Compound interest calculator
Enter your initial amount, rate, years and monthly contribution to project your balance.
Compound interest calculator: how your money grows
This compound interest calculator shows what a starting balance grows into, with optional regular contributions, when interest earns interest of its own. It returns the final balance, the total you paid in and the interest earned, instantly and without sending anything to a server.
The compound interest formula
For a lump sum with no extra contributions, the final balance is:
Where A is the final amount, P the principal, r the annual rate (as a decimal), n the number of times interest compounds per year (12 for monthly) and t the number of years. The exponent is the point: because interest is reinvested, growth is exponential, not linear.
The bottom band is your principal; the upper area is interest, which is barely visible early on and then takes off. That is why time is the most powerful lever in compounding.
Simple vs compound interest
Simple interest is always charged on the original principal, so it grows in a straight line. Compound interest is charged on the principal plus the interest already earned, so each period starts from a bigger base. Over a few years the gap is small; over 20 or 30 years it is huge.
Compounding frequency and regular deposits
The compounding frequency (yearly, monthly, daily) matters: more often means slightly more interest. Regular contributions (saving every month) boost the outcome sharply, because each deposit also starts to compound.
Worked example
You invest £10,000 at 6% a year, compounded monthly, for 20 years, adding nothing more. The final balance is about £33,100 — roughly £23,100 of interest, more than double what you put in, without touching the money. Add £100 a month and the result comfortably tops £79,000.
Frequently asked questions
What is compounding? It is the moment interest is added to the balance and starts earning interest itself. The more often it happens, the larger the result.
Does it work for a savings account or a fund? Yes, for any product that reinvests returns. Real fund returns vary year to year; here a constant average rate is assumed for an estimate.
Does it allow for tax or inflation? No. The figure is gross; savings returns are usually taxed and inflation erodes buying power, so the real value will be a little lower.