Proportion Notebook

PROPORTION CALCULATOR / MATH YOU CAN SEE

Proportionin balance.

Three known values. One missing piece.
See the relationship, then follow the steps.

YOUR EQUATIONa / b = c / d
3 of 4 parts
The left ratio.
15 of 20 parts
The right ratio.

Find the missing piece.

Leave one box empty. Or fill all four to check.

TRY A QUICK EXAMPLE
WORKED EXAMPLE · 3/4 = x/20
x = 15

The denominator is ×5. The numerator is, too.

  1. Cross multiply: 3 × 20 = 4 × x.
  2. Divide both sides by 4: x = 60 ÷ 4.
  3. Check: 3/4 = 15/20. Both simplify to 3/4.
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KEEP EXPLORING

Small steps.
Big ideas.

  1. 01Solving proportions
  2. 02Formula reference
  3. 03Fractions & mixed numbers
  4. 04Decimal proportions
  5. 05Equivalent ratios
  6. 06Word problems
  7. 07Recipe scaling

What is a proportion?

A proportion is an equation stating that two ratios are equal. For example, 3/4 = 15/20 because each side represents the same fraction, even though the written numbers differ.

a/b = c/d → a × d = b × c

Both denominators must be nonzero. Enter any three values to solve for the fourth, or enter all four to check equality. This is an equal-ratio calculator, not a statistical proportion estimator.

Students can check homework and practice algebra. Teachers can print worked examples. Cooks and makers can use it for scaling, provided the quantities really are proportional.

A missing denominator?

The unknown does not have to be on top. Consider 5/8 = 15/x.

  1. Cross multiply: 5x = 8 × 15.
  2. Simplify: 5x = 120.
  3. Divide by 5: x = 24.
  4. Verify: 15/24 reduces to 5/8.

More examples: all four formula arrangements, fractional inputs, and word problems.

Questions & answers

How do I use this proportion calculator?

Enter three values and leave the unknown box empty. Choose Solve proportion to find the missing value. Fill all four boxes to check whether the two ratios are equal.

Can I enter fractions or mixed numbers?

Yes. Use 3/4 for a fraction or 1 1/2 for a mixed number. Negative numbers and finite decimals are supported. Use a period for decimals and do not include units or commas.

Why is the answer a fraction?

The main answer is exact. A decimal is also displayed to up to eight decimal places. Keep the fraction if you need to use the result in another calculation.

Can a denominator be zero?

No. The bottom value in either ratio must be nonzero. Some equations have no solution or infinitely many valid solutions; the calculator identifies these cases.

Is this a statistical sample proportion calculator?

No. This tool solves equal-ratio equations such as a/b = c/d. It does not estimate population proportions, confidence intervals, or sample sizes.

Do I need an account?

No. Calculations run in your browser, with no account or subscription. Your entries are not sent to a calculation server.