Compound Interest Calculator
Grow a lump sum with optional regular contributions at annual, monthly, or daily compounding.
Written by Golam Rabbani, Founder & Lead Engineer
How to use this compound interest calculator
- Enter the principal (initial amount) and pick a currency.
- Type the annual interest rate as a percent and the number of years.
- Pick the compounding frequency: annual, semi-annual, quarterly, monthly, or daily.
- Optionally add a recurring contribution, its frequency, and whether it lands at the start or end of each period.
- Press Calculate to see the future value, total contributed, and interest earned.
- Use Copy to share the summary or Reset to model a different scenario.
About this compound interest calculator
The compound interest calculator implements the standard time-value-of-money formula A = P(1 + r/n)^(nt), 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 time in years. When you add a recurring contribution PMT, the calculator also adds the future value of an annuity: FV_annuity = PMT × ((1 + i)^N − 1) ÷ i, with i = r/n_contrib and N = n_contrib × t; multiplying by (1 + i) converts an ordinary annuity into an annuity-due if contributions land at the start of the period. These formulas are the ones taught in CFA Institute curriculum (Quantitative Methods: Time Value of Money) and described in the U.S. SEC's Investor.gov compound-interest explainer.
Worked example: principal USD 5,000, annual rate 6%, 10 years, monthly compounding (n = 12), with USD 200 contributed at the end of each month. Lump-sum portion: 5,000 × (1 + 0.06/12)^(12·10) ≈ USD 9,096.98. Annuity portion: 200 × ((1.005)^120 − 1) ÷ 0.005 ≈ USD 32,775.87. Future value ≈ USD 41,872.85; total contributed = 5,000 + 200·120 = USD 29,000; interest earned ≈ USD 12,872.85. Currency is a label only — there is no exchange-rate conversion.
FAQ
- What is the compound interest formula?
- A = P(1 + r/n)^(nt). P is the principal, r is the annual rate as a decimal, n is compounding periods per year, t is time in years. The calculator also adds a future-value-of-annuity term if you contribute regularly.
- Does more frequent compounding really make a difference?
- It increases the final balance, but the effect plateaus quickly. At 6% over 10 years, switching from annual to monthly compounding changes the result by less than 2%; switching from monthly to daily barely registers.
- What is an annuity-due?
- An annuity-due deposits each contribution at the START of the period instead of the end, so every payment earns interest for one extra period. The calculator multiplies the ordinary-annuity formula by (1 + i) to switch modes.
- Why is "interest earned" lower than I expected?
- Interest earned = future value − total contributed. With heavy regular contributions, total contributed grows fast and interest looks smaller as a share of the final balance, even though the dollar interest is large.
- Does this account for inflation or taxes?
- No — it is a pure nominal projection. Subtract inflation from the rate to get a real-terms projection; the tool does not deduct income tax on interest.
- Is the calculation private?
- Yes. Everything is computed in your browser; no data is uploaded.