Compound Interest Calculator

$
%

Contributions and compounding frequency are in the inspector.

Try a scenario
Projection ready

Projected future balance

$222,711.72

Starting with $10,000.00 and adding $350.00 each compounding period produces this estimate.

7% nominal rate · 12 periods/year · contributions at period end

Balance over time

The curve separates the slow early years from the point where compounding does more of the work.

Interest earned

$128,711.72

57.8% of the ending balance

Money added

$94,000.00

Principal plus recurring contributions

Effective annual rate

7.23%

After compounding frequency

Year-by-year growth

What each year contributes to the total: money you added, interest the balance earned, and the closing figure the next year compounds on.

Year-by-year growth — 20 years · $128,711.72 of the ending balance is interest
YearOpeningbalanceAddedthis yearInterestthis yearClosingbalance
Year 1$10,000.00$4,200.00$860.31$15,060.31
Year 2$15,060.31$4,200.00$1,226.12$20,486.42
Year 3$20,486.42$4,200.00$1,618.37$26,304.79
Year 4$26,304.79$4,200.00$2,038.98$32,543.77
Year 5$32,543.77$4,200.00$2,490.00$39,233.77
Year 6$39,233.77$4,200.00$2,973.62$46,407.39
Year 7$46,407.39$4,200.00$3,492.20$54,099.58
Year 8$54,099.58$4,200.00$4,048.27$62,347.85
Year 9$62,347.85$4,200.00$4,644.54$71,192.39
Year 10$71,192.39$4,200.00$5,283.91$80,676.30
Year 11$80,676.30$4,200.00$5,969.50$90,845.80
Year 12$90,845.80$4,200.00$6,704.65$101,750.45
Year 13$101,750.45$4,200.00$7,492.95$113,443.41
Year 14$113,443.41$4,200.00$8,338.24$125,981.64
Year 15$125,981.64$4,200.00$9,244.63$139,426.27
Year 16$139,426.27$4,200.00$10,216.54$153,842.81
Year 17$153,842.81$4,200.00$11,258.71$169,301.53
Year 18$169,301.53$4,200.00$12,376.23$185,877.75
Year 19$185,877.75$4,200.00$13,574.52$203,652.28
Year 20$203,652.28$4,200.00$14,859.44$222,711.72

20 years · $128,711.72 of the ending balance is interest

Starting principal

$10,000.00

Contribution

$350.00

Waiting one year costs

$19,059.44

Growth overtakes deposits

Year 9

Doubles in year 2

Finance

Back to the tool

Compound Interest Calculator

Project how a starting balance and regular contributions could grow under a fixed return. Compare contribution timing, nominal and effective rates, total money added, interest earned, and the year-by-year curve behind the final balance.

Compounding periods
1–365
Contribution timing
Start/end
Effective rate shown
APY
Growth schedule
Yearly

Step by step

How to use it

  1. 01

    01Enter the starting principal

    Use the amount available at the beginning of the projection.

  2. 02

    02Set rate, frequency, and time

    The annual rate is treated as nominal and divided across the selected compounding periods.

  3. 03

    03Add recurring contributions

    Choose whether each deposit arrives before or after interest for that period.

  4. 04

    04Separate deposits from growth

    Use the schedule to see how much of the ending balance came from your money and how much from the assumed return.

Worked example

$10,000 growing for 20 years

Given

Starting balance
$10,000
Annual rate
7% nominal
Monthly deposit
$350 at period end

Monthly recurrence

monthly rate = 0.07 / 12
next balance = balance × (1 + rate) + 350
repeat for 240 months
Money added
$94,000
Ending balance
≈ $237,000
Growth
≈ $143,000

The result is a projection from a smooth fixed rate. Real savings rates and market returns change over time, so use several rates rather than treating one curve as a forecast.

Why this one

What compounding can—and cannot—tell you

Time changes the source of growth

Early balances are driven mainly by deposits. Later, returns apply to prior returns as well as principal.

Nominal rate and APY differ

More frequent compounding raises the effective annual yield when the quoted nominal rate stays fixed.

Earlier contributions get one more period

A deposit at the start of a period earns that period’s return; an end-of-period deposit does not.

A fixed return hides volatility

Two paths with the same average can finish differently when withdrawals or contributions occur along the way.

The judgement call

Which assumption belongs in the calculator?

  • Bank account quoting APY

    Use the APY carefully

    Do not compound an already-effective rate again as though it were nominal.

  • Regular deposit made on payday

    Start of period

    The deposit is present for that period’s growth.

  • Deposit made after interest posts

    End of period

    It begins earning in the following period.

  • Long-term market investment

    Test a range

    A single fixed return is an assumption, not a promise.

Reference

The numbers behind it

Periodic rate
Annual nominal rate / periods per year
Effective annual rate
(1 + r/n)ⁿ − 1
Compound growth
Principal × (1 + r/n)ⁿᵗ
Projection
No taxes, fees, or changing returns

FAQ

Questions, answered plainly

What is compound interest?

It is interest calculated on the starting principal plus interest already credited, so the amount earning a return can grow each period.

How often should interest be compounded?

Use the frequency stated by the account or scenario. Monthly means 12 periods per year; daily commonly means 365.

What is the difference between APR and APY?

APR is usually a nominal annual rate before intra-year compounding. APY is the effective one-year result after compounding.

Do contributions compound too?

Yes. Once added, each contribution becomes part of the balance and earns returns in later periods.

Is the final balance guaranteed?

No. It is the mathematical result of the inputs. Actual rates, returns, fees, taxes, and deposit timing can change the outcome.

Inputs and projections remain in your browser. Results are illustrations based on fixed assumptions, not financial advice or guaranteed returns.

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