Methods and reference

Interpreting calculator results

A calculator is only as reliable as its assumptions and inputs. Match your units, understand whether percentages are applied before or after tax, and distinguish nominal interest rates from real purchasing-power changes. Numerical rounding affects display, and some formulas require positive denominators or specific valid ranges. Where results involve taxes, loans or investments, verify authoritative terms before relying on an estimate.

Each additional tool has its own method and worked example. Browse all tools.

Explore the tools

• Discount plus sales tax — Apply a discount before calculating sales tax.
• Bill and tip split — Divide a restaurant bill including a percentage tip.
• Monthly loan payment — Estimate the fixed monthly principal-and-interest payment on an amortizing loan.
• Savings growth projection — Model monthly deposits with fixed hypothetical interest.
• Percentage change — Compare an earlier value with a new value.