How to use the interest calculator
- Choose what to calculate. Final balance works out what your money grows to. The other four work backwards from a target balance: the rate needed, the time needed, the starting amount or the deposit needed each period.
- Enter the amount, rate and time. Give the time in years and months, or pick two dates for interest between dates. With dates you can count 365 days a year or the 360-day banker's year some loans and court judgments use.
- Say whether the rate is an APR or an APY. Savings accounts and CDs usually quote an APY, which already includes compounding. Choose how often interest compounds: annually, monthly, daily or continuously.
- Add regular deposits or withdrawals if you have them, how often they happen and whether they come at the start or end of each period.
The results show compound and simple interest side by side, then a chart, a year-by-year or month-by-month schedule you can download as CSV, and the formulas with your numbers filled in. Share result saves every input in the link, so it opens with the same numbers on another device.
Simple interest vs compound interest
Simple interest is paid only on the money you put in: I = P × r × t. 10,000 at 5% a year earns 500 a year, 5,000 over 10 years. Many car loans, bond coupons and court judgments use simple interest.
Compound interest is also paid on the interest already earned: A = P × (1 + r/n)^(n×t), where n is the number of times a year interest is added. The same 10,000 at 5% compounded monthly grows to 16,470.09 in 10 years, 1,470.09 more than simple interest. Savings accounts, CDs and investment returns compound. So do credit card balances, which is why they grow so quickly.
How often interest compounds matters less than people expect. At 5%, compounding yearly gives an APY of 5%, monthly 5.116%, and daily or continuous compounding both about 5.127%. The rate and the time matter far more.
Deposits, withdrawals and the rate per period
Each deposit earns interest from the day it goes in. When deposits and compounding happen on different schedules, for example monthly deposits into an account that compounds daily, the calculator uses the equivalent rate per deposit period, i = (1 + r/n)^(n/m) − 1, a common convention for savings calculators. Regular deposits then grow like an annuity: FV = PMT × ((1 + i)^N − 1) / i, one period more when you deposit at the start of each period.
Withdrawals come out of the balance each period. If they are larger than the interest, the balance shrinks, and the calculator tells you when it runs out. With simple interest, withdrawals come out of the interest earned first, then the principal.
Working backwards: rate, time or deposit needed
The reverse modes answer planning questions. Time needed tells you how long it takes to double your money or reach a goal. The exact doubling time is ln 2 ÷ ln(1 + APY), 11.9 years at 6% compounded yearly, which is where the rule of 72 (72 ÷ 6 = 12) comes from. Deposit needed gives the monthly saving that reaches a target by a date. Each mode also shows what simple interest would need, so you can see what compounding is worth.
Reading the results
- Money put in is the starting amount plus every deposit. Everything above it is interest.
- APY is the yearly rate after compounding:
(1 + r/n)^n − 1. Use it to compare accounts that compound differently. - The schedule shows what was added, the interest earned in each year or month, the running total and both balances.
Rates change. On September 21, 2026 the FDIC's national average was 0.37% APY for savings accounts and 1.73% for 12-month CDs, and rates at individual banks vary widely, so use the rate you are actually offered. Interest is usually taxable income, and inflation lowers what the final balance will buy; the calculator shows the amounts before either.
If you are paid by the hour, the time card calculator works out a week's pay, and the pay period calendar shows every payday of the year, which helps you time regular deposits.
Sources, checked September 2026: SEC Office of Investor Education, Investor.gov compound interest calculator; CFPB, Regulation DD (Truth in Savings), 12 CFR 1030, Appendix A, the annual percentage yield formula; FDIC, National Rates and Rate Caps, rates as of September 21, 2026. This calculator is for planning and learning, not financial advice.