Compound interest calculator
See how savings grow with compound interest and regular contributions.
Runs 100% in your browser- Future value
- —
- You contributed
- —
- Interest earned
- —
How to calculate compound interest
- Enter your starting amount. Type your initial principal and any monthly contribution.
- Set rate, years and frequency. Add the annual rate, how many years, and the compounding frequency.
- Read the breakdown. See the future value split into what you put in versus interest earned.
What compound interest actually is
Simple interest pays only on your original deposit. Compound interest pays on your deposit and on the interest it has already earned — so each period you earn a little on a slightly larger balance, and that balance snowballs. The mechanism is just interest-on-interest repeated many times, and it's the single most important force in long-term saving and investing. This calculator applies the standard formula, future value = principal × (1 + r/n)^(n·t) plus the growth of your regular contributions, and splits the result so you can see exactly how much is money you put in versus money the compounding earned.
Why time matters more than amount
Compounding is exponential, so its biggest gains arrive at the end of a long horizon — which means the most valuable ingredient is years, not dollars. A modest sum that compounds for 30 years routinely beats a much larger sum that compounds for ten. The rule of 72 makes this tangible: 72 ÷ your rate ≈ the years to double, so money at 8% doubles roughly every nine years, then doubles again the longer you leave it. Steady contributions made early each get the full runway to compound, which is why the "interest" portion of your balance eventually overtakes the "contributions" portion.
Reading the result honestly
Two caveats keep the projection grounded. First, it's nominal: it doesn't subtract inflation, so a large future number buys less than it appears — run it through the inflation calculator to see its value in today's money. Second, it assumes a constant rate, while real investment returns vary year to year. Treat the output as a planning estimate, not a promise. For goal-based planning ("how much per month to reach a target"), use the savings calculator; to see the same compounding work against you on a loan, the amortization calculator.
Educational tool only — not financial advice. Assumes a constant rate and excludes taxes and fees. Real returns vary.
Frequently asked questions
- Compound interest is interest earned on both your original money and the interest it has already earned. Over time the growth accelerates, which is why starting early matters so much.
- Future value = principal × (1 + r/n)^(n·t), plus the future value of any regular contributions. Here r is the annual rate, n the times it compounds per year, and t the number of years. This tool does it for you and splits the result into contributions vs interest.
- A quick mental shortcut: divide 72 by your annual rate to estimate the years it takes money to double. At 8% that's about 9 years; at 6%, about 12. It is an approximation, but it makes the cost of a lower rate or the value of starting earlier easy to feel without a calculator.
- Yes — more frequent compounding (daily or monthly vs annually) earns a little more for the same nominal rate, because interest starts earning interest sooner. The gap between daily and annual is real but modest; the rate and the time horizon matter far more. You can change the frequency below.
- Because each contribution starts its own compounding clock. A dollar added in year one compounds for the whole period; a dollar added near the end barely compounds at all. That is why steady early contributions usually beat larger ones made late, and why the "interest" slice of your result grows faster than the "contributions" slice over time.
- No. It shows nominal growth. To see what the result is worth in today's money, use the inflation calculator — at 3% inflation, money loses roughly half its purchasing power over ~24 years, so a big nominal number is worth less than it looks.
- No — the calculation runs entirely in your browser.