Math & Engineering
Ratio Calculator
Calculate ratios and find equivalent ratios with this easy-to-use calculator.
Reviewed by the calculator.uk.com Team · Last reviewed 17 May 2026 · Our methodology
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Related to Ratio Calculator
The ratio calculator helps you work with ratios in three main ways: finding the fourth number in a proportion, simplifying ratios, and converting ratios to decimals. A ratio compares two or more related quantities, often written as a:b or a/b.
Finding the Fourth Number
When you have three numbers in a proportion (a:b = c:d), the calculator finds the fourth number using the formula: d = (b × c) ÷ a. This is based on the principle that corresponding ratios are equal in a proportion.
Simplifying Ratios
The calculator simplifies ratios by finding the greatest common divisor (GCD) of the two numbers and dividing both numbers by it. For example, 8:12 simplifies to 2:3 because 4 is the GCD of 8 and 12.
Decimal Conversion
The calculator converts ratios to decimals by dividing the second number by the first (b÷a). This is useful for comparing ratios or using them in calculations.
The calculator provides three key results to help you understand and work with ratios effectively. Each result has its own practical applications and interpretations.
Fourth Number (d)
This is the missing value in your proportion (a:b = c:d). It represents the quantity that maintains the same ratio as the original numbers. For example, if 2:3 = 6:d, then d=9 because 2:3 = 6:9.
Simplified Ratio
The simplified ratio shows the relationship between quantities in its most basic form. For instance, 50:100 simplifies to 1:2, making it easier to understand and work with while maintaining the same proportional relationship.
Decimal Form
The decimal form (b/a) shows the ratio as a single number, useful for comparing different ratios or performing calculations. For example, 3:4 as a decimal is 1.333..., which means the second number is 1.333... times the first.
1. What is a ratio?
A ratio is a comparison between two or more related quantities, typically expressed as a:b or a/b. It shows how much of one thing there is compared to another thing.
2. How do I know if two ratios are equivalent?
Two ratios are equivalent if they simplify to the same ratio or if their decimal forms are equal. For example, 2:3 and 4:6 are equivalent because they both simplify to 2:3 and both equal 1.5 in decimal form.
3. Can ratios have decimal numbers?
Yes, ratios can include decimal numbers. For example, 1.5:2.5 is a valid ratio. The calculator can handle decimal inputs and will provide accurate results for proportions, simplification, and decimal conversion.
4. Why do we simplify ratios?
Simplifying ratios makes them easier to understand and work with while maintaining the same proportional relationship. It helps in comparing different ratios and identifying equivalent ratios more quickly.
5. What is the scientific source for this calculator?
Ratio simplification here uses the Euclidean algorithm for finding the greatest common divisor, one of the oldest algorithms in mathematics, described in Euclid's Elements (circa 300 BCE) and still the standard method taught today. Proportions and decimal conversions follow the same cross-multiplication and division rules used throughout basic algebra.