as a decimal: 1920 ÷ 1080 ≈ 1.78. Comparing that decimal to known ratios (1.33 for 4:3, 1.78 for 16:9, 2.35 for cinematic widescreen) tells you the format at a glance.

Where each ratio shows up

16:9 is today's dominant format for televisions, computer monitors, and online video. 4:3 was the shape of older TVs and remains common in some tablets and presentation slides. 3:2 is a classic photography ratio from 35 mm film, while 1:1 squares suit social-media grids and 9:16 vertical video suits phone-first platforms.

Knowing the target ratio matters when you resize or crop: stretching content to a different ratio distorts it, so designers either crop, add letterbox bars, or pick source material that already matches the destination shape.

Frequently asked questions

Is 16:9 the same as 1920×1080?

1920×1080 has a 16:9 aspect ratio, but so do 1280×720 and 3840×2160. The ratio describes the shape, while the pixel dimensions describe the resolution — many resolutions share the same ratio.

What is the difference between aspect ratio and resolution?

Resolution is the actual pixel count, such as 1920×1080, and controls sharpness. Aspect ratio is the proportion between those numbers and controls shape. Two screens can share a ratio but differ enormously in resolution.

Why do videos sometimes have black bars?

Black bars, called letterboxing or pillarboxing, appear when the video's aspect ratio does not match the screen's. The bars preserve the original proportions instead of stretching the picture, avoiding distortion.

How do I keep proportions when resizing an image?

Scale width and height by the same factor. If you know one new dimension, multiply the other by the ratio: to make a 16:9 image 1280 wide, the height is 1280 × 9 ÷ 16 = 720.