Proportion Calculator
Find the unknown term in a proportion such as 3/4 = 9/x by cross multiplying.
How to use the Proportion Calculator
- Choose the unknown term.
- Enter the a.
- Enter the b.
- Enter the c.
- Enter the d.
- Press Calculate. The result updates automatically as you type.
About the Proportion Calculator
A proportion says two ratios are equal. Whenever one quantity changes at a constant rate with another, you can set up a proportion and solve for the piece you do not know. This "rule of three" covers unit pricing (if 3 lb costs $4, how much do 9 lb cost?), map and blueprint scales, recipe conversion, dosage per body weight and similar triangles in geometry.
Keep the units in matching positions: if A is pounds and B is dollars, then C must be pounds and D dollars. The calculator checks for zero denominators, since a ratio with zero on the bottom is undefined. Proportions only work for quantities that scale directly; doubling the number of painters does not exactly halve the job time in practice.
Formula
A / B = C / D ⇔ A × D = B × C
Cross multiply, then divide by the number paired with the unknown. For example, 3/4 = 9/D gives 3 × D = 4 × 9 = 36, so D = 36 ÷ 3 = 12.
Example calculation
Using the default values:
- Unknown term = D
- A = 3
- B = 4
- C = 9
the calculator returns:
- Missing value: 12
- Completed proportion: 3/4 = 9/12
Frequently asked questions
How do you solve a proportion?
Cross multiply to remove the fractions, then divide both sides by the number next to the variable.
What is the rule of three?
It is the shortcut for solving A/B = C/D when three values are known: the missing value equals the product of the diagonal pair divided by the remaining value.
How can I check my answer?
Divide A by B and C by D. If the two decimals are equal, the proportion is correct.
Last updated: · Category: Math calculators