Compound Interest Calculator
Calculate the future value of an investment from a principal, interest rate, compounding frequency, time, and optional monthly contributions.
How to use the compound interest calculator
- 1
Enter your initial principal and expected annual interest rate.
- 2
Choose a compounding frequency (annually, semiannually, quarterly, monthly, or daily) and a time period in years.
- 3
Optionally add an additional monthly contribution you plan to keep making.
- 4
Your future value, total contributions, total interest earned, and a year-by-year growth table update instantly.
The compound interest formula, explained
Formula: A = P(1 + r/n)nt, where A is the future value, P is the principal, r is the annual interest rate as a decimal, n is the number of compounding periods per year, and t is the time in years.
Worked example: $10,000 invested at 7% annual interest, compounded annually, for 20 years, with no further contributions: A = 10,000 × (1 + 0.07)20 = $38,696.84 — earning $28,696.84 in interest on a $10,000 principal.
Why compound beats simple interest: simple interest on the same $10,000 at 7% for 20 years would only reach $10,000 + (10,000 × 0.07 × 20) = $24,000 — over $14,600 less than the compound result, because simple interest never earns interest on its own interest.
Adding monthly contributions: the same $10,000 principal at 7%, compounded monthly, plus $200 contributed every month for 20 years, grows to $144,572.72 — of which $58,000 came from contributions (principal + deposits) and $86,572.72 was interest earned.
How compounding frequency changes your return
| Compounding frequency | $10,000 at 7% for 20 years |
|---|---|
| Annually | $38,696.84 |
| Monthly | $40,387.39 |
| Daily | $40,546.56 |
Switching from annual to daily compounding adds about $1,850 over 20 years on this example — a real but modest difference compared to the effect of the interest rate itself or the length of time invested.
Why starting early beats contributing more
Because compound growth accelerates the longer money stays invested, an early start can outperform a much larger total contribution made later — this is one of the most counter intuitive and important lessons in personal finance.
Worked example: at 7% annual growth, compounded monthly:
- Person A contributes $300/month for 10 years (ages 25-35), then stops entirely and lets the balance grow untouched for 30 more years. Total contributed: $36,000. Final balance at age 65: $421,452.72.
- Person B waits until age 35 and contributes the same $300/month continuously for 30 years (ages 35-65). Total contributed: $108,000 — three times as much as Person A. Final balance at age 65: $365,991.30.
Despite contributing $72,000 less in total, Person A ends up with about $55,000 more — purely from the extra 10 years those early contributions had to compound. This is the core argument for starting retirement or investment contributions as early as possible, even in small amounts.
Model your own timeline in the calculator above, or compare it against paying down debt instead with our Mortgage & Loan Calculator.
Frequently asked questions
Related guides
The compound interest formula broken down piece by piece, with a full worked example and a direct comparison against simple interest.
Read the guide →How the Rule of 72 works, how accurate it really is compared to the exact compound interest formula, and worked examples at several interest rates.
Read the guide →Why starting to invest early beats contributing more money later, with a full worked comparison of two savers at different starting ages.
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