Free Online Rule of Three Calculator 2026
The rule of three calculator is an essential tool for solving proportion problems quickly and accurately. Whether for everyday math, unit conversions, or financial planning, this free calculator solves any simple rule of three in seconds.
Just enter the three known values and the calculator automatically finds the unknown value (X).
Rule of Three Calculator
Solve proportions quickly and accurately
If
is to
,
then
is to
How to Use the Rule of Three Calculator
- Value A – Enter the first value of the proportion (e.g. 2)
- Value B – Enter the second value that relates to A (e.g. 5)
- Value C – Enter the third value (e.g. 10)
- Click “Calculate” and the value of X will appear automatically
What is the Rule of Three?
The rule of three is a mathematical method used to find an unknown value from directly or inversely proportional quantities. The formula used is: A / B = C / X, where X = (B × C) ÷ A.
Examples of Use
- Cooking recipes: If a recipe serves 4 people with 200g of flour, how much flour do I need for 7 people?
- Fuel: If 30 liters cover 400 km, how many liters do I need for 650 km?
- Conversions: If 1 dollar equals $1.10, how much is $100 in euros?
- Proportions: If 5 workers do the job in 12 days, how many days do 8 workers need?