Rule of three calculator
Three values in, the fourth out — with the working shown.
Your files stay on your device
This tool runs entirely in your browser. What you paste is processed on your device and is never sent to a server, so it is safe to use with tokens, keys and unpublished text.
How it works
- Enter A and the value B that goes with it (e.g. 4 kg cost 10).
- Enter the new value C (e.g. 6 kg).
- Choose direct if the two grow together, inverse if one grows as the other shrinks.
- X appears with the cross-multiplication and a check.
Frequently asked questions
- How does the rule of three work?
- A is to B as C is to X, so X = B × C ÷ A: if 4 kg cost 10, 6 kg cost 10 × 6 ÷ 4 = 15.
- When is it inverse?
- When one quantity grows as the other shrinks: 6 workers take 12 days, so 9 take 6 × 12 ÷ 9 = 8. Then X = A × B ÷ C.
- Is anything uploaded?
- No. It is worked out in your browser.
Related tools
％ Percentage and discount calculator Discounts, markups, shares and changes, typed the way you would say them. 日 Prorated rent calculator Part-month rent or pay, worked out line by line, the way your lease or contract counts days. ㎡ Tsubo, m² and tatami converter Japanese floor areas made clear: tsubo, jō and m², with totals and the price per tsubo. % VAT calculator VAT added or taken out, for one price or a whole list.