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Compound & Simple Interest Calculator

Enter your principal, rate and duration to see projected growth, all computed locally, nothing uploaded.

How Compound & Simple Interest Calculator works

Compound interest is the mechanism by which interest earns interest: after each compounding period, the interest already earned is added to the principal and the next period's interest is calculated on the larger total. This calculator projects the future value of a lump sum, optionally supplemented by regular monthly contributions, at any compounding frequency from daily to annual. A simple-interest comparison mode is also included. All the arithmetic runs in your browser, so the figures you enter never leave your device.

Model any scenario: a 30-year retirement account, a 5-year savings plan, or a 36-month loan. The projection is a mathematical estimate that ignores taxes, fees and inflation and is not financial advice. Treat the output as a guide to understand the underlying arithmetic, not a guaranteed outcome.

Written by Bastien Sulyan

How to use Compound & Simple Interest Calculator, step by step

  1. Enter the initial principal (the amount you start with or borrow).
  2. Enter the annual interest rate as a percentage.
  3. Select the compounding frequency: daily, monthly, quarterly or annual.
  4. Enter the term in years.
  5. Optionally add a monthly contribution for recurring deposits.
  6. Read the future value, total contributions and total interest earned from the result panel.

Common use cases

  • Projecting how much a 10,000 deposit grows to over 20 years at 5 percent annual interest compounded monthly, to plan a long-term savings goal.
  • Comparing daily compounding versus annual compounding on the same rate to see how much difference the frequency makes over time.
  • Estimating the total interest cost of a loan with a fixed rate to understand the true cost of borrowing beyond the principal.
  • Illustrating to a teenager how starting to save 100 per month at age 20 versus age 30 changes the outcome at age 65.

Frequently asked questions

Is my data sent to any server?

No. All calculations happen locally in your browser using JavaScript arithmetic. No inputs or results leave your device, and the calculator works offline once the page has loaded.

What is the compound interest formula?

For a lump sum without contributions: FV = P times (1 + r/n) to the power (n times t), where P is the principal, r is the annual rate (as a decimal), n is the number of compounding periods per year, and t is the term in years. Example: 1000 at 6 percent compounded monthly for 5 years: FV = 1000 times (1 + 0.06/12) to the power 60 = 1000 times 1.34885 = 1348.85.

What is the difference between compound and simple interest?

Simple interest is calculated only on the original principal: I = P times r times t. Compound interest is recalculated each period, so prior interest earns further interest. Over long periods the gap is large: 1000 at 6 percent simple interest for 20 years earns 1200 in interest. At 6 percent compounded monthly the same 1000 grows to 3310, earning 2310 in interest.

How are monthly contributions handled in the formula?

Monthly contributions are modelled as a future-value annuity: FV_annuity = PMT times ((1 + r_m) to the power n_months minus 1) divided by r_m, where r_m is the monthly rate and PMT is the monthly payment. The total future value is the lump-sum FV plus the annuity FV. This is an approximation when contributions do not exactly align with the compounding frequency.

Does compounding frequency matter much?

Yes, increasingly so over time. At 10 percent annual rate over 30 years: annual compounding on 1000 gives 17 449; monthly compounding gives 19 837; daily compounding gives 20 085. The difference between monthly and daily is modest, but annual versus monthly is substantial over long horizons.

Is this financial advice?

No. The output is a mathematical projection based on a fixed rate and the inputs you provide. It ignores taxes, fees, inflation, variable rates and any regulatory rules. Consult a qualified financial adviser before making investment or borrowing decisions.

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