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Work out simple or compound interest on a principal, with the formula shown.
Each period also earns interest on the interest already added: A = P(1 + r/n)^(nt).
Choose Simple interest or Compound interest, then enter the principal, annual percentage rate, and time in years. For compound interest, also enter a whole-number compounding frequency from 1 to 365 and press Calculate to see the total amount and interest separately.
Simple interest uses only the starting principal: interest = principal × (annual rate ÷ 100) × years. Compound interest adds interest to the balance each period: amount = principal × (1 + annual rate ÷ 100 ÷ periods per year)^(periods per year × years). Interest earned is the final amount minus the principal; this model assumes a constant rate and no deposits, withdrawals, fees, or tax.
Choose Simple interest with principal 1000, rate 5%, and time 3 years. Interest = 1000 × 0.05 × 3 = 150; total amount = 1000 + 150 = 1150.
Choose Compound interest with 1000, 5%, 3 years, and 1 compound per year. Amount = 1000 × 1.05³ = 1157.625; interest = 157.625. Round to 1157.63 and 157.63 if reporting money to the nearest cent.
Simple interest applies the rate to the original principal only. Compound interest also applies it to previously added interest; with a positive rate, that makes the same balance grow faster over time.
Enter 12. Annual compounding is 1, quarterly is 4, and daily is 365. Time is still entered in years: six months is 0.5 years.
The compound formula expects a nominal annual rate divided across the chosen periods, not an already-compounded APY. If you use an effective annual yield as an annual growth rate, choose 1 compound per year.
No. This tool models one starting balance, not amortization or contributions. Values are currency-neutral and are not automatically rounded to cents. For typical savings estimates use a nonnegative rate; unusual negative rates can produce results outside this model’s useful range.
Use the Percentage Calculator for a single increase or discount, and the General Calculator for custom expressions. The percentages lesson explains the rate arithmetic behind both interest models. Related tools: Percentage Calculator, General Calculator. Read the Percentages lesson or practice percentages to build confidence without relying on a calculator.