Compound Interest Calculator
Calculate how your money grows with compound interest over time.
โน๏ธAbout This Calculator
Put $10,000 in an account earning 7% and walk away. In 10 years you'll have $20,097. In 20 years, $40,387. In 30 years, $76,123 โ and you never added a single dollar. That's compound interest doing the heavy lifting. Now add $200 every month and the numbers get wild: that same 30-year window turns into $283,382. The gap between "started at 25" and "started at 35" is often $200,000+ on identical monthly contributions. This calculator lets you plug in your actual numbers โ principal, monthly deposit, rate, and compounding frequency โ and see the year-by-year growth table so there's no guessing.
Frequently Asked Questions
What is compound interest?
You earn interest on your original deposit, then earn interest on that interest, then interest on that interest-on-interest. It snowballs. $1,000 at 8% simple interest earns $80/year forever. $1,000 at 8% compound interest earns $80 the first year, $86.40 the second, $93.31 the third โ and $10,063 total after 30 years vs. $3,400 with simple interest.
What is the compound interest formula?
A = P(1 + r/n)^(nt). P is your starting amount, r is the annual rate as a decimal, n is how many times per year it compounds, t is years. Quick example: $5,000 at 6% compounded monthly for 15 years = $5,000 ร (1 + 0.005)^180 = $12,271. The formula doesn't include monthly contributions โ those require a separate future value of annuity calculation, which this calculator handles automatically.
Does compounding frequency matter much?
Less than you'd think. $10,000 at 7% for 20 years: annually = $38,697, monthly = $40,387, daily = $40,552. The jump from annual to monthly is meaningful ($1,690). Monthly to daily? Only $165. Don't pick a worse savings account just because it compounds daily โ the rate matters 10x more than the frequency.
What is the Rule of 72?
72 รท interest rate = years to double your money. At 6%, about 12 years. At 8%, about 9 years. At 12%, about 6 years. Works surprisingly well for rates between 4-20%. I use this constantly for back-of-napkin math โ if someone offers you 9% returns, you know your money doubles roughly every 8 years without touching a calculator.
What rate should I use for projections?
Depends where your money sits. High-yield savings: 4-5% right now, but rates float. S&P 500 historical average: about 10% before inflation, 7% after. A balanced 60/40 portfolio: roughly 7-8% pre-inflation. If you're being conservative for retirement planning, use 6%. One thing people miss: a fund charging 1% annual fees turns your 7% return into 6%. Fees compound too โ against you.
How does compound interest work against you on debt?
Credit cards charge 20-29% APR compounded daily. A $5,000 balance at 24% with $100 minimum payments takes about 9 years and costs $6,300 in interest โ more than the original debt. Paying off a 24% credit card gives you a guaranteed 24% return. No investment can promise that.
What if I can only start with a small amount?
$100/month at 8% for 30 years = $149,036. $100/month at 8% for 40 years = $349,101. The last 10 years generated $200,065 โ more than the first 30 years combined. That's compounding in action. Starting small but starting early beats starting big but starting late almost every time.