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.
a / b = c / x → x = b × c / a
Worked example
Show example inputs and result
a — first value: 3 · b — corresponding value: 5 · c — new first value: 9.
- Missing value (x)
- 15
Questions and answers
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.
Method reviewed: October 8, 2026 · Methods & accuracy