Proportion calculator

Solve direct and inverse proportions using the rule of three.

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Set your data

Value a

Value b

Value c

Relationship

For a : b = c : x choose Direct. For inverse relationships use a × b = c × x.

Result

Enter your data and run the tool. Your result will appear here.

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How to use the tool

Fill in these fields: Value a, Value b, Value c, Relationship.
Select “Calculate” to obtain the result.
For a : b = c : x choose Direct.
For inverse relationships use a × b = c × x.

Method and principles

Formula

x = \frac{b \times c}{a}

Method, examples and limits

The direct formula is x = b × c ÷ a: if 2 items cost 10, then 5 cost 25. The inverse formula is x = a × b ÷ c: 2 people working 10 hours correspond to 5 people working 4 hours, assuming equal productivity. The divisor must be nonzero. Use a dot or comma for decimals, without thousands separators.

For a direct proportion, enter three values using consistent units. If 3 items cost 12 euros, 5 items cost 20 euros: use a=3, b=12 and c=5. Choose inverse proportion when increasing one quantity decreases the other. Decide which relationship describes your problem before using the answer.

Practical example

Input

Value a: 2
Value b: 10
Value c: 5
Relationship: Direct · a : b = c : x

Result

Calculation steps

Substitute the example values into the formula.
$x = \frac{10 \times 5}{2}$

Limits and checks

Limits and checks
Parameter	Value
Value a	≥ -1000000000000; ≤ 1000000000000
Value b	≥ -1000000000000; ≤ 1000000000000
Value c	≥ -1000000000000; ≤ 1000000000000

Frequently asked questions

When should I use inverse proportion?

When the product of the two quantities stays constant: the model uses a × b = c × x.

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