Proportion Calculator
Last updated: November 4, 2026
Solve a / b = x / c for the missing second numerator, showing the ratio used.
On this page
Quick summary: Solve a / b = x / c for the missing second numerator, showing the ratio used.
Read the complete guide
Updated reference sections in this guide are in English.
Using this calculator
Use this form when two ratios are intended to be equal and the missing value is the numerator of the second ratio. Enter a, b and c in the displayed equation a/b = x/c. Denominators must be positive. This fixed arrangement makes the unknown and the reference quantities explicit.
Calculation method
x = (a / b) × c
Dividing a by b gives the quantity per one reference unit. Multiplying that ratio by c scales the quantity to the new reference amount. The calculation assumes direct proportionality: doubling the reference amount doubles the numerator.
Worked examples
Scale a recipe
If 3 cups of an ingredient serve 4 equal portions, 20 portions need 3 ÷ 4 × 20 = 15 cups under the same recipe ratio.
Keep unit order consistent
If a drawing uses 2 cm for 5 m, enter a = 2, b = 5 and c = 12 to find the drawn length for 12 m: 4.8 cm.
Check this scenario in the calculator:
- Known numerator a
- 3
- Known denominator b
- 4
- Second denominator c
- 20
Understanding the result
The output inherits the numerator’s unit. The b and c inputs must share a reference unit, and any required unit conversion must happen before entry. A negative numerator can represent a signed quantity; it does not establish that a real recipe, price or distance can be negative.
Boundaries and assumptions
This is a direct-proportion model, not an inverse-proportion solver or a general equation parser. Quantity discounts, fixed setup charges and curved relationships do not follow a constant ratio. Inputs are limited to magnitudes of 10¹², with positive denominators, and results beyond 10¹⁸ are rejected. A zero numerator produces a zero missing numerator.
Questions and reference
Can I solve for a denominator?
Rearrange your known relationship into the displayed a/b = x/c form first. This form always returns x.
Why does a fixed delivery fee break the model?
A total containing a fixed fee does not scale in direct proportion to quantity.