Compound Interest Calculator

How compound interest works

Compound interest is interest earning interest. Each period, your returns are added to your balance, so the next period earns on a bigger number. Over years, that compounding curve bends sharply upward — which is why starting early matters more than almost anything else in investing.

How this is calculated

We grow your balance month by month: each month we add your contribution and apply one-twelfth of your annual return to the running total. Your future value is the result; total contributed is your starting amount plus every contribution; interest earned is the difference. Returns are assumed constant for simplicity — real markets move up and down, so treat this as a long-run estimate.

Educational estimate, not investment advice — see our disclaimer.

The Rule of 72

The Rule of 72 is a quick mental shortcut for estimating how long it takes money to double at a given annual return. Just divide 72 by the rate. At a 6% return, your money often doubles in about 12 years (72 ÷ 6 = 12). At 8%, it doubles in roughly 9 years. It is not exact, but it is close enough to sanity-check a result in your head before you trust a calculator.

The same trick works in reverse: if you want to know what return you would need to double your money in a set number of years, divide 72 by that number. To double in 10 years, you would need about a 7.2% return.

A compounding example

Imagine you start with $5,000 and add $200 every month for 30 years at a 7% annual return. Over that period you personally contribute $5,000 plus $72,000 in monthly deposits, for $77,000 of your own money. Thanks to compounding, the balance often grows to roughly $250,000 — meaning more than two-thirds of the ending balance came from growth, not from your deposits. The longer your money compounds, the more the growth portion dwarfs what you put in.

Time beats amount

Starting early with a small amount frequently beats starting late with a large one. Suppose one person invests $200 a month from age 25 to 35 and then stops, while another waits and invests $200 a month from age 35 to 65. The early starter contributes for only 10 years, yet often ends up with a comparable or larger balance at 65 — because that first decade of growth keeps compounding for 30 more years. The lesson: the most valuable ingredient is time in the market, and you can rarely buy it back later. Our FIRE calculator can show how an early head start shapes your path to financial independence.

What affects your results

• Rate of return — even a one or two percent difference compounds into a large gap over decades.
• Time — the single biggest lever; more years means more compounding periods.
• Contribution size — steady deposits add fuel that growth then multiplies.
• Compounding frequency — monthly compounding edges out annual, though the difference is modest.
• Fees — expense ratios and account fees quietly subtract from your return every year.

Common mistakes to avoid

• Waiting to start — delaying even a few years sacrifices some of your most powerful compounding time.
• Stopping contributions — pausing deposits during tough years can set you back further than the missed amounts suggest.
• Assuming unrealistic returns — plugging in 15% may look exciting, but historically broad-market returns have been more modest.
• Ignoring fees and inflation — both erode real growth, so consider returns net of costs and rising prices.

Once you know your target, a savings goal calculator can help you work out the monthly contribution needed to get there on time.