Compound Interest Calculator
See how your money grows with compound interest plus regular contributions. Real math using the standard compound interest formula with multiple compounding frequencies, and a year-by-year breakdown that shows exactly how much of your final balance came from principal vs contributions vs interest.
Last updated: April 2026
7% is the standard real (inflation-adjusted) stock market return
Year-by-year growth breakdown
Watch how the contributions vs interest split shifts over time. In the early years, most of your balance is contributions. In the later years, interest becomes the dominant component — that is compound growth in action.
| Year | Total contributed | Interest earned | Balance |
|---|---|---|---|
| Year 1 | $16,000 | $919 | $16,919 |
| Year 5 | $40,000 | $9,973 | $49,973 |
| Year 10 | $70,000 | $36,639 | $106,639 |
| Year 15 | $100,000 | $86,971 | $186,971 |
| Year 20 | $130,000 | $170,851 | $300,851 |
| Year 25 | $160,000 | $302,290 | $462,290 |
The compound interest formula explained
The standard compound interest formula is:
Where A is the final amount, P is the principal, r is the annual rate as a decimal, n is the number of compoundings per year, and t is time in years. With periodic contributions added, the formula extends to:
Worked example. $10,000 principal, $500/month contribution, 7% annual return, monthly compounding, 25 years:
r = 0.07 (7%)
n = 12 (monthly compounding)
t = 25 years
PMT = $500/month
Principal alone: $10,000 × (1.00583)^300 = $57,308
Contributions: $500 × ((1.00583^300 - 1) / 0.00583) = $407,166
Final balance: $464,474
Total contributed: $10,000 + $150,000 = $160,000
Interest earned: $304,474
Of the final $464,474 balance, only $160,000 is your money — the other $304,474 is pure compound growth. Interest earns more than 65% of the final balance. That ratio gets even more dramatic over longer time horizons.
Three real compound interest scenarios
1. The early starter at 25
Sarah starts investing $500/month at age 25 in a Roth IRA, earns 7% real return, and stops contributing at age 35 — 10 years of contributions totaling $60,000. She lets it grow untouched until age 65 (30 more years). Final balance: about $625,000. She contributed $60,000. The other $565,000 came from compound growth. Compare to Tom who waits until 35 to start the same $500/month and contributes for 30 years ($180,000 total). Tom ends up with about $605,000 at 65. Sarah contributed 1/3 as much money but ended up with more — purely because of starting earlier.
2. The college fund
When Maya is born, her parents open a 529 college savings plan with $5,000 and add $200/month. They earn 6% real return (a bit conservative for stocks). At age 18: $90,000. They contributed $5,000 + ($200 × 12 × 18) = $48,200. The other $41,800 is interest. By starting at birth, they more than doubled their contributions through compound growth — and that money is tax-free for qualified education expenses.
3. The savings account vs the index fund
Marcus saves $300/month for 30 years. In a 4% high-yield savings account (a generous long-term assumption), he ends up with about $208,000 — of which $108,000 is contributions and $100,000 is interest. In a 7% real-return stock index fund, he ends up with about $367,000 — same $108,000 contributed but $259,000 in growth. The difference: roughly $159,000 just from earning 7% instead of 4% over the same period. Three percentage points compounded for 30 years is enormous.
Common compound interest mistakes
1. Underestimating the power of time
Most people focus on the contribution amount and ignore the time horizon. The math heavily rewards starting early. A 10-year head start can outperform 20 years of catch-up contributions.
2. Using nominal returns without adjusting for inflation
A 10% nominal return at 3% inflation is really only 7% in purchasing power. For long-term planning, use real returns (typically 6-7% for stocks) so projections show what your money will actually buy.
3. Stopping contributions during market downturns
Market drops are when each contribution buys the most shares. Stopping during a downturn turns a temporary loss into a permanent one by missing the recovery.
4. Confusing compounding frequency with rate
Daily compounding at 5% is barely better than annual compounding at 5%. The rate matters far more than the frequency. Do not get distracted by "daily compounding" marketing.
5. Ignoring fees
A 1% expense ratio compounded over 30 years can cost you 25% of your final balance. Vanguard, Fidelity, and Schwab index funds with sub-0.10% fees are dramatically better than 1%+ actively managed funds.
Frequently asked questions
What is compound interest?
Compound interest is interest calculated on both the original principal and on the accumulated interest from previous periods. Unlike simple interest (which only earns on the original principal), compound interest earns on a growing base — interest earns interest, which earns more interest. Over long time horizons this creates exponential growth. A $10,000 investment at 7% earns $700 in year one, but in year 30 it earns about $4,800 because the base has grown to $69,000+. This is why time horizon is the most important variable in long-term investing.
What is the compound interest formula?
The basic formula is A = P(1+r/n)^(nt), where A is the final amount, P is principal, r is the annual interest rate (as a decimal), n is the number of times interest compounds per year, and t is time in years. With periodic contributions added, the formula expands to: A = P(1+r/n)^(nt) + PMT × [((1+r/n)^(nt) - 1) / (r/n)]. This calculator uses the full version with contributions. Most retirement and savings projections use monthly compounding (n=12).
How does compounding frequency affect growth?
More frequent compounding produces slightly more growth, but the effect is smaller than most people think. $10,000 at 7% over 30 years grows to $76,123 with annual compounding, $77,007 with monthly compounding, and $77,072 with daily compounding. The difference between annual and daily compounding is less than 1.3% over 30 years. Continuous compounding (the mathematical limit) produces only marginally more than daily. The interest rate matters far more than the compounding frequency.
What is the difference between APR and APY?
APR (Annual Percentage Rate) is the simple interest rate without accounting for compounding. APY (Annual Percentage Yield) is the effective rate after compounding is factored in. A 6% APR with monthly compounding produces a 6.17% APY. For loans, lenders advertise APR (which understates your real cost). For savings accounts, banks advertise APY (which makes their offer look better). Always compare like with like — compare loan APRs against each other and savings APYs against each other.
How long does money take to double at 7%?
About 10.3 years at 7% compound interest. The Rule of 72 gives a quick estimate: divide 72 by the annual rate. 72 / 7 = 10.3 years. The exact formula is ln(2) / ln(1+r), which gives 10.24 years at 7%. For comparison: at 4%, doubling takes 18 years (72/4). At 10%, it takes 7.2 years. At 12%, it takes 6 years. The Rule of 72 works best for rates between about 5% and 15% — outside that range it loses accuracy.
What rate should I use for long-term projections?
For long-term retirement projections, 7% is the standard "real return" assumption (i.e., already adjusted for inflation). This is based on historical S&P 500 averages of about 10% nominal minus roughly 3% inflation. Using 7% gives a conservative-realistic projection in today's dollars. Some advisors use 6% to be even more conservative. Using 10% nominal will overstate growth in today's purchasing power. For shorter horizons (under 5 years), no equity assumption is reliable — short-term returns are essentially random.
Does the timing of contributions matter?
A small amount, yes. Contributions made at the beginning of each period have slightly more time to compound than contributions at the end. For monthly contributions over 30 years, this difference adds up to about 1% of the final balance — not negligible but not huge. Most calculators (including this one) assume end-of-period contributions, which is the more conservative assumption. The biggest factor is whether you contribute at all and how much, not the exact timing within a period.
Can I include irregular contributions?
This calculator assumes regular periodic contributions of a fixed amount. For irregular contributions, you can either average them into a monthly equivalent (e.g., a $6,000 annual bonus = $500/month average) or run multiple scenarios. The math gets more complex with irregular cash flows — the proper approach is the IRR (internal rate of return) calculation, which the Investment Return Calculator handles for completed periods.
How does inflation affect compound interest?
Inflation erodes purchasing power over time. A 7% nominal return with 3% inflation produces a 4% real return — meaning your money grows by 4% per year in actual purchasing power, not 7%. For long-term projections, use real returns (subtract expected inflation from nominal returns) to see what your money will actually be worth in today's dollars. The default 7% in this calculator is a real return assumption based on historical averages.
What is the rule of 72?
The Rule of 72 is a mental math shortcut for estimating how long it takes money to double at a given interest rate. Divide 72 by the annual rate (as a percent) to get the doubling time in years. At 6% it takes 12 years. At 8% it takes 9 years. At 12% it takes 6 years. The rule works because of the math of natural logarithms: ln(2) ≈ 0.693, and 72 happens to be conveniently divisible. The rule is most accurate for rates between 5% and 15%.
Why does starting early matter so much?
Because compound growth is exponential. The first 10 years of compounding produce the smallest absolute gains, but those early gains become the base for much larger gains in later years. Example: investing $5,000/year from age 25-35 ($50K total) and then nothing else, vs investing $5,000/year from age 35-65 ($150K total). At 7% real return, the early starter ends up with about $562,000 at age 65 — while the late starter (despite contributing 3x more money) ends up with about $505,000. Time matters more than the amount.
What is the difference between simple and compound interest?
Simple interest only earns on the original principal. If you invest $10,000 at 5% simple interest for 20 years, you earn $500/year for a total of $10,000 in interest — your final balance is $20,000. Compound interest earns on the growing balance. The same $10,000 at 5% compound interest for 20 years grows to about $26,533 — an extra $6,533 from interest earning interest. Simple interest is rare today; most loans, bonds, and savings accounts use compound interest.
Methodology: Calculator uses the standard compound interest formula A = P(1+r/n)^(nt) plus the future value of an annuity for periodic contributions. Default 7% rate is the historical S&P 500 real (inflation-adjusted) average. Contributions are assumed at end of period.
Last updated: April 2026.
Disclaimer: This calculator provides estimates for educational purposes only and is not investment advice. Actual investment returns vary based on market performance and are not guaranteed. Past performance does not predict future results.