Savings & Banking

Compound Interest Calculator

Compound interest pays you interest on your interest — the engine behind long-term wealth. Enter a starting balance, a monthly contribution, and a rate to see exactly what you'd have in 5, 10, or 40 years, how much of it is your money, and how much the compounding earned for you.

Formula shown below · Tested against worked examplesHow we verify

Starting balance

Monthly contribution

Annual interest rate

Compounded monthly. Long-run US stock returns have averaged ~7% after inflation; savings accounts pay less.

Years to grow

Final balance

$170,619

You put in: $70,000
Compound interest earned: $100,619
Growth multiple: 2.44

Growth by year

Year	Balance	Contributed	Interest earned
1	$13,821	$13,000	$821
2	$17,918	$16,000	$1,918
3	$22,312	$19,000	$3,312
4	$27,023	$22,000	$5,023
5	$32,074	$25,000	$7,074
6	$37,491	$28,000	$9,491
7	$43,300	$31,000	$12,300
8	$49,528	$34,000	$15,528
9	$56,206	$37,000	$19,206
10	$63,368	$40,000	$23,368
11	$71,047	$43,000	$28,047
12	$79,281	$46,000	$33,281
13	$88,110	$49,000	$39,110
14	$97,578	$52,000	$45,578
15	$107,730	$55,000	$52,730
16	$118,616	$58,000	$60,616
17	$130,289	$61,000	$69,289
18	$142,806	$64,000	$78,806
19	$156,227	$67,000	$89,227
20	$170,619	$70,000	$100,619

Compare scenariosTry three values of one input

Vary

Compound Interest Calculator results for three values of Starting balance
Starting balance
Final balance	$166,580	$170,619+$4,039	$174,658+$8,077
You put in	$69,000	$70,000+$1,000	$71,000+$2,000
Compound interest earned	$97,580	$100,619+$3,039	$103,658+$6,077
Growth multiple	2.41	2.44+0.03	2.46+0.05

Every other input stays at the value you set above — currently $10,000 for starting balance. Differences are measured against the first column.

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How this calculator works

The simulation credits interest monthly at rate ÷ 12 on the running balance, then adds your contribution at month's end (an ordinary annuity). This matches the closed-form future-value formula FV = PV(1+r)^n + PMT((1+r)^n − 1)/r within cent-rounding.

All amounts are tracked in exact cents, the way a real account posts interest. Results are estimates for education — actual account terms, fees, and market returns will differ.

Formula

FV = P(1 + r/m)^(m·t) + C × ((1 + r/m)^(m·t) − 1) ÷ (r/m)

FV: Future value
P: Starting principal
C: Contribution added each period
r: Annual rate, as a decimal
m: Compounding periods per year
t: Years

The first term grows the money you start with; the second is the future value of a series of equal contributions. Growth accelerates because each period earns on the accumulated total, not on the original principal.

What this assumes

A constant rate of return, applied evenly every period. Real markets do not behave this way — they deliver the same average through years that look nothing alike.
Contributions arrive on schedule and nothing is withdrawn.
No tax on growth, and no investment fees. Both are real and both compound against you.

What changes this number

Time: The dominant input by a wide margin. Of a 40-year result at 7%, nearly half arrives in the final decade — and those years exist only if you started early.
Contribution amount: Controls the early balance almost entirely. Growth does not overtake contributions until well into the projection.
Rate of return: Compounds its own difference. One percentage point over thirty years is a materially different outcome, which is why fees matter so much.

A worked example

Take the emergency fund: $5k + $200/mo at 4.5% for 5 years scenario. These figures are produced by the calculator above, not written alongside it, so they always match what the tool returns.

What you enter

Starting balance: $5,000
Monthly contribution: $200
Annual interest rate: 4.5%
Years to grow: 5 years

What it returns

Final balance: $19,688
You put in: $17,000
Compound interest earned: $2,688
Growth multiple: 1.16

Sources

This calculator uses no external data — the result follows entirely from the formula above and the values you enter, so there is nothing to cite beyond the arithmetic.

Calculator last reviewed August 8, 2026. How we verify

Try an example

Emergency fund: $5k + $200/mo at 4.5% for 5 years Retirement start: $10k + $500/mo at 7% for 30 years The latte test: $0 + $150/mo at 7% for 40 years

Frequently asked questions

What is compound interest?

Compound interest is interest calculated on both your original money and the interest it has already earned. Each period's earnings are added to the balance, so the next period earns interest on a bigger number — growth accelerates over time instead of staying linear.

How often is interest compounded in this calculator?

Monthly, which matches most savings accounts and is a close approximation for investment growth. Interest is credited each month on the current balance, then your monthly contribution is added.

What rate should I use?

Use your account's actual APY for savings (high-yield accounts have recently paid 4–5%). For long-term stock investing, planners commonly model 6–8% nominal returns. Lower your assumption to stress-test the plan — the habit of contributing matters more than the exact rate.

What is the Rule of 72?

A quick mental shortcut: divide 72 by your annual rate to estimate how many years money takes to double. At 7%, that's roughly 72 ÷ 7 ≈ 10 years. This calculator shows the exact path, including your ongoing contributions.

Does this account for taxes or inflation?

No — results are nominal and pre-tax, like your account statement. In taxable accounts, interest is taxed as income each year; retirement accounts defer or eliminate that. To think in today's purchasing power, subtract expected inflation (~2–3%) from your rate.