Ratio Calculator

Solve proportions, simplify ratios, and scale A:B to any size.

What is a ratio?

A ratio compares two quantities by division. Written as A : B, it tells you how much of one thing there is relative to another — “3 cups of flour to 5 cups of water” is a 3 : 5 ratio. Ratios appear everywhere: recipes, scale models, screen dimensions, financial statements, map distances, mixing paint, and tuning engines.

Ratio calculator and ratio converter in one tool

This page combines a full ratio calculator with a built-in ratio converter, so you can solve proportions, convert a decimal to a ratio, convert a ratio to its simplest form, and convert between scaled versions of the same ratio — all without leaving the page.

Solve a proportion. If you know three of the four values in A : B = C : D, you can find the fourth. Type the three numbers you have, leave the unknown field blank, and press Calculate. The calculator uses cross-multiplication internally — for example, if C is missing, it returns C = (A × D) / B. The filled-in proportion shows alongside the result so you can double-check at a glance.

Convert and scale a ratio. Enter any ratio like 15 : 25 in the Scale tab and the converter instantly shows the simplified form (3 : 5) plus its decimal equivalent (0.6). From there you can convert the ratio to a new size three ways:

  • Multiply by a factor — convert 3 : 5 to 6 : 10 by scaling with factor 2.
  • Set A to a target value — pin A to 10 and B is converted proportionally.
  • Set B to a target value — pin B to 100 and A is converted proportionally.

Converting a decimal to a ratio

To convert a decimal like 0.6 to a ratio, rewrite it as 0.6 : 1, multiply both sides by 10 to clear the decimal (6 : 10), then reduce by the greatest common divisor to get 3 : 5. The Scale tab does all of this automatically — just type the decimal value in the A field and 1 in the B field.

Simplifying a ratio (converting to lowest terms)

Simplifying means dividing both sides of a ratio by their greatest common divisor (GCD) so the numbers are as small as possible while preserving the relationship. 15 : 25 converts to 3 : 5 because the GCD of 15 and 25 is 5. Simplified ratios are easier to read, compare, and scale.

Common uses for a ratio calculator & converter

  • Cooking & baking: convert a recipe from 4 servings to 10 while keeping every ingredient proportional.
  • Screen & image design: convert between aspect ratios like 4 : 3 and 16 : 9, or resize mockups.
  • Scale models & maps: convert real-world distances at a fixed scale like 1 : 87.
  • Finance: compute debt-to-equity, price-to-earnings, and similar ratios.
  • Mixing solutions: dilute concentrates, mix paint, or prepare chemical solutions to an exact ratio.
  • Education: solve proportion word problems step by step.

Tips

  • Use decimal numbers freely — both the calculator and converter handle them.
  • To compare two ratios, convert both to simplest form: if the reduced ratios match, the originals are equivalent.
  • Results update live; nothing is sent to a server, so your numbers stay private in your browser.

Frequently Asked Questions

What is a ratio?
A ratio compares two quantities by division, written as A : B. For example, 3 : 5 means for every 3 units of the first quantity there are 5 units of the second. Ratios are used in cooking, scale models, finance, and design.
How do I solve A:B = C:D for a missing value?
Enter the three values you know and leave the unknown field blank. The calculator uses cross-multiplication to solve: if A is missing, A = (B × C) / D; if B is missing, B = (A × D) / C, and so on.
How do I simplify a ratio like 15:25?
Open the Scale tab and enter 15 and 25. The simplified form updates automatically using the greatest common divisor (GCD), so 15:25 becomes 3:5 with a decimal form of 0.6.
Can I use decimal numbers?
Yes. Both tabs accept positive decimals. When simplifying a decimal ratio the calculator multiplies both sides by the appropriate power of ten to produce an integer pair, then reduces.
What is the difference between a ratio and a fraction?
A fraction A/B is a single numerical value representing a part of a whole. A ratio A:B compares two quantities and can be thought of as the fraction A/B, but it is typically read as a comparison rather than a value.
How do I scale a ratio to a larger size?
In the Scale tab, enter your starting ratio, then choose a scaling mode: multiply both sides by a factor, fix A to a target value and let B scale proportionally, or fix B and let A scale. For example, scaling 3:5 by a factor of 2 gives 6:10.