Compound Interest Calculator
Enter a starting balance, monthly contribution, interest rate and term to see your final balance, a year-by-year breakdown, and a growth chart.
Compound interest is what happens when the interest your money earns starts earning interest of its own. This calculator takes a starting balance, an optional monthly contribution, an annual interest rate, and a term in years, then grows the balance using whichever compounding frequency you pick — annually, semi-annually, quarterly, monthly, or daily. Change any field above and the final balance, the year-by-year table, and the growth chart all update immediately, entirely on your device — nothing is sent anywhere.
Worked examples
A retirement contribution left to grow untouched
Someone deposits a bonus into a retirement account and makes no further contributions, letting compound interest do the rest of the work for a decade.
- Principal
- $10,000
- Rate
- 7% / year
- Compounding
- Annually
- Term
- 10 years
≈ $19,671.51 after 10 years — $9,671.51 of that is interest
Building a nest egg $500 a month
A saver starts from zero and commits to a fixed $500 monthly deposit into an account that compounds monthly, tracking how much of the balance ends up being interest.
- Principal
- $0
- Monthly
- $500
- Rate
- 7% / year
- Compounding
- Monthly
- Term
- 10 years
≈ $86,542.40 after 10 years — about $26,542.40 of that is interest
How the formula works
Split the final balance into two pieces that both grow, just differently. The starting principal P compounds on its own: multiply it by (1 + r/n) once for every one of the n×t compounding periods in the term, where r is the annual rate as a decimal and n is how many times per year interest is credited. The monthly contributions PMT form a separate, growing stream of deposits, each one compounding from the month it lands until the end of the term. Adding the two pieces together gives the final balance FV.
The compounding-frequency selector above changes n for the principal piece only. The contribution piece always uses a monthly rate (r/12) over 12×t months, because deposits are inherently monthly — this is the same simplification used by most online compound interest calculators, and the gap it introduces versus a full daily simulation is tiny at typical savings rates.
Quick estimate — the Rule of 72
72 ÷ interest rate ≈ years for money to double, with no further contributions. A quick mental-math shortcut, not a replacement for the exact formula above.
Frequently asked questions
What's the real difference between compound interest and simple interest?
Simple interest only ever pays interest on your original principal, so a balance earning simple interest grows in a straight line. Compound interest pays interest on the principal and on all the interest that has already accumulated, so each period's interest becomes part of the base the next period's interest is calculated from. The two look similar for the first year or two, but the gap widens quickly — over 20-30 years, compound interest can produce a meaningfully larger balance from the same rate and principal.
Why does choosing monthly compounding instead of annual compounding change my final balance at the same interest rate?
The stated annual rate gets divided across however many compounding periods you pick, and interest is credited that many times per year. Compounding monthly credits interest 12 times a year instead of once, so interest starts earning its own interest sooner. At a given nominal annual rate, more frequent compounding always produces a slightly higher effective annual return — the difference is small at everyday rates, but it grows with the interest rate and the number of periods.
Why do my monthly contributions always compound monthly, even when I pick annual or quarterly compounding above?
The compounding-frequency selector controls how interest accrues on your principal balance. Monthly deposits, on the other hand, are inherently monthly events, so this calculator always grows them using a monthly rate (annual rate divided by 12) over the matching number of months — regardless of what you've chosen for the principal. That's the same simplification most online compound interest calculators use, and at typical savings rates the gap between this and a full day-by-day simulation is negligible.
What is the Rule of 72, and how accurate is it?
Dividing 72 by an annual interest rate gives a fast mental-math estimate of how many years it takes an investment to double, with no contributions and no calculator required. It comes from approximating the exact formula, ln(2) divided by ln(1 + r), which is more accurate but harder to do in your head. The Rule of 72 is closest in the 6-10% range and drifts further off at very low or very high rates — use the calculator above for anything that needs to be precise.
Does picking a different currency symbol convert my numbers?
No — the currency selector only changes which symbol is shown in front of your numbers; it doesn't apply an exchange rate or convert anything. Compound interest math doesn't care what currency you're in, so just enter your principal and monthly contribution already in whichever currency you're planning in, and pick the matching symbol for display.
Does this calculator assume my contributions happen at the start or end of each month?
End of month. That means a deposit made in a given month hasn't earned any interest yet by the time that month closes — it starts compounding the following period. This is the more common, slightly more conservative convention (sometimes called an "ordinary annuity"); depositing at the start of each month instead (an "annuity due") would produce a marginally higher final balance, since every contribution gets one extra period to grow.