Compound Interest Calculator
Enter a starting balance, a monthly contribution, a return rate, and a time horizon — then watch what compounding actually does to money over decades. The results change how people think about saving.
Project your savings growth
Why compound interest feels like a trick — and is not
Compound interest means your money earns returns, and then those returns earn returns. In month one, your $10,000 earns interest. In month two, that interest also earns interest. By month three, interest is compounding on the original principal plus two months of accumulated returns. It does not sound dramatic. For the first few years, the effect is underwhelming. Then somewhere around year ten, the math starts to look like a mistake. It is not. The thing that changed is the base — there is now enough accumulated growth that a 7% return on it dwarfs what the monthly contributions add. This is why people who start at 25 retire wealthy and people who start at 45 retire stressed, even if they contribute the same total dollars.
The formula — and what each part means
P is your starting amount. PMT is the monthly contribution. r is the monthly rate (annual rate ÷ 12). n is the total number of months. The first term is your lump sum compounding. The second is every monthly contribution independently compounding from the month it was made. The calculator compounds monthly, which matches how most investment and savings accounts actually work.
Three real scenarios — the numbers that change behavior
Scenario 1: The Early Starter. Age 25, $5,000 starting balance, $400/month, 7% for 40 years. Total contributed: $197,000. Future value: $1,097,000. Growth: $900,000 — the market did 4.6x more work than you did.
Scenario 2: The Late Starter. Same person, but starts at 40. $5,000, $400/month, 7% for 25 years. Total contributed: $125,000. Future value: $330,000. Growth: $205,000. Starting 15 years later — while contributing only $72,000 less — produces a $767,000 smaller result. Those 15 years cost 15 years of compounding on an increasingly large base.
Scenario 3: The $200 Extra. Age 35, $20,000 starting, $600/month vs. $800/month, 7% for 30 years. At $600/month: $738,000. At $800/month: $929,000. An extra $200/month — $72,000 more contributed over 30 years — produces $191,000 more in the account.
The Rule of 72 — mental math for doubling time
Divide 72 by your annual return rate to get the approximate years to double your money. At 6%: 12 years. At 7%: 10.3 years. At 9%: 8 years. At 12%: 6 years. It works because 72 is close to the natural log of 2 scaled for easy mental arithmetic. Accurate within a year for rates between 2% and 20%.
Choosing an honest return assumption
High-yield savings / money market: Use your actual current APY. As of mid-2026, quality HYSAs pay 4.5–5.0%. These rates will change — do not model them as permanent for long-horizon projections.
CDs: Use the certificate rate. Simple, reliable, FDIC-insured. Good for money you will need within 1–5 years.
Diversified stock market (index funds, 401k, IRA): 6–8% is the most widely used long-term planning range based on historical US market averages after fees and inflation effects. Run your numbers at 5%, 7%, and 9% to understand the range — never anchor to a single rate.
The fee warning: A 1% annual management fee quietly reduces a 40-year compound interest result by roughly 20–25%. Low-cost index funds with expense ratios of 0.03–0.20% keep that money compounding for you. Over decades, the difference is not cosmetic.
Frequently asked questions
What return rate should I use?
For stock market investments: 6–8% for long-term planning. For high-yield savings: your current APY. For CDs: the certificate rate. Always model a range — run 5%, 7%, and 9% to see the spread.
Does this account for inflation?
No — results are in future nominal dollars. For a real-dollar estimate, subtract expected inflation from your return assumption: 7% growth minus 3% inflation equals 4% real rate. Enter 4% to see approximate purchasing power in today dollars.
What is the Rule of 72?
Divide 72 by your annual return to estimate doubling time. At 7%: about 10.3 years. At 9%: about 8 years. Accurate within a year for rates between 2% and 15%.
Monthly vs. annual compounding — does it matter?
Modestly. $10,000 at 7% for 20 years: roughly $38,700 annually vs. roughly $40,300 monthly — about 4% more. This calculator compounds monthly, matching how most real accounts work.
What matters more — rate or contributions?
Early on, contributions dominate. After 15–20 years, the accumulated base takes over and rate matters more. Maximize contributions first, minimize fees to protect the rate, and start as early as possible.
Is 7% realistic?
It is the most common long-term planning assumption for diversified stock investments based on historical data. Real portfolios experience volatile individual years, not smooth lines. 5% is more conservative; 9% is optimistic but historically achievable for long periods. Model all three.
How much does a 1% fee cost?
More than it appears. On $10,000 starting balance with $500/month for 30 years at 7%, a 1% annual fee reduces the final balance by roughly $100,000–$150,000. This is the primary mathematical argument for low-cost index funds.
Lump sum or spreading it out — which is better?
Lump sum historically outperforms about two-thirds of the time because markets trend upward. Dollar-cost averaging reduces regret if markets drop right after investing. Both are reasonable — whichever gets the money invested is better than waiting for the right moment.
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