Compound Interest Calculator

See how your money grows with compound interest — principal, rate, compounding frequency and regular contributions, with a year-by-year table.

Inputs

Initial investment

Annual interest rate

Years to grow

Monthly contribution

Compounding

Results

Future value

$54,713.58

Interest earned

$20,713.58

Total contributed

$34,000.00

Effective annual rate

7.2%

Year-by-year growth

Year	Contributed	Balance
1	$12,400.00	$13,122.90
2	$14,800.00	$16,471.56
3	$17,200.00	$20,062.29
4	$19,600.00	$23,912.59
5	$22,000.00	$28,041.23
6	$24,400.00	$32,468.34
7	$26,800.00	$37,215.48
8	$29,200.00	$42,305.79
9	$31,600.00	$47,764.07
10	$34,000.00	$53,616.94

Compounding frequency matters less than time and rate — but starting early beats everything. Money invested at 25 is worth roughly double the same amount invested at 35.

Comprehensive Guide & Formulas

By CalcSuite Editorial TeamUpdated 17 October 20269 min read

Direct Summary / AI Overview

Growth & future value

Key Takeaways & Benchmarks

✓A = P(1 + r/n)^(nt) — interest earns interest, so growth accelerates.
✓At 7%, money doubles roughly every 10 years (Rule of 72).
✓Time and contributions matter more than compounding frequency.
✓A 1% annual fee can confiscate nearly a third of 30-year returns.
✓High-interest debt compounding against you usually beats investing.

How compounding builds wealth

Simple interest pays you only on your original deposit. Compound interest pays you on your deposit plus everything it has already earned — and that recursive structure is why the growth curve bends upward so aggressively over time. The first decade feels slow; the third feels like magic. It isn't magic, it's exponents.

Two levers matter enormously: time and contributions. Starting ten years earlier roughly doubles the outcome at typical market returns, and regular monthly contributions often contribute more dollars than the initial lump sum over a long horizon. Rate matters too, but chasing an extra 1% of return is usually less powerful — and far riskier — than starting earlier or saving more.

1
Enter your starting amount
The lump sum invested today.
2
Set the annual rate
Your expected average yearly return.
3
Choose years and contributions
Time horizon plus monthly additions.
4
Pick compounding frequency
Monthly, quarterly, annual or daily.

The formula

"A = P × (1 + r/n)^(n×t)"

"A = future value, P = principal, r = annual rate"

"n = compounds per year, t = years"

"With contributions: add PMT × [((1 + r/n)^(n×t) − 1) ÷ (r/n)]"

Worked example

$10,000 invested at 7% compounded monthly for 10 years grows to about $20,097 on its own. Add $200/month and the total reaches roughly $44,665 — of which $24,000 was contributed and about $10,665 is pure compounding on top of the principal's growth.

Extend to 30 years and the picture transforms: the same $200/month becomes roughly $303,000, with contributions of only $82,000 total. Time did most of the work — which is exactly why starting beats timing.

Making compounding work for you

▸Start early — a decade of delay roughly halves the long-term outcome at 7%.
▸Automate monthly contributions; consistency beats cleverness.
▸Keep fees low — a 1% annual fee quietly confiscates nearly a third of 30-year returns.
▸Kill high-interest debt first; 24% APR compounding against you dwarfs 7% for you.
▸Use realistic return assumptions (5–7% real) and stress-test the downside.

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Frequently asked questions

How does compound interest work?+

Each period, interest is calculated on your balance — which already includes previously earned interest. So growth accelerates: you earn interest on your interest. At 7% annual compounding, $10,000 becomes about $19,672 in 10 years without adding a cent. Add $200/month and it reaches roughly $44,000 — contributions plus compounding working together.

What is the compound interest formula?+

A = P(1 + r/n)^(nt), where P is principal, r the annual rate, n compounding periods per year, and t years. Regular contributions add a second term — the future value of an annuity. This calculator applies both, so you don't need to wrestle the algebra.

How often should interest compound?+

More frequent compounding grows money slightly faster, but the difference is modest: at 7%, monthly vs annual compounding differs by about 0.17 percentage points of effective yield. Rate and time dominate — don't chase compounding frequency while ignoring fees that cost far more.

What is the Rule of 72?+

A mental shortcut: divide 72 by your annual rate to estimate doubling time. At 7%, money doubles in about 10.3 years; at 10%, about 7.2 years. It's an approximation, but it makes the cost of delay visceral — every decade you wait roughly halves what compounding can do for you.

Is 7% a realistic return assumption?+

The US stock market has averaged roughly 7% annual returns after inflation over long periods — but 'average' hides brutal decades. Use 5–7% for long-horizon planning illustrations and stress-test with lower numbers. Anyone promising guaranteed double-digit returns is selling something, not advising you.

Does compound interest work against me with debt?+

Absolutely — it's the same math in reverse. Credit card balances compounding at 24% APR double in about 3 years if unpaid. That's why the Credit Card Payoff Calculator exists: killing high-interest debt is usually a better 'investment' than chasing market returns.

Should I invest a lump sum or contribute monthly?+

Lump sums win on average because markets rise more often than they fall — money invested earlier compounds longer. But monthly contributions win behaviourally: they're automatic, painless, and keep you investing through downturns. The best strategy is the one you'll actually stick with for decades.

How does inflation affect compound growth?+

Nominal returns overstate real purchasing power. At 3% inflation, money doubling nominally in 10 years only grows about 34% in real terms. For retirement planning, either use real (inflation-adjusted) return assumptions around 4–5%, or inflate your target alongside your projection.