How to Calculate Aspect Ratio: The Complete Guide

The Aspect Ratio Formula

The aspect ratio of any rectangle is simply: Width ÷ Height. To express it as a clean W:H ratio (e.g., 16:9 rather than 1.778:1), you need to find the Greatest Common Divisor (GCD) of the width and height and divide both by it.

Step-by-Step: How to Find the Aspect Ratio

Here is how to manually calculate the aspect ratio of any image:

• →Step 1: Write down the width and height in pixels (e.g., 1920 and 1080)
• →Step 2: Find the Greatest Common Divisor (GCD) of both numbers. For 1920 and 1080, the GCD is 120.
• →Step 3: Divide both numbers by the GCD. 1920 ÷ 120 = 16; 1080 ÷ 120 = 9.
• →Step 4: Write the result as W:H — in this case, 16:9.

Worked Examples

Here are some common dimensions and their aspect ratios:

How to Calculate a Missing Dimension

If you know the original dimensions and want to find a new size at the same ratio, use this formula: New Height = (Original Height ÷ Original Width) × New Width. For example, to find the height of a 16:9 image at 1280px wide: (1080 ÷ 1920) × 1280 = 720px. Our calculator does this automatically in both directions.

The Fastest Method: Use a Calculator

Manually calculating aspect ratios is straightforward for round numbers, but quickly becomes tedious for irregular dimensions like 1847×923. Our free Aspect Ratio Calculator handles any width and height instantly — enter your values and get the simplified ratio, decimal, closest standard match, and CSS values in one click.