Compound Interest Calculator
Investment Growth Over Time
Compound interest is the engine behind long-term investing: each period your balance earns a return, and that return is added to the balance so it earns its own return next time. This calculator combines a starting lump sum with recurring monthly contributions and grows both at your chosen rate and compounding frequency, separating how much you put in from how much the market growth added. An area chart traces total value against cumulative contributions across the full investment horizon.
Formula
FV = P(1 + i/n)^(n·t) + C · ((1 + i/n)^(n·t) − 1) / (i/n)
- FV
- Future value of the investment
- P
- Initial principal (starting lump sum)
- i
- Annual interest rate as a decimal
- n
- Compounding periods per year (1, 2, 4, or 12)
- t
- Number of years invested
- C
- Contribution per compounding period = monthly amount × 12 ÷ n
How it works
- Enter your initial investment and a fixed monthly contribution. The lump sum grows on its own while the contributions are converted internally into per-period deposits based on the compounding frequency.
- Set the annual interest rate and the investment period in whole years.
- Choose how often interest compounds: annually, semi-annually, quarterly, or monthly. The results show future value, total contributions, and total interest earned, with the growth chart updating live.
Worked example
A $10,000 starting balance plus $200 per month, earning 6% per year compounded monthly for 10 years.
- Periodic rate i/n = 0.06 ÷ 12 = 0.005; total periods n·t = 12 × 10 = 120.
- Contribution per period C = 200 × 12 ÷ 12 = $200.
- Lump-sum growth: 10,000 × 1.005^120 = $18,193.97.
- Contribution growth: 200 × (1.005^120 − 1) ÷ 0.005 = $32,775.87.
- Future value = 18,193.97 + 32,775.87 = $50,969.84.
Future value is about $50,969.84 from $34,000 of total contributions, meaning roughly $16,969.84 was earned as compound interest.
Frequently asked questions
- What is the difference between total contributions and total interest?
- Total contributions is the money you personally put in: the initial investment plus every monthly deposit. Total interest is the extra growth the calculator computes by subtracting your contributions from the final future value.
- Does compounding frequency really change the outcome?
- Yes, but usually modestly at typical rates. More frequent compounding lets interest start earning interest sooner, so monthly compounding produces a slightly higher future value than annual compounding for the same rate and term.
- Are the monthly contributions added before or alongside compounding?
- The calculator converts your monthly amount into a deposit for each compounding period and grows it with the annuity portion of the formula. Each contribution then compounds for the remaining time until the end of the term.
- What happens if the interest rate is zero?
- With a 0% rate there is no growth, so the future value simply equals your total contributions and the total interest earned is zero.