Ratio & Percentage Calculator
This tool bundles three related calculations: reducing a ratio to its simplest whole-number form, solving for a missing term in a proportion (A:B = C:D), and converting between a part, a whole, a percentage, and a rate of change. Everything happens in your browser, and nothing you type is ever sent to a server. If you want to work out a tax-included/excluded price or a discounted price, try the Consumption Tax & Discount Calculator as well.
🔒 Runs in your browser · Your data is never sent to a server
1. Simplify a ratio
Reduce the ratio A:B to its simplest whole-number form.
2. Solve a proportion
Given the proportion A:B = C:D, calculate the value of whichever term you leave blank.
3. Percentage calculations
Choose what you want to calculate.
Notes
- "Simplify a ratio" can't calculate if either value is 0, negative, or not recognized as a valid number.
- For "Solve a proportion", enter a value greater than 0 for all three terms other than the one you leave blank.
- The rate of change in "Percentage calculations" is relative to the old value: positive for an increase, negative for a decrease.
- This tool provides a rough simulation only. Double-check accuracy separately before relying on the results for anything official.
Glossary
Ratio
A comparison of the sizes of two or more quantities, written as A:B. For example, "4:6" represents the same relationship as "2:3" — dividing both sides by their greatest common divisor reduces it to the simplest whole-number ratio.
Proportion
An equation stating that two ratios are equal, such as "A:B = C:D". Because the product of the extremes equals the product of the means (A×D = B×C), knowing any three terms lets you solve for the fourth.
Percentage
The size of a part relative to a whole, expressed out of 100. It's calculated as "part ÷ whole × 100", and conversely the part can be found as "whole × percentage ÷ 100".
Rate of change
How much a value changed from an old value to a new one, expressed as a percentage of the old value: "(new value − old value) ÷ old value × 100". It's positive for an increase and negative for a decrease, and is commonly used for year-over-year or month-over-month comparisons.
FAQ
How does ratio simplification work?
Each number is divided by the greatest common divisor to give the simplest whole-number ratio. For example, 12:18 becomes 2:3, and decimals like 1.5:2.5 become 3:5.
How do I solve for a missing term in a proportion?
Leave the unknown term of A:B = C:D empty and enter positive numbers in the other three. The tool uses the fact that cross products are equal.
How is the percent change calculated?
It is the change relative to the original value: (new − original) ÷ original × 100. Increases are positive and decreases are negative.
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