Compound Interest Calculator - Free Online Tool

Who Uses Compound Interest Calculator

Use it when: comparing savings accounts or CDs by APY, projecting a retirement or index-fund balance from monthly contributions, deciding how much to save each month for a goal, checking a compound interest homework problem, or seeing how much starting earlier changes the outcome.

Compound Interest Calculator Features

• Lump sum and monthly deposits — Combine a starting amount with regular contributions, the way most savings and investing actually works.
• Any compounding — Yearly, every six months, quarterly, monthly or daily - with the effective annual rate (APY) for each.
• Year-by-year growth — A chart and table of deposits versus interest for every year, downloadable as a CSV.
• Inflation-adjusted — Add an inflation rate to see what the final balance is worth in today's money.

How to Use Compound Interest Calculator

Three steps, results update live.

1. Enter your savings — A starting amount, a monthly deposit, or both.
2. Set rate and time — Annual interest rate, number of years and how often interest compounds.
3. Read the growth — Final balance, interest earned and a yearly chart update as you type.

Frequently Asked Questions

How compound interest works

With simple interest, you earn the same amount every year: 5% of $10,000 is $500, year after year. With compound interest, each year's interest is added to the balance, and the following year's interest is worked out on the new, larger total. In the first year you earn $500; in the second, 5% of $10,500, which is $525; in the tenth, about $776. The difference looks small at first and becomes enormous over decades.

Three things drive the result. The rate is the most obvious. Time is the most powerful, because the growth is exponential: the balance does not grow by a fixed amount each year but by a fixed percentage of an ever larger number. Regular deposits are the third, and for most people the largest single source of the final balance in the early years, before interest takes over.

The chart above shows this directly. Each bar is the balance at the end of a year, split into what you deposited and the interest earned so far. On a long enough horizon, the interest part grows until it is larger than everything you put in, which is the point at which your money is earning more than you are adding.

A quick way to feel the scale is the rule of 72: divide 72 by the annual rate to get the years it takes money to double. At 6% that is twelve years, at 9% eight. A sum left for 36 years at 6% doubles three times, ending at eight times what you started with, while the same sum at 3% only doubles once in 24 years. Small differences in rate, held for long enough, turn into very large differences in outcome.

How this calculator works it out

The calculator steps through every month of the period. Each month the balance grows by the monthly equivalent of the chosen compounding, and then that month's deposit is added. For monthly compounding the monthly growth is simply the annual rate divided by twelve; for yearly, quarterly or daily compounding it is the monthly rate that produces exactly the same yearly growth, so a lump sum lands precisely on the textbook formula A = P(1 + r/n)^(nt).

Deposits are made at the end of each month, which is how most automatic transfers and payroll contributions work. That is the same convention as the standard future-value-of-an-annuity formula, so the results match spreadsheet functions such as FV with the payment at the end of the period.

Alongside the final balance you get the total you deposited, the total interest and the share of the balance it represents, the effective annual rate (APY) for your compounding, and the time a lump sum takes to double. If you enter an inflation rate, the final balance is also shown in today's money, by discounting it at that rate for the full period. The yearly table can be downloaded as a CSV.

The calculator works in US dollars, euros, British pounds, Canadian and Australian dollars and Indian rupees. The currency only changes how amounts are displayed, since compounding works the same way everywhere, but it makes the summary you copy ready to paste into a message or a budget. Your inputs are remembered in your own browser, so the numbers are still there when you come back to compare a different rate or a longer horizon.

Making compound interest work for you

The single biggest lever is time. Because the last years of compounding contribute the most, starting ten years earlier can roughly double a retirement balance even when the extra money deposited is modest. If you can only change one thing, start sooner rather than waiting until you can save more.

The second lever is the rate you actually keep. Fees and taxes come straight off the return, and a one-percentage-point fee on an investment can cost a quarter or more of the final balance over thirty years. Low-cost index funds, and tax-advantaged accounts such as a 401(k) or IRA in the US, an ISA or pension in the UK, or a TFSA or RRSP in Canada, let more of the compounding stay with you.

Compounding works against you too. Credit card balances typically compound daily at 20% or more, which is why paying off high-interest debt is usually the best return available. And when you project investment growth, remember that returns are not steady: a 7% average includes years far above and below it. Use the calculator to compare a cautious, a middle and an optimistic rate, and plan around the cautious one.

Finally, automate it. A fixed transfer on payday into savings or an investment account means the monthly deposit happens without a decision each month, which is the assumption the calculator makes and the habit that makes the projections come true. Raising that deposit a little every time your income rises, even by 1% of pay, compounds in its own right: each increase is invested for every remaining year, and the effect by retirement is often larger than any change in the rate you could realistically find.