How it works
Keep corresponding quantities in the same order. If 3 items cost 5, then 9 items cost 5 × 9 / 3 = 15, assuming a constant unit price.
Inputs and units
| Input | Example value | What to enter |
|---|---|---|
| a — first value | 3 | Enter a numeric value. Must be greater than zero. |
| b — corresponding value | 5 | Enter a numeric value. |
| c — new first value | 9 | Enter a numeric value. |
Worked example
Show example inputs and results
This example is generated by the same calculation engine using the starting values in the form. Your own inputs produce your own result.
- a — first value
- 3
- b — corresponding value
- 5
- c — new first value
- 9
Complete example results
- Missing value (x)
- 15
Reading the result
Use quantities on the same basis. The starting value in percentage change, the denominator in a ratio and the length unit in geometry determine the meaning of the answer. Check the formula conditions alongside the example.
How do I solve for the missing fourth value?
Write the matching quantities as a : b = c : x, then calculate x = b × c / a. If 3 items cost 5, the same unit price gives 6 items a cost of 10 and 9 items a cost of 15. Keep item counts in the first positions and costs in the corresponding second positions.
Direct or inverse proportion?
In a direct proportion, doubling one quantity doubles the other. A fixed task shared by twice as many equally productive workers is an inverse-proportion scenario: the time decreases. Use the inverse proportion calculator for that assumption.
Compare different scenarios
Inputs held constant: a — first value: 3 · b — corresponding value: 5.
| c — new first value | Missing value (x) |
|---|---|
| 6 | 10 |
| 9 | 15 |
| 12 | 20 |
Questions and answers
Can I use the example values for my own calculation?
The example values illustrate the calculation. Replace every input with your own information, especially dates, rates and the selected operation.
Does this work for inverse proportions?
No. This tool assumes direct proportionality. Problems where one quantity increases as another decreases require a different equation.
Why are some results rounded?
Results display up to 8 decimal places. Calculations use the unrounded values; very small values use scientific notation. JavaScript floating-point arithmetic can introduce tiny rounding differences.
Are my inputs sent to a server?
The calculator processes inputs in your browser. Its code does not send your values to a server.