Ratio Calculator
Enter a known ratio A : B and one term of a second ratio. This calculator reduces A : B by its greatest common divisor and cross-multiplies to solve A : B = C : X, so X = B × C ÷ A. It also shows the ratio as a decimal and as a percentage split of the whole.
- Accurate
- Real-time
- Easy to use
- 100% free
Missing term X
12.0000
Decimal ratio (A ÷ B)
0.7500
Details
Updates as you typeThe left side of the known ratio A : B. Must be greater than 0.
The right side of the known ratio A : B. Must also be greater than 0.
The left side of the proportion C : X, solved against A : B.
Summary
Missing term X
12.0000
A + B
7.0000
- Term A3.000043%
- Term B4.000057%
- Simplified ratio
- 3:4
- Decimal ratio (A ÷ B)
- 0.7500
- Share of A
- 42.9%
- Share of B
- 57.1%
- Scale factor (C ÷ A)
- 3.0000
- A ratio compares parts to each other, not to the whole: 3 : 4 means seven parts in total, of which three are A.
- A : B and C : X are equivalent ratios, so cross-multiplying gives A × X = B × C, and therefore X = B × C ÷ A.
- Terms are read to 15 significant digits before reducing, so a value typed with more precision than a decimal number can hold is rounded before the common factor is taken.
How this is calculated
- Term A
- 3.0000
- Term B
- 4.0000
- Term C
- 9.0000
- Common factor removed
- 1.0000
- A : B simplified
- 3:4
- Scale factor C ÷ A
- 3.0000
- X = B × C ÷ A
- 12.0000
- Solved proportion
- 9:12
Compare scenarios
See how one change moves the result
- CurrentYour inputs as they stand12.0000Current
- Term A49.0000
- Term B515.0000
- Term C1114.6667
Frequently asked questions
How do you simplify a ratio?
Divide both terms by their greatest common divisor. For 18 : 24 the GCD is 6, so the ratio reduces to 3 : 4. Decimal terms are first scaled up to whole numbers — 1.5 : 2 becomes 15 : 20, which reduces to 3 : 4.
How do you solve a proportion for the missing term?
Two equivalent ratios cross-multiply: if A : B = C : X then A × X = B × C, so X = B × C ÷ A. With 3 : 4 = 9 : X, the missing term is 4 × 9 ÷ 3, which is 12.
What is the difference between a ratio and a fraction?
A ratio compares two parts to each other, while a fraction compares a part to the whole. The ratio 3 : 4 describes seven total parts, so as fractions of the whole the sides are 3/7 and 4/7, or about 42.9% and 57.1%.
Why must the first term be greater than zero?
Solving for X divides by term A, and dividing by zero is undefined, so a zero first term has no solution. A zero second term is also rejected because a side with no parts cannot hold a meaningful share of the whole.