Aspect Ratio Calculator
Enter a width and height to get the simplified aspect ratio and its decimal value, or lock a target ratio such as 16:9 and let the calculator solve the missing dimension as you type. Everything updates live, with a preview of the shape you are describing.
Enter a width and height, or lock a ratio to solve a missing dimension.
What an aspect ratio is
An aspect ratio describes the proportional relationship between an image's width and its height, written as two numbers separated by a colon, such as 16:9. It says nothing about the absolute size in pixels or inches — a 1280×720 frame and a 3840×2160 frame share the same 16:9 shape. Because it is a proportion, the ratio is what stays constant when you scale content up or down, which is why it is the number to match when you resize, crop, or fit media into a container.
Common ratios and where they are used
- 16:9 — the standard for modern monitors, laptops, TVs, and most online video (720p, 1080p, 1440p, and 4K are all 16:9).
- 21:9 — ultrawide monitors and cinematic gaming. Marketed as 21:9, the pixel dimensions usually reduce to 64:27 (about 2.37:1).
- 4:3 — the legacy standard for older TVs, early computer displays, and many tablets and document scans.
- 9:16 — vertical video for phones and social platforms such as Stories, Reels, TikTok, and Shorts.
- 1:1 — the square format common in profile pictures and some social feed images.
- 2.39:1 — modern anamorphic widescreen cinema, the widest of the common theatrical formats.
How the calculation works
To simplify width and height into a ratio, divide both by their greatest common divisor (GCD). For 1920×1080 the GCD is 120, so the ratio reduces to 16:9. The decimal form is simply width divided by height: 1920 ÷ 1080 = 1.78, meaning the image is 1.78 times as wide as it is tall. When you lock a ratio and enter one dimension, the calculator does the inverse — it multiplies or divides by the ratio to find the other side, rounding to the nearest whole pixel.
Resizing, letterboxing, and pillarboxing
To resize without stretching or squashing, keep the ratio fixed: change one dimension and recompute the other rather than editing both freely. When source and display ratios differ, the content is fitted with bars instead of being distorted. Letterboxing adds horizontal bars above and below when wider content sits inside a narrower frame — for example a 2.39:1 film on a 16:9 screen. Pillarboxing adds vertical bars on the sides when narrower content, such as 4:3 footage, plays on a 16:9 display. If you want to relate these ratios to a real screen's physical size or pixel density, the What Is My Screen Size and PPI Calculator tools cover those measurements.