Aspect Ratio Calculator

What this tool does

Aspect Ratio Calculator works out the third value whenever you know two of width, height and ratio, reduces any width/height pair to its simplest whole-number ratio such as 16:9, and scales an original box to a target maximum width or height while keeping its proportions. Built-in presets cover common video, photo and paper ratios.

Resizing an image, video frame or UI panel without distorting it means keeping the relationship between width and height fixed. Doing that math by hand for odd numbers, such as fitting a 4000×3000 photo into a 1200px-wide thumbnail, is slow and easy to get slightly wrong.

This calculator covers three everyday jobs: reducing a pixel pair to a clean ratio like 16:9, filling in a missing width or height once you know the ratio, and scaling an original size down (or up) to a target maximum dimension while the other side follows automatically.

How to use Aspect Ratio Calculator

1. To find a ratio, type a width and height into Find the ratio from width & height and read the simplified form and decimal value.
2. To solve for a missing side, type or pick a ratio such as 16:9, choose whether you already know the width or the height, then enter that known value.
3. Read the computed width or height under Find a missing dimension from a ratio.
4. To fit an existing image or frame into a new size, enter its original width and height under Scale to fit.
5. Choose whether your target is a maximum width or height and enter that target value.
6. Read the resulting pair; copy either number with the small copy buttons next to each result.
7. Click a preset in the side panel to load a common ratio such as 21:9 or A4 paper into the ratio field.

Tips for better results

• Non-integer inputs still simplify correctly: the tool scales both sides up before reducing, so 1.5 by 1 becomes 3:2.
• A4 and US Letter paper are not whole-number ratios, so they are listed here as decimals (1:1.4142 and 1:1.2941) rather than rounded fractions.
• 2.39:1, often called Cinemascope, is one of several anamorphic widescreen ratios used in film; 2.35:1 and 1.85:1 are also common.
• When scaling to fit a maximum width on a vertical photo, double-check whether you actually meant to cap the height instead.

Common use cases

• Working out the height for a banner or hero image once the design calls for a fixed width and a 16:9 ratio.
• Checking what ratio a camera or screenshot actually used before cropping it to a standard size.
• Generating a thumbnail size that fits inside a 1200px box without stretching the original photo.
• Picking consistent video export dimensions for 9:16 vertical clips from a 16:9 source.

Common mistakes to avoid

• Stretching an image to an unrelated width and height instead of computing the matching dimension first, which distorts faces and straight lines.
• Rounding an intermediate ratio before using it in a second calculation, which compounds small errors across a batch of images.
• Assuming paper sizes reduce to tidy whole numbers the way screen ratios do; A4 and Letter are irrational and near-irrational proportions.