Compound Interest Calculator
Compound interest lets your savings earn interest on interest. Enter a starting balance and an optional regular contribution, and this calculator shows the future value, total interest earned, a year-by-year table and a chart.
Compound interest calculator
Illustrative only, not financial advice. Assumes a constant interest rate for the whole period; actual returns are not guaranteed and will vary.
Contributions vs. interest, year by year
Year-by-year breakdown
| Year | Deposits | Interest | Balance |
|---|
How to use the compound interest calculator
- Enter your initial deposit and, if you have one, a regular contribution amount.
- Choose how often you contribute (monthly or yearly) and whether deposits happen at the start or end of each period.
- Set the annual interest rate and how often it compounds (annually, quarterly, monthly or daily).
- Enter the number of years you plan to save and pick a currency symbol.
- Optionally add an expected inflation rate to see the future value in today's money.
- Results update instantly: future value, total contributions, interest earned and effective annual rate (APY), plus a year-by-year table and chart.
How the calculation works
The calculator treats your money as two parts that grow the same way but on different schedules: the lump sum you start with, and the stream of regular contributions.
Lump sum. An initial deposit P grows to a future value
A after t years at a nominal annual rate r
compounded n times per year:
A = P × (1 + r/n)n×t
Effective annual rate (APY). Compounding more often than once a year
makes the real annual return higher than the nominal rate:
APY = (1 + r/n)n − 1. A nominal 5% rate compounded monthly
(n = 12) has an APY of 5.116%, because interest earned during the year starts earning
its own interest before the year is over.
Regular contributions. Each deposit compounds for a different length
of time, so the total is a geometric series (an annuity). If contributions happen
f times a year, the calculator first converts the nominal rate into the
matching periodic rate — i = (1 + r/n)n/f − 1, the equivalent
periodic rate method, so a monthly deposit under quarterly compounding compounds at the
true monthly-equivalent of your quarterly rate, not an approximation. The future value
of m = f×t contributions of size c is
FV = c × [(1 + i)m − 1] / i, times (1 + i) if each
contribution lands at the start of a period rather than the end, since it then has one
extra period to grow. The calculator adds this to the lump-sum result for the total
future value; at 0% the total is simply everything you put in. The year-by-year table
and chart use the same two formulas, so contributions plus interest always add up to
the running balance. Calculations run entirely in your browser; nothing is sent to a
server.
Examples
Example 1 — lump sum only. $10,000 deposited once, no further contributions, at a 5% annual rate compounded annually for 10 years grows to $16,288.95 — $6,288.95 of that is interest.
Example 2 — starting from zero. $0 to start, contributing $100 at the end of every month, at a 6% annual rate compounded monthly, for 10 years: the balance reaches $16,387.93. You will have deposited $12,000 in total, so $4,387.93 came from interest.
Example 3 — deposit plus contributions, with inflation. $5,000 to start, plus $200 added at the end of every month, at a 7% annual rate compounded quarterly, over 20 years: the future value is $123,707.21 on $53,000 of contributions ($70,707.21 in interest, an effective annual rate of 7.186%). If inflation averages 3% a year over those 20 years, that $123,707.21 would buy about what $68,493.69 buys today — nominal and real, inflation-adjusted returns are different questions.
Compounding frequency, time and inflation
Why compounding frequency matters
More frequent compounding produces a higher balance for the same nominal rate, because interest starts earning its own interest sooner. $10,000 at a 5% nominal rate for 10 years, no further contributions:
| Compounding | Future value |
|---|---|
| Annually | $16,288.95 |
| Quarterly | $16,436.19 |
| Monthly | $16,470.09 |
| Daily | $16,486.65 |
The gap shrinks as compounding gets more frequent — annual to monthly adds $181.14, but monthly to daily adds only $16.56 more, because each extra period spreads the same rate over a shorter subperiod. Advertised rates should always state the compounding frequency; two accounts quoting "5%" are not necessarily paying the same amount.
The rule of 72
A quick mental estimate for doubling time: divide 72 by the annual rate in percent. At 6% a year, 72 ÷ 6 = 12 years to roughly double — the actual figure, compounding annually, is 2.0122× your starting balance after 12 years, close enough as an estimate, though it gets less accurate at very low or very high rates.
Time matters more than the rate or the amount
Because interest compounds, starting earlier usually beats contributing more later. Saving $200 a month for 40 years at 7% (monthly compounding) grows to $524,962.68 from $96,000 of contributions. Waiting 10 years and then contributing twice as much — $400 a month for 30 years at the same rate — reaches only $487,988.40 from $144,000 of contributions: more money in, for a smaller result. The extra decade of compounding outweighed the extra $48,000 of deposits.
Nominal returns, real returns and no guarantees
The rate you enter is nominal; if prices rise over the same period, part of that growth
just offsets inflation. The optional inflation field converts the future value into
today's purchasing power by dividing it by (1 + inflation)t (see
Example 3 above). This calculator assumes a constant rate for the whole period — real
rates on savings accounts, CDs and investments change over time and are never
guaranteed, and past performance never guarantees future returns.
Frequently asked questions
What's the difference between compound interest and simple interest?
Simple interest is earned only on your original principal, so it grows in a straight line. Compound interest is earned on the principal plus all interest already added, so growth accelerates — the more often it compounds, the faster.
Which compounding frequency should I choose if my bank doesn't say?
Check the account's disclosure statement. In the US, banks must state the Annual Percentage Yield (APY), which already reflects the compounding frequency, so you can compare accounts by APY instead of guessing the schedule.
Should contributions be at the beginning or end of the period?
End of period is the more common assumption and matches how most automatic transfers land relative to when interest is calculated. Use beginning of period if your deposit posts first — it gives every contribution one extra period to grow, so the total is slightly higher.
Does this calculator account for taxes or fees?
No. It shows gross growth based only on the rate, contributions and time you enter. Taxes, account fees and any employer match are not included, since they vary by country, account type and provider.
Is the future value shown guaranteed?
No. It is a projection based on a constant rate you supply. Real interest rates and investment returns fluctuate, and past results never guarantee future performance.
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