Rule of Three Calculator
Everyday
The rule of three solves a proportion: two ratios that are equal, a/b = c/d. Enter the three values you know and leave the unknown field empty — it's filled in instantly by cross-multiplication (a·d = b·c). Use it to scale a recipe, convert map distances, work out unit prices, adjust quantities, or any 'if this much costs that, how much does…' problem. The unknown can be in any of the four positions. Everything is computed locally in your browser.
Enter three values and leave one field empty — it's solved automatically.
Result
d = 8
3 / 4 = 6 / 8
How to use
- Type the three numbers you know into the proportion a / b = c / d.
- Leave the one you want to find empty.
- Read the answer and the completed proportion below.
Frequently asked questions
- What is the rule of three?
- It's a way to find an unknown value in a proportion. If a/b = c/d and one of them is unknown, you cross-multiply: a·d = b·c, then divide to isolate the missing value.
- Where is the unknown allowed?
- Anywhere — leave a, b, c or d empty. The calculator detects which field is blank and solves for it as long as that doesn't require dividing by zero.
- Can I use decimals?
- Yes. Any of the values can be a decimal (e.g. 2.5), and the result keeps full precision, trimmed to a readable number of digits.
- Why do I get a division-by-zero message?
- Solving for one position divides by another value. If that value is zero, the proportion has no finite solution — change the inputs so you're not dividing by zero.
Related tools
Tile Calculator
Work out how many floor or wall tiles you need for a room — from its length and width and the tile size, with an adjustable waste allowance and boxes required.
Compound Interest Calculator
Project how a starting balance grows with regular contributions, your interest rate, and your compounding frequency.
Age Calculator
Calculate exact age in years, months, days — and find your next birthday.
BMI Calculator
Calculate Body Mass Index from height and weight, with a healthy range.
Tip Calculator
Calculate the tip and split the total bill between people.
Percentage Calculator
Work out percentages, what-percent, and percent change in one place.