Proportion Calculator

Solve a proportion for any missing value by cross multiplication, with steps.

Cross multiply: if A/B = C/D then A × D = B × C, so the missing value is the product of the diagonal pair divided by the remaining number. For 3/4 = x/20, x = 3 × 20 ÷ 4 = 15.

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How to use it

1. Enter three of the four values in A/B = C/D.
2. Leave the unknown blank.
3. Read the missing value and the cross multiplication steps.

What you enter

• Any three of A, B, C and D

What you get

• Missing value
• Cross multiplication steps

Formula

A/B = C/D ⇔ A × D = B × C

Worked example

Input: 3/4 = x/20

Result: x = 15

3 × 20 = 60 and 60 ÷ 4 = 15.

Questions

How do you solve a proportion?

Cross multiply to get A × D = B × C, then divide both sides by the number next to the unknown.

What is a proportion?

An equation saying two ratios are equal, such as 2/3 = 8/12.

How do I check if two ratios are proportional?

Cross multiply. If A × D equals B × C, the ratios are equal.

Where are proportions used?

Scaling recipes, converting units, reading map scales, and working out doses or dilutions in the lab.

Limitations

• Denominators cannot be zero.

References

• OpenStax, Prealgebra 2e, §6.5.

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All free tools · By Dr. Zubair Khalid, DVM, MS, PhD.