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Ratio Calculator

Fill in three numbers of A : B = C : D and get the fourth — plus the simplified ratio.

Runs locally — files never leave your device

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How to use Ratio Calculator

  1. 1Enter three of the four values in A : B = C : D and leave the unknown one empty.
  2. 2The missing value is calculated instantly and highlighted.
  3. 3See the ratio in its simplest form, such as 1920 : 1080 = 16 : 9.
  4. 4Use the presets for common aspect ratios, or Clear to start again.

Solve ratios and proportions

Ratios are everywhere: screen and photo dimensions, recipes, mixing paint or concrete, map scales, gear ratios, exchange rates and school maths. This ratio calculator solves the classic proportion A : B = C : D for whichever value you don't know and shows the ratio in its simplest form. It's the quickest way to keep an image's aspect ratio, scale a recipe or check your homework.

Features

  • Solve for any of the four values — just leave one box empty.
  • Simplify ratios to whole numbers, including decimal ratios.
  • Aspect ratio presets: 16:9, 4:3, 3:2, 1:1, 21:9 and 9:16.
  • Instant results as you type.
  • Decimal support for precise measurements.

Common aspect ratios

Ratio Typical use Example sizes
16:9 HD video, monitors, YouTube 1280 × 720, 1920 × 1080, 3840 × 2160
9:16 Phone stories and short videos 1080 × 1920
4:3 Older screens, presentations, many cameras 1024 × 768, 4000 × 3000
3:2 DSLR photos, prints 6000 × 4000, 6 × 4 in
1:1 Square social media posts 1080 × 1080
21:9 Ultrawide monitors, cinema 2560 × 1080, 3440 × 1440

Related tools

Resize a picture to the dimensions you calculated with Resize Image, work with fractions in the Fraction Calculator, or convert percentages with the Percentage Calculator.

Frequently asked questions

How do I solve a proportion?
Cross-multiply. In A : B = C : D, A × D = B × C. If D is missing, D = B × C ÷ A. For 16 : 9 = 1920 : D, D = 9 × 1920 ÷ 16 = 1080.
How do I simplify a ratio?
Divide both numbers by their greatest common divisor. 1920 : 1080 → both divide by 120 → 16 : 9. Decimal ratios are first multiplied up to whole numbers, so 1.5 : 2.5 becomes 3 : 5.
How do I keep an image's aspect ratio when resizing?
Put the original width and height in A and B, the new width in C, and leave D empty — D is the new height. For a 4000 × 3000 photo resized to 1200 pixels wide, the height is 900.
How do I scale a recipe?
Enter the original servings and an ingredient amount as A and B, the new number of servings as C, and the calculator gives the new amount as D. For 4 servings with 300 g flour scaled to 6 servings, use 4 : 300 = 6 : D → 450 g.
Why do I see no result?
Exactly one of the four boxes must be empty, and all entered values must be greater than zero.

Related tools

Fraction CalculatorCalculate with fractions and mixed numbers and see the simplified answer with step-by-step working.Percentage CalculatorSolve the four everyday percentage problems — percent of, what percent, percent change and original value.Resize ImageResize photos to exact pixel sizes or by percentage — one image or a whole batch, without uploading.Unit ConverterConvert between metric and imperial units across ten kinds of measurement, with all results at a glance.Age CalculatorFind your exact age in years, months and days — and how many days until your next birthday.Average CalculatorPaste a list of numbers to get the mean, median, mode, range and standard deviation — or calculate a weighted average.