Try a worked example
Choose a situation to fill the fields and solve it. The note shows how its units become A/B = C/D.
Select an example to see its setup and units.
Proportion Calculator: Solve for X with Steps
Enter the proportion
X · Missing
May be zero.
X · Missing
Cannot be zero.
X · Missing
May be zero.
X · Missing
Cannot be zero.
Supported: integers, signed values, decimals, fractions (3/4), and scientific notation (1.2e3). Use one slash at most.
Privacy: Everything runs in your browser. Nothing is uploaded.
Result
Ready to calculate
Enter values and choose Solve or Check.
A/B = C/D
- Ratio decimals:
- —
- Simplified ratios:
- —
- Cross-product A × D:
- —
- Cross-product B × C:
- —
- Verification:
- —
When to use this calculator
Use a proportion when two ratios should be equal: scale a recipe, read a map, resize a design, compare similar shapes, or convert a percentage. A ratio compares two quantities; a proportion states that two ratios are equivalent. For example, 3/4 is a ratio, while 3/4 = 6/8 is a proportion.
Equivalent, direct, and inverse relationships
Equivalent ratios multiply or divide both terms by the same nonzero scale factor. In a direct proportion, the quantities scale together and their ratio remains constant, so A/B = C/D fits. In an inverse proportion, one rises as the other falls and their product remains constant; set that relationship up as xy = k instead of using this calculator.
Common mistakes and quick checks
- Keep corresponding quantities and units in the same numerator or denominator position.
- Multiply diagonally: A × D and B × C, not A × C and B × D.
- Never put zero in B or D. Division by zero is undefined.
- Estimate first. If a recipe grows 2.5 times, the ingredient amount should also grow about 2.5 times.
- Substitute the answer and check that both cross-products agree.
All four proportion formulas
Starting from A/B = C/D, cross multiplication gives A × D = B × C. In every completed proportion, B ≠ 0 and D ≠ 0. These rearrangements solve each position:
A = (B × C) / DRequires D ≠ 0.
B = (A × D) / CRequires C ≠ 0 and the result B ≠ 0.
C = (A × D) / BRequires B ≠ 0.
D = (B × C) / ARequires A ≠ 0 and the result D ≠ 0.
If the divisor is zero, the original equation may have no solution or many solutions rather than one missing value.
How to set up a proportion word problem
- Identify the two corresponding kinds of quantity.
- Choose an order and keep it on both sides—for example, servings over cups.
- Convert to matching units before entering values.
- Write the known pair as A/B and its corresponding pair as C/D.
- Estimate the scale, solve, then verify the cross-products.
Recipe example
A recipe uses 2 cups of flour for 4 servings. For 10 servings, keep servings/cups in the same order: 4 servings / 2 cups = 10 servings / x cups. Cross-products give 4x = 20, so x = 5 cups.
Map example
A map scale says 1 cm represents 500 m. A 3 cm route uses map centimetres/real metres: 1 cm / 500 m = 3 cm / x m. Cross-products give x = 1,500, so the route is 1,500 m (1.5 km).
Calculation method and review
The calculator parses each entered integer, decimal, fraction, or scientific-notation value as an exact reduced fraction. It applies A × D = B × C, simplifies with integer greatest-common-divisor arithmetic, and creates a rounded decimal string only for the labeled approximation. Completed proportions are compared exactly and also receive a relative-tolerance check for rounded inputs across very large or small scales.
Method reference: OpenStax, “Solve Proportions and their Applications”.
Authored by: Starlight Robotics · Last reviewed: 17 July 2026
Representative calculation tests cover every missing position, complete-ratio checks, exact fractions, decimals, negative values, zero numerators, rejected zero denominators, malformed entries, scientific notation, and large numbers.
Proportion FAQs
How do I solve a proportion for x?
Put x in the missing position, enter the other three values in the same order, and use cross multiplication. For A/B = C/D, the cross-products are A × D and B × C; isolate the missing term by dividing by its coefficient.
Can I enter all four values to check a proportion?
Yes. Select Check four values, complete A, B, C, and D, and the calculator compares both ratios and the cross-products A × D and B × C.
Can I use fractions, decimals, or scientific notation?
Yes. The calculator accepts integers, signed values, decimals, fractions such as 3/4, and scientific notation such as 1.2e3. It keeps exact fractional results when possible and shows a decimal approximation separately.
Can proportion values be zero or negative?
A and C may be zero, and negative values are allowed. B and D cannot be zero because division by zero is undefined. Some zero combinations do not have a unique missing value, and the calculator explains those cases.
What is the difference between a ratio and a proportion?
A ratio compares two quantities, such as 3/4. A proportion is an equation stating that two ratios are equivalent, such as 3/4 = 6/8.
What is the difference between direct and inverse proportion?
In a direct proportion, quantities scale by the same factor and their ratio stays constant. In an inverse proportion, one quantity decreases as the other increases and their product stays constant. This calculator solves direct proportions in A/B = C/D form.
What are common cross-multiplication mistakes?
Common mistakes include reversing one ratio, mixing units between numerator and denominator positions, multiplying straight across instead of diagonally, dividing by the wrong coefficient, and allowing a zero denominator.
Is my proportion data private?
Yes. Calculations run in your browser; values are not uploaded or stored by this tool.