Aspect Ratio Calculator
FreeCompute missing dimensions from a ratio, simplify pixel dimensions to a ratio, or resize proportionally, with a live preview and common ratio presets.
Runs entirely in your browser.
Common ratios
Recent Activity
Visible to everyone. Last 20 uses across all visitors, newest first.
16:9 -- At width 1920px, height = 1080px
1 day ago
What this tool does
This tool works with aspect ratios in three ways, right in your browser: compute a missing width or height from a ratio, simplify a pixel width and height into a ratio, or resize dimensions proportionally, with a live preview rectangle, 8 common ratio presets, and a reference table of standard resolutions.
How It Works
Given a ratio W:H and one known dimension, the other is found by cross-multiplication: if width is known, height = width × (H / W), and if height is known, width = height × (W / H). To simplify a pixel width and height into a ratio, both numbers are divided by their Greatest Common Divisor (GCD), found via the Euclidean algorithm. To resize proportionally, the same scale factor is applied to both dimensions: new height = new width × (original height / original width), or the equivalent when starting from a new height instead.
Problems it solves
- Finding the correct height for a target width (or vice versa) to match a specific ratio like 16:9.
- Identifying what standard ratio a set of pixel dimensions corresponds to.
- Resizing an image or video frame without distorting it.
- Choosing the right ratio for a specific platform, like YouTube, Instagram, or TikTok.
Frequently asked questions
›What is an aspect ratio?
It is the proportional relationship between a width and a height, written as two numbers separated by a colon, like 16:9. A 1920×1080 screen and a 1280×720 screen share the same 16:9 ratio, just at different sizes.
›How is a ratio simplified, like 1920×1080 becoming 16:9?
Both numbers are divided by their Greatest Common Divisor (GCD), the largest number that divides evenly into both. For 1920 and 1080, the GCD is 120, and 1920/120 = 16 while 1080/120 = 9, giving 16:9.
›Why does resizing keep width and height proportional?
To avoid stretching or squashing an image or video, the new width and new height must have the same ratio as the original. Multiplying (or dividing) both dimensions by the same scale factor guarantees that.
›Which ratio should I use for YouTube, Instagram, or TikTok?
YouTube standard video is 16:9, Instagram portrait posts and Reels commonly use 4:5, and TikTok's full-screen vertical video uses 9:16. These are included as quick preset buttons.
›What does the decimal ratio number mean?
It is the width divided by the height as a single number, for example 16:9 is approximately 1.778:1. It is a quick way to compare two ratios or check how close a custom ratio is to a standard one.
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