Significant figures express the precision of a measurement.
Rules: leading zeros don't count, trailing zeros after decimal do.
Use this calculator whenever mental arithmetic introduces uncertainty — particularly when the result will inform a decision rather than just satisfy curiosity. Even confident mathematicians use calculation tools when precision matters.
Many errors with this type of calculation stem from unit inconsistency: mixing metric and imperial, or mixing annual and monthly rates. Ensure all inputs use the same unit system before running the calculation.
A shopper sees two versions of the same product at different prices: one at £14.50 for 400g and another at £11.99 for 300g. Running both price-per-gram calculations reveals the larger pack is 7% cheaper per unit — the kind of comparison that is impossible to do reliably in your head at the shelf.
Three: 4, 5, and the trailing 0.