Compound Interest Calculator
See what a starting balance and a steady monthly contribution turn into over time. Adjust the rate, the years, and the compounding frequency and watch the split between what you put in and what the interest earns. Runs entirely in your browser — nothing you enter is uploaded.
Year-by-year breakdown
| Year | Start | Added | Interest | End |
|---|
The two forces: time and rate
Every compound-growth number comes down to two levers. The rate sets how fast your balance grows each year. But time is the one that does the heavy lifting, because growth compounds on itself — each year's interest joins the pile that earns next year's interest. That's why the year-by-year table looks almost flat at first and then bends sharply upward near the end.
Why starting early beats saving more
The classic example: two people each contribute the same total amount, but one starts 10 years earlier. The early starter usually ends up with far more, despite identical contributions, purely because their money had more years to compound. Put two scenarios into the calculator — $200/month for 30 years vs $400/month for 15 years (same $72,000 contributed) — and compare the final balances. The extra time almost always wins.
The practical takeaway isn't "save more later." It's "start now, even small." A modest contribution that compounds for decades beats a large one that only has a few years to grow.
A future dollar is not a today dollar
Long-horizon compounding produces big nominal numbers, and it's easy to plan around a balance that won't buy what you think. The inflation input deflates the final balance into today's buying power, and the ±1% band under the headline shows how much the result swings if your return assumption is off by a single point — over 30 years, that one point can move the outcome by a third. Treat the band as the honest range, not the headline as a promise.
How the math works
The calculator converts your annual rate and compounding frequency into an effective annual yield, then into an equivalent monthly growth factor. Each month it grows the current balance by that factor, then adds your contribution. Repeating that for every month of every year gives the exact balance — no shortcut formula, just the same arithmetic a savings account does, run forward.
- Total contributed is your starting amount plus every monthly contribution.
- Total interest is the final balance minus everything you put in — the part the money earned on its own.
- Effective yield (APY) is your nominal rate adjusted for how often it compounds.
Related
- Personal finance hub — all our money calculators and guides
- How compound interest works (and why time beats amount)
- Savings goal calculator — work backward from a target
- FIRE calculator — when compounding makes work optional
- Debt payoff calculator — the same math working against you
FAQ
Is anything I enter sent to a server?
No. The calculator runs entirely in your browser — open DevTools → Network and confirm. Nothing you type is uploaded, ever. Your numbers stay in the tab.
What rate of return should I use?
Be honest and a little conservative. A globally diversified stock portfolio has historically returned roughly 6–7% per year after inflation over long periods, but with big swings. A high-yield savings account or CD might be 3–5% in nominal terms. Bonds sit in between. If you're modelling decades, using a real (after-inflation) return like 5–7% keeps the future number in today's-dollars terms and avoids fooling yourself with a scary-looking nominal balance.
How is compound interest different from simple interest?
Simple interest pays you only on your original principal. Compound interest pays you on your principal and on the interest you've already earned — interest on interest. Over one year the difference is tiny; over 30 years it's the whole game. That's why the year-by-year table starts slow and then bends sharply upward.
When is the monthly contribution added — start or end of the month?
Your choice — that's the contribution timing toggle. "End of month" (the default, an "ordinary annuity") applies that month's growth first, then adds your contribution; it's the conservative, standard convention. "Beginning" adds the contribution first, so every contribution earns one extra month of growth and the final balance is slightly higher. Flip the toggle and watch how small the gap actually is.
What does the inflation input do?
It adds a "buying power in today's $" figure: the future balance divided by (1 + inflation)years. A nominal $500,000 in 30 years buys far less than $500,000 does today — at 2.5% inflation it's about $238,000 of today's purchasing power. The default is 2.5%; set it to 0 to switch the adjustment off. If you'd rather bake inflation in directly, use a real (after-inflation) return rate and set inflation to 0.
What is the annual contribution increase?
Most people don't contribute a flat amount forever — contributions tend to rise with income. Setting the increase to, say, 3% grows your monthly contribution by 3% at the start of each new year ($200/mo in year 1, $206 in year 2, and so on). The year-by-year table's "Added" column shows the escalation, and "total contributed" counts the higher contributions correctly.
Does the compounding frequency really change much?
Less than people expect. At 7%, compounding daily instead of annually raises the effective yield from 7.00% to about 7.25% — real but small. The frequency selector here genuinely affects the result (it converts your rate to an effective annual yield first), but the two forces that dominate your final number are time and the rate, not how often interest posts.
What's the rule of 72?
A shortcut: divide 72 by your annual return to estimate the years for money to double. At 8% → about 9 years to double; at 6% → about 12 years. It's an approximation, but a useful gut-check against this calculator's exact numbers.