Compound Interest Calculator

Money you put in and money your money earned are two different numbers, and most calculators blur them together. This one doesn't: enter a starting amount and a monthly contribution, and watch the balance, the interest, and the real value after inflation update as you type.

Here's the honest version: compounding feels boring for the first few years, then it goes vertical. Simple interest pays only on your original deposit, so its growth is a straight line. Compounding pays on your deposits and on every dollar of interest already credited — and that feedback loop is why the same $200 a month is worth dramatically more at 30 years than at 10. The hard part was never the math. It's believing the curve before you can see it.

Worked example: $10,000 at 7% for 10 years

A $10,000 lump sum earning 7% compounded monthly, with nothing added, ends at $20,097 — roughly double, matching the Rule of 72's estimate of 72 ÷ 7 ≈ 10.3 years to double. Add $200 a month and the ending balance climbs to about $54,700, of which $34,000 is your own money and roughly $20,700 is interest. That last figure is the one worth staring at: the contributions are only 62% of the outcome.

Compounding frequency matters less than most people think

Going from annual to daily compounding sounds enormous. On $10,000 at 7%, it's $25 a year. Daily compounding is the most oversold feature in consumer finance: the nominal rate does almost all the work. Where frequency genuinely matters is comparing products that quote rates differently: a savings account advertising 4.30% compounded daily is not quite the same as a bond paying 4.30% semi-annually. The effective annual rate (EAR) in your results is how you catch that difference.

Why recurring contributions change the shape of the curve

A lump sum compounds from day one, so its curve is smooth. Contributions arrive over time, so each deposit gets less compounding than the one before it and the curve starts flat before bending upward. Contribution timing is a smaller effect that nonetheless shows up in real numbers: depositing at the start of each month rather than the end adds roughly one extra month of growth per deposit, which at 7% over 30 years is worth about $1,400 on a $200 monthly contribution — and scales with the size of the deposit.

Nominal growth versus what it actually buys

A balance that triples over 30 years feels like a win — until you price it in today's dollars. At 3% inflation, prices roughly double in 24 years, so a nominal 3× is closer to 1.2× in purchasing power. That's not a reason to give up. It's a reason to look at the real-value figure beside the headline instead of scrolling past it. The honest comparison for any investment is return minus inflation, never return alone.

Mistakes that quietly cost you money

• Using an average return as if it were guaranteed — a portfolio that returns +20%, −20%, +20% does not average out to its arithmetic mean in compounded terms.
• Ignoring fees. A 1% annual fee on a 7% return removes about 24% of your 30-year ending balance — the fee compounds against you for the entire period, not just in the years you pay it.
• Stopping contributions during downturns, which is precisely when the shares you buy do the most future compounding.
• Comparing a nominal return against an inflation-adjusted goal.
• Assuming a high APY is safe — check whether the account is FDIC-insured and whether the rate is promotional.