Compound Interest Calculator
See what a starting balance grows to over time, with or without regular contributions, and how much of the final figure is interest rather than money you paid in.
Contributions are added at the end of each period.
| Year | Paid in | Interest | Balance |
|---|---|---|---|
| 1 | $0.00 | $0.00 | $0.00 |
| 2 | $0.00 | $0.00 | $0.00 |
| 3 | $0.00 | $0.00 | $0.00 |
| 4 | $0.00 | $0.00 | $0.00 |
| 5 | $0.00 | $0.00 | $0.00 |
| 6 | $0.00 | $0.00 | $0.00 |
| 7 | $0.00 | $0.00 | $0.00 |
| 8 | $0.00 | $0.00 | $0.00 |
| 9 | $0.00 | $0.00 | $0.00 |
| 10 | $0.00 | $0.00 | $0.00 |
A fixed rate is assumed throughout. Real savings rates move, and investment returns vary year to year — treat this as a projection, not a promise.
How to use this calculator
- Enter your starting balance. Leave it at zero if you are starting from nothing and only want to model contributions.
- Enter the annual interest rate your account or investment is expected to earn, and how many years it has to grow.
- Choose how often it compounds. Savings accounts are usually monthly; some bonds and funds are quarterly or annual.
- Add a regular contribution if you pay in steadily. The field label follows the compounding frequency you picked, so the two always match.
- Read the year-by-year table to see the point where interest starts outpacing what you are paying in.
The formula
Compound growth on a starting balance:
A = P × (1 + r ÷ n)^(n × t)
A = final amount
P = starting balance
r = annual rate as a decimal
n = compounding periods per year
t = years
Regular contributions add the future value of an ordinary annuity on top:
A = PMT × ((1 + i)^N − 1) ÷ i
PMT = contribution each period
i = r ÷ n, the rate for one period
N = n × t, the number of periods
The effective annual rate converts any compounding schedule into a single comparable number:
effective rate = (1 + r ÷ n)^n − 1
The calculator steps through one period at a time rather than applying the closed forms in one go. The answers are the same, but stepping keeps the year-by-year table exactly consistent with the headline total instead of drifting by a cent or two.
Worked examples
$1,000 at 5% for 10 years
- Compounded annually: 1,000 × 1.05¹⁰ = $1,628.89
- Compounded monthly: 1,000 × (1 + 0.05 ÷ 12)¹²⁰ = $1,647.01
- Simple interest would have paid only $500, against $628.89 and $647.01
$100 a year for 10 years at 5%
Starting from nothing, paying in at the end of each year.
- Future value: 100 × ((1.05¹⁰ − 1) ÷ 0.05) = $1,257.79
- You paid in $1,000, so $257.79 is interest
- The first deposit compounds for 9 years; the last one earns nothing at all
Why starting early beats saving more
Two people each pay $200 a month into an account earning 7%, compounded monthly.
- One starts at 25 and stops at 35 — ten years of deposits, then leaves it alone
- The other starts at 35 and pays in every month until 65 — thirty years of deposits
- The first pays in $24,000 and the second $72,000, yet by 65 their balances are remarkably close. The extra twenty years of compounding does what the extra $48,000 could not.
Frequently asked questions
Simple interest is only ever charged on the original amount. Compound interest is charged on the balance, which includes the interest already added — so the interest itself starts earning interest.
Over $1,000 at 5% for 10 years, simple interest pays $500. Compounded annually it pays $628.89. Over 30 years the gap widens to $1,500 against $3,322.
A little, and it matters more at higher rates. $1,000 at 5% for 10 years grows to $1,628.89 compounded annually and $1,647.01 compounded monthly — about $18 apart.
At 12% for a year, annual compounding earns 12% while monthly compounding earns 12.68%. That second figure is the effective annual rate, which is what the calculator shows so you can compare accounts quoting different compounding schedules.
At the end of each compounding period, which is the standard "ordinary annuity" assumption. A deposit made at the end of a period earns nothing during that period.
If you actually pay in at the start of each period, your real balance will be slightly higher than the projection — by roughly one period's interest on each contribution.
Because mixing them quietly introduces error. Monthly deposits into an account that compounds annually need a different formula, and using the simple one anyway produces a number that looks precise but is wrong.
Picking one frequency for both keeps every figure on this page exactly right. If your account genuinely mixes the two, treat the result as a close estimate.
A mental shortcut: divide 72 by the annual rate to estimate the years it takes to double your money. At 6% that is about 12 years, at 8% about 9 years.
It is an approximation that works best between roughly 5% and 12%. The calculator shows it as a sanity check alongside the exact figure.
No. The figures are nominal, meaning they ignore inflation, tax on interest or gains, and any account or fund fees. All three reduce what the money is actually worth to you.
A rough way to see the real picture is to subtract expected inflation from the rate before entering it: 7% growth with 3% inflation behaves like about 4% in today's money.