About this calculator
Leave D empty to solve for it, or fill all four to check.
How to use it
- Enter A, B and C.
- Leave D empty to solve, or enter it to check.
- Read the result.
The formula
A ÷ B = C ÷ D, so D = B × C ÷ A
- cross products
- A × D and B × C are equal in a proportion
Worked example
3 pens cost ₹4… or 3 : 4 = 15 : D
- D = 4 × 15 ÷ 3 = 20.
- So 3 : 4 = 15 : 20.
What the result means
For inverse proportion (more workers, fewer days), multiply instead: A × B = C × D.
Assumptions
- Direct proportion.
Limitations
- A cannot be zero when solving for D.
Frequently asked questions
What is the rule of three?
Solving a proportion by cross multiplication.
How do I check a proportion?
The two cross products must be equal.
What is inverse proportion?
When one quantity rises as the other falls, so their product stays the same.
Last reviewed on 3 October 2026. Found a mistake? Tell us.