Compound Interest Calculator

What is compound interest?

Compound interest is interest earned on both your original money and on the interest it has already earned. Unlike simple interest, which only ever pays out on your starting balance, compounding creates a snowball effect: each period your balance is a little bigger, so the next batch of interest is a little bigger too. Over long periods this effect becomes enormous, which is why Albert Einstein is often quoted — probably apocryphally — as calling it the eighth wonder of the world.

The formula

For a lump sum, compound interest follows A = P(1 + r/n)^nt, where A is the final amount, P is the principal, r is the annual rate, n is how many times per year interest compounds, and t is the number of years. This calculator goes a step further by adding your regular monthly contributions, compounding each one from the moment it is deposited. That makes the result far more realistic for anyone who saves or invests a fixed amount every month.

Why time matters more than amount

The single biggest driver of compound growth is time. Because growth builds on previous growth, money invested early has many more years to multiply than money invested later. Someone who saves a modest amount in their twenties often ends up with more than someone who saves far more aggressively starting in their forties. The chart above makes this visible: the balance curve starts almost flat and then bends sharply upward in the later years, as compounding takes over from your contributions as the main source of growth.

This is also why starting today, even with a small amount, usually beats waiting until you can afford to save "properly." The years you spend waiting are the most valuable years your money will ever have, because they sit at the very base of the compounding curve where everything else is built on top.

How compounding frequency changes things

The more often interest is compounded, the more you earn, because interest starts earning its own interest sooner. Daily compounding beats monthly, which beats annual — though the difference between them shrinks at lower interest rates. For most savings accounts and investments the headline annual rate matters far more than the compounding frequency, so do not lose sleep over it, but it is worth understanding when comparing products.

Tips for making compounding work for you

• Start as early as you can, even with small amounts — time is the most powerful ingredient.
• Automate your monthly contributions so saving happens without willpower.
• Reinvest interest and dividends rather than spending them, so they keep compounding.
• Be patient: the dramatic growth almost always happens in the final third of the timeline.
• Remember that real-world returns vary year to year — this tool assumes a steady average rate.