Aspect Ratio Calculator

Frequently Asked Questions

What is an aspect ratio?

It is the proportional relationship between width and height, written as W:H. Common examples are 16:9 (HDTV), 4:3 (old TV), 21:9 (ultrawide), and 3:2 (35mm photos).

How do I keep an image's aspect ratio when resizing?

Pick a new width, then new height = new_width × (original_height ÷ original_width). Most image tools have a "lock aspect" option to do this automatically.

What aspect ratio should I use for social media?

Instagram square 1:1, Reels and TikTok 9:16, YouTube 16:9, Twitter 16:9 or 1:1. Vertical 9:16 dominates mobile-first platforms.

How do I simplify an aspect ratio like 1920×1080?

Divide both numbers by their GCD. GCD(1920, 1080) = 120, so it simplifies to 16:9.

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How This Calculator Works

An aspect ratio is the proportional relationship between an image's width and its height. The simplify mode reduces any width × height pair to its smallest whole-number ratio using the Greatest Common Divisor (GCD). The resize mode takes an original W×H and a new dimension and computes the missing dimension while preserving the original ratio - essential for resizing images, videos, or design assets without distortion.

Common Aspect Ratios & Where They Appear

16:9 (≈1.78): The dominant video standard since the late 1990s. HDTV, YouTube, most laptop and desktop monitors, modern smartphone landscape video.

4:3 (≈1.33): The original TV and early-computer ratio. Still used for some tablets (iPad), classic films, and presentations.

21:9 (≈2.39): Ultrawide cinematic ratio used by many movies and increasingly by gaming monitors. Lets you fit two full pages side-by-side.

3:2 (1.5): The 35mm film ratio still used by most digital cameras (DSLRs, mirrorless) and some Surface devices.

1:1: Square. Instagram's original format, vinyl record sleeves, profile photos.

9:16 (≈0.5625): Vertical video for TikTok, Instagram Reels, YouTube Shorts.

2.39:1 (Cinemascope): Theatrical anamorphic widescreen - the "letterbox" black bars you see on a 16:9 TV when watching a movie.

How the Math Works

To simplify a ratio, divide both numbers by their GCD. For 1920 × 1080: GCD(1920, 1080) = 120, so the simplified ratio is 16:9. To preserve a ratio when resizing, the rule is:

new_height = new_width × (original_height ÷ original_width)

And vice versa for solving for the new width. This guarantees the image is scaled rather than stretched.

Why Aspect Ratio Matters

Mismatched aspect ratios are the source of most "stretched" or "letterboxed" images on the web. Upload a 16:9 photo into a 1:1 social-media slot and you either lose pixels (cropping) or get bars (padding). Designers solve this by either: (1) preparing multiple aspect-ratio crops up front, (2) using `object-fit: cover` in CSS to fill a container while respecting the ratio, or (3) shooting/editing with a target ratio in mind.

Pixel Aspect Ratio vs Display Aspect Ratio

Modern screens use square pixels - the displayed ratio matches the pixel-count ratio. But some legacy video formats (DV, anamorphic) use non-square pixels: a frame stored as 720×480 with rectangular pixels actually displays as 16:9, not 3:2. If you're working with broadcast video, check the file's PAR (Pixel Aspect Ratio) flag in addition to its dimensions.

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