PercentKit

Every percentage, tip, discount, tax and ratio calculation you need — fast, offline, and remembered.

Percentages break intuition in a specific place: they do not commute across sequential changes. A price raised 20 percent then cut 20 percent does not return to where it started — it lands 4 percent lower, because the second percentage applies to a larger base.

The same asymmetry explains why a 50 percent loss needs a 100 percent gain to recover. Tip and tax calculations add their own wrinkle, since applying tax then a tip on the taxed total gives a different answer from tipping on the pre-tax amount, and both conventions exist in practice.