Answer: Enter your values and the Compound Interest Calculator returns the exact result instantly — formula, worked example, and a plain-English explanation are included below the tool.
A compound interest calculator shows how an investment grows when interest earns interest, projecting future value from principal, rate, time, and contribution schedule.
See how your investments grow with the power of compound interest
| Year | Balance | Interest Earned | Total Contributions |
|---|
Enter a starting balance, a monthly contribution, an annual return, and a time horizon, and the calculator projects the balance year by year with the compounding frequency you choose. The growth table shows every early year and each five-year checkpoint, splitting the balance into what you put in and what the compounding earned on your behalf. Everything runs in your browser — no account, no data leaving the page.
One input deserves a second thought: the rate. Seven percent is the long-run average often used for a diversified stock portfolio, but it is an average of good decades and terrible ones, not a promise of any single year. Model a range — run the same plan at 5%, 7%, and 9% — and you will see the honest spread of outcomes rather than a single confident line.
A = P(1 + r/n)^(nt)
Where A = final amount, P = principal, r = annual rate, n = compounds per year, and t = years. With recurring contributions the calculator adds the future value of the monthly deposit, PMT × ((1 + r/n)^(nt) − 1) / (r/n), where PMT is the per-period contribution. Contributions compound alongside the principal — that is why $500 a month does so much more than $6,000 once a year deposited at the end.
Simple interest pays only on the original principal. Compound interest pays on the principal plus everything already earned. The difference sounds like a technicality and is in fact the whole subject:
| $10,000 at 7% for... | Simple interest | Compounded annually | Advantage |
|---|---|---|---|
| 10 years | $17,000 | $19,672 | +15.7% |
| 20 years | $24,000 | $38,697 | +61.2% |
| 30 years | $31,000 | $76,123 | +145.6% |
Simple interest grows in a straight line; compounding curves upward, and the curve steepens with time. This is why the advice is always to start early — not because early dollars are magic, but because early dollars are the ones that reach the steep part of the curve.
A $10,000 lump sum, compounded annually, no further contributions:
| Rate | 10 years | 20 years | 30 years |
|---|---|---|---|
| 4% (high-grade bonds) | $14,802 | $21,911 | $32,434 |
| 6% (moderate portfolio) | $17,908 | $32,071 | $57,435 |
| 8% (stock-heavy portfolio) | $21,589 | $46,610 | $100,627 |
| 10% (S&P 500 long-run avg.) | $25,937 | $67,275 | $174,494 |
Read down the 30-year column: the jump from 4% to 10% turns $32,434 into $174,494 — more than five times the money from two and a half times the rate. Meanwhile the row-level difference between 6% and 8% over 30 years ($57,435 vs $100,627) is exactly what a 1-2% fee or tax drag costs you, which is the argument for watching expense ratios.
Less than most people guess. The same $10,000 at 7% for 20 years:
| Frequency | Final balance |
|---|---|
| Annually | $38,696.84 |
| Semi-annually | $39,592.60 |
| Quarterly | $40,063.92 |
| Monthly | $40,387.39 |
| Daily | $40,546.56 |
Going from annual to daily compounding — a 365-fold increase in frequency — adds about 4.8% to the outcome. Rate and time dominate; frequency is a rounding-level refinement, which is why banks advertise it louder than it deserves.
The oldest mental shortcut in finance: divide 72 by the annual return percentage to estimate how many years money takes to double.
| Annual return | Years to double | What earns about that |
|---|---|---|
| 3% | 24.0 | Inflation (prices double) |
| 4% | 18.0 | High-grade bonds |
| 6% | 12.0 | Moderate portfolio |
| 7% | 10.3 | Stock-heavy portfolio |
| 10% | 7.2 | S&P 500 long-run average |
Put the shortcut to work with real contributions:
Every figure above is nominal — before inflation takes its share. At 3% inflation, prices double every ~24 years, so the same doubling that takes money 10 years at 7% takes spending power the other direction meanwhile. The quick estimate for a real (after-inflation) return: divide the growth factors — 1.07 ÷ 1.03 ≈ 1.039, about 3.9% real. A plan that looks comfortable at 7% nominal is really growing at roughly 4% in tomorrow's dollars, and long-horizon projections should be judged on that number.
For the full derivation, worked examples by hand, and how contribution timing changes results, see our guide on how to calculate compound interest.
Interest that earns interest. Each period, your earnings are added to the balance, and the next period's interest is calculated on that larger balance. At 7% a year, $10,000 becomes $76,123 after 30 years without a single additional deposit — versus $31,000 if the interest were simple and never reinvested. The gap between those two numbers is the entire concept.
Simple interest pays only on the original principal: $10,000 at 7% for 30 years collects $21,000 in interest, for a total of $31,000. Compound interest pays on the principal plus all accumulated interest, so the same money ends at $76,123 — the balance grows exponentially rather than in a straight line. Every investment vehicle worth having compounds.
Less than most people expect. $10,000 at 7% for 20 years grows to $38,697 compounded annually, $40,387 compounded monthly, and $40,547 compounded daily. Monthly beats annually by about 4.4% over two decades. Frequency helps at the margin, but the rate you earn and the years you stay invested dominate the outcome.
A mental shortcut for doubling times: divide 72 by the annual return percentage. At 7.2%, money doubles in about 10 years; at 10%, about 7.2 years; at 3% inflation, prices double in about 24 years. It is an approximation, accurate within a few percent for rates between 4% and 12%.
They subtract from the rate that actually compounds, and the drag compounds too. Knock 1% off a 7% return and $10,000 lands at $57,435 after 30 years instead of $76,123 — the fee costs $18,688, nearly a quarter of the outcome. Interest taxed every year compounds at the after-tax rate, which is why tax-advantaged accounts such as a 401(k) or IRA compound faster than the same investment in a taxable account.