Ratio calculator
Simplify a ratio to its lowest terms, or find the missing number in an equivalent ratio.
Fill in the fields and press Calculate to see your result here.
How this calculation works
Simplifying a ratio divides both sides by their greatest common divisor (GCD), the same way as reducing a fraction. Solving a proportion a : b = c : x uses cross-multiplication, rearranged to x = (b × c) ÷ a.
Worked example
A recipe uses a 3:2 ratio of flour to sugar. For 500g of flour: sugar = (2 × 500) ÷ 3 ≈ 333g.
Tips
A ratio simplifies like a fraction
12:18 and the fraction 12/18 reduce the same way — both divide by their GCD, 6, to give 2:3.
Order matters in a proportion
a:b = c:x keeps the same order on both sides. Swapping a and b changes what x solves for.
Handy for scaling recipes and plans
If a recipe uses 2 cups of rice for every 5 cups of water, the proportion solver tells you the water needed for 8 cups of rice: 20.
Compare ratios by simplifying both
3:4 and 9:12 look different but both simplify to 3:4, so they describe the same relationship.
Check a proportion by cross-multiplying
For a:b = c:x, the products a × x and b × c must match. It's a quick way to sanity-check any answer.
Frequently asked questions
What if a and b share no common factor?
Then the ratio is already in its simplest form, and the calculator will return the same two numbers you entered.
Can I use decimals?
The simplify tab expects whole numbers to find a clean GCD; the proportion solver accepts decimals for a, b and c.