Aspect Ratio Glossary: GCD, DAR, PAR & Ratio Notation
Bikash Roy
Online Research Specialist · SEO Strategist · Web Developer
Bikash personally verifies every calculator against official standards (WHO, NIH, government data). Online research specialist, SEO strategist, and web developer with 10+ years building accurate digital tools.
Full bio →Why This Vocabulary Matters
Aspect ratio terminology spans photography, video production, and web design, and mixing up a few key terms is a common source of confusion when resizing or exporting media. This glossary defines them, so a result from the aspect ratio calculator is fully clear.
Aspect Ratio
An aspect ratio is the proportional relationship between width and height, written as width:height (16:9, 4:3, 1:1). It describes shape, not size — a 16:9 poster and a 16:9 phone screen have the same proportions even though their actual dimensions are completely different.
Greatest Common Divisor (GCD)
The greatest common divisor is the largest number that divides evenly into both the width and height of a set of pixel dimensions. Dividing both numbers by their GCD produces the simplified aspect ratio — 1920÷120 and 1080÷120 (where 120 is the GCD of 1920 and 1080) gives 16:9. The aspect ratio calculator computes this automatically for any dimensions you enter.
Display Aspect Ratio (DAR)
Display aspect ratio is the aspect ratio an image or video is meant to be displayed at — the ratio viewers actually see, regardless of how the underlying pixel data is stored. For most modern digital photos and videos, DAR matches the simplified pixel ratio directly.
Pixel Aspect Ratio (PAR)
Pixel aspect ratio describes the shape of an individual pixel itself — square (1:1) in virtually all modern digital video and photography, but non-square in some older broadcast and DVD formats. When PAR isn’t 1:1, the stored pixel dimensions and the actual displayed aspect ratio diverge, a legacy consideration mostly irrelevant for current web, social, and streaming content.
Letterboxing and Pillarboxing
Letterboxing adds horizontal black bars above and below content to preserve its original aspect ratio when displayed on a wider or taller screen (common when showing a 4:3 video on a 16:9 screen). Pillarboxing is the reverse — vertical black bars on the sides, common when showing a 9:16 vertical video on a 16:9 horizontal screen.
Cropping vs Letterboxing
Cropping changes the aspect ratio by cutting off part of the original image or video to fill a target frame completely, losing some content at the edges. This is the alternative to letterboxing/pillarboxing when a platform doesn’t support displaying black bars — see how to resize proportionally without distortion for when each approach makes sense.
Proportional Resize
A proportional resize scales width and height by the same factor, preserving the original aspect ratio exactly. Scaling only one dimension without adjusting the other produces a non-proportional (distorted) resize, stretching or squashing the image.
Common Named Ratios
16:9 (widescreen), 4:3 (classic television/presentations), 9:16 (vertical/mobile video), 1:1 (square), and 21:9 (ultrawide/cinematic) are the ratios most platforms and devices standardize around — see common video and photo aspect ratios by platform for exact pixel dimensions each one uses.
Putting the Terms Together
Once you know the difference between a display ratio and a pixel ratio, and between cropping and letterboxing, using the aspect ratio calculator to find, convert, or resize a ratio makes a lot more sense. See common aspect ratio mistakes for the errors that most often trip people up.