Aspect Ratio Calculator

How to use Aspect Ratio Calculator

1. Enter the original width and height in pixels, or tap a preset such as 16:9 or 4:3.
2. Read the simplified ratio, decimal value and orientation.
3. Type a new width to get the height that keeps the same shape, or type a new height to get the width.
4. Round to whole pixels and use the numbers in your editor, video tool or CSS.

What an aspect ratio means

An aspect ratio describes the shape of a rectangle as the relationship between its width and its height. It says nothing about actual size. A phone screen and a cinema screen can both be 16:9, even though one is a few centimetres wide and the other several metres. Because only the proportion matters, ratios are written as the smallest whole numbers that preserve it, such as 16:9 rather than 1920:1080.

The formula used here is simple. Find the greatest common divisor g of the width W and the height H, then the ratio is (W ÷ g):(H ÷ g). To resize while keeping the shape, use new height = new width × H ÷ W, or new width = new height × W ÷ H.

Worked example: a 1920 by 1080 video has g = 120, so it is 16:9. To make it 1280 pixels wide, the height is 1280 × 1080 ÷ 1920 = 720, so the result is 1280 by 720.

Common ratios and where they are used

• 16:9: standard for HD and 4K video, most monitors and TVs, and YouTube.
• 4:3: older television and many tablets and compact cameras.
• 1:1: square posts and profile pictures.
• 9:16: vertical video for phone-first formats such as stories and short clips.
• 21:9: ultrawide monitors and many films.
• 3:2: classic 35 mm photography and many DSLR sensors.

Tips and limits

Always scale from the original dimensions rather than from an already-rounded copy, otherwise small rounding errors accumulate. If you need an exact fit inside a fixed frame, decide whether to crop or to add bars: scaling alone cannot change the shape. Some platforms apply their own limits or re-encode files, so check their current size guidance. In CSS, the aspect-ratio property accepts the simplified pair directly, for example aspect-ratio: 16 / 9. All calculations happen in your browser and nothing is uploaded.

Why use this aspect ratio calculator tool

• Simplified ratio. Divides both sides by their greatest common divisor, so 1920 by 1080 becomes 16:9.
• Scale either way. Start from a target width or a target height and get the other side instantly.
• Common presets. One tap for 16:9, 4:3, 1:1, 9:16, 21:9 and 3:2.
• Visual preview. A proportional box shows what the shape will look like before you resize anything.

Frequently asked questions

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