How to use this compound interest calculator
Fill in five numbers: what you are starting with, what you will add each month, the annual rate, how long for, and how often interest is credited. Everything recalculates as you type, and the table below breaks the result down year by year so you can see the moment when interest starts outpacing your own deposits.
That crossover point is the whole reason compound interest is worth understanding. In the default example — $5,000 to start, $300 a month, 7% a year — you pay in $77,000 over twenty years and finish with roughly $170,000. The extra $93,000 was never yours to deposit; it was earned by money that was already earning.
The compounding frequency setting
Compounding frequency is how often earned interest is added to the balance and starts earning interest itself. Monthly compounding on a 7% rate produces slightly more than annual compounding, because each month's interest joins the pot immediately.
The difference is smaller than people expect. On $10,000 at 7% for one year: annual compounding gives $700, monthly gives $722.90, daily gives $725.01. Beyond daily, the gain is negligible — continuous compounding, the mathematical limit, gives $725.08. So do not agonise over the setting; the rate and the time horizon dominate everything else.
Use the setting that matches your account. Savings accounts usually compound monthly or daily. Bonds pay semi-annually. Index funds do not "compound" in the banking sense at all — growth comes from price appreciation and reinvested dividends — but monthly is a reasonable model for them.
Contribution timing
Depositing at the start of the month rather than the end gives every contribution one extra month of growth. Over twenty years, on the default figures, that is worth around $2,000. Small, but free — if you can set your standing order for the 1st rather than the 28th, do.
Reading the year-by-year table
The "paid in" column is the sum of your starting amount and every contribution to that point. The interest column is everything else. Watch how the relationship changes: interest is a rounding error in year one and typically overtakes total contributions somewhere between year twelve and year eighteen at moderate rates.
This is why starting early beats contributing more later. Someone investing $200 a month from age 25 to 35 and then stopping entirely will usually finish ahead of someone investing $200 a month from 35 to 65 — three times the money, less time to work. The first person's early contributions had thirty extra years to compound.
What the calculator deliberately does not model
Being clear about this matters more than the arithmetic, because the arithmetic is trivial and the omissions are where real money goes.
Inflation. Every figure is nominal. At 3% inflation, $170,000 in twenty years buys roughly what $94,000 buys today. To think in today's money, enter your real rate instead of the nominal one — if you expect 7% growth and 3% inflation, enter 4%. The result is then in purchasing-power terms and is a far more honest number.
Fees. A fund charging 1% a year does not reduce your return by 1%; it compounds against you for the entire period. On the default scenario, a 1% annual fee costs about $26,000 over twenty years — more than a quarter of all the interest earned. Subtract the fee from the rate before you enter it.
Tax. Interest, dividends and capital gains are taxable in most jurisdictions unless held inside a shelter such as an ISA, a 401(k) or an equivalent. Tax rules vary enormously and change, so nothing is assumed here.
Volatility. This is the big one. The calculator applies a smooth, identical return every single period. Real markets do not. A portfolio averaging 7% over twenty years might return +22%, −14%, +9%, −3% and so on. The final balance from a volatile path that averages 7% is generally lower than the smooth 7% shown here, because losses need larger gains to recover from — down 20% then up 20% leaves you at 96%, not 100%. Treat the output as a central estimate, not a forecast.
Choosing a realistic rate
For historical context rather than prediction: developed-market equities have returned roughly 7–8% a year in real terms over very long periods, government bonds 2–3%, and cash close to zero after inflation. Ordinary savings accounts pay whatever the current rate environment allows. Anything promising a guaranteed double-digit return deserves scepticism proportional to the number.
Not financial advice. This calculator is an educational tool that models a simplified scenario. Real outcomes depend on fees, taxes, inflation and market behaviour that no calculator can predict. Speak to a regulated adviser before making a financial decision.