Value D
15
Calculated value that preserves A/B = C/D.
Mathematics · Numbers & everyday maths
Proportion calculator — rule of three and cross multiplication
Choose A, B, C or D as the unknown, then enter the three known values. The diagram substitutes your numbers directly into the two fractions.
Created v1.0
By Thibaut Grzelak, Mechanical Analysis Engineer
03
Results
Results show up to four significant digits for readability; internal calculations retain full precision.
See calculation details
01Value D
Formula
D = B × C / ASubstitution
10 × 6 / 4Result
15
01Formulas and symbols
Formulas used
A
A/B = C/D
A·D = B·C
The two fractions A/B and C/D are equal; cross multiplication gives A·D = B·C.
| Symbol | Meaning |
|---|---|
| A | Value A |
| B | Value B |
| C | Value C |
| D | Value D |
02Validation example
Reference numerical case
- A = 4, B = 10 and C = 6 give D = 15. The inverse modes recover A = 4, B = 10 or C = 6 from the other three values.
03Cross multiplication
The diagram keeps A/B and C/D as stacked fractions. After your three known values are substituted, A·D = B·C isolates the selected unknown directly.
04Limits and conventions
The calculator solves a direct algebraic proportion. It does not decide whether a real-world relationship is actually proportional.
05Common mistakes
Keep corresponding quantities in matching positions: A relates to B as C relates to D. Reversing only one fraction changes the relation.
06Practical questions
Can I choose any value as the unknown?
Yes. Select A, B, C or D; the calculator hides that input and uses the other three values.
Is this the same as the rule of three?
Yes. A/B = C/D gives the cross product A·D = B·C, then the selected letter is isolated.
07Technical references
- Prealgebra 2e — proportions and applications — OpenStax