Compound Interest Calculator

How Compound Interest Works

A compound interest calculator helps you calculate investment growth by modeling one key principle: you earn interest on your interest, not just on your original principal. It's why Albert Einstein allegedly called compounding "the eighth wonder of the world." The longer your money compounds, the more dramatically it grows, because each period's interest becomes part of the new principal for the next period.

Compound vs. Simple Interest

With simple interest, $10,000 at 7% earns exactly $700 per year, every year — totaling $31,000 after 30 years (principal + $21,000 interest). With compound interest at 7%, the same $10,000 grows to $76,123 after 30 years — $45,000 more than simple interest, all from the same single investment.

Compounding Frequency Matters

Daily compounding gives slightly higher returns than monthly, which beats quarterly and annual. The difference is small on short time horizons but meaningful over decades. A $10,000 investment at 7% for 30 years: annually = $76,123; monthly = $81,165; daily = $81,645. Monthly vs. annual compounding adds about $5,000 extra on this example.

The Power of Starting Early

Two investors each invest $200/month at 7% annually. Investor A starts at age 25 and stops at 35 (10 years, $24,000 total). Investor B starts at 35 and invests until 65 (30 years, $72,000 total). At age 65 (monthly compounding): Investor A has about $281,000. Investor B has about $244,000 — despite investing 3× as much money. Time, not amount, is the most powerful variable in compounding.

What Interest Rate to Use

Use a rate that reflects your actual investment vehicle: savings accounts currently offer 4–5% APY; diversified stock index funds have returned an average of 10% nominal, 7% inflation-adjusted, over long historical periods. For conservative long-term planning, 6–7% is a reasonable assumption.

The Compound Interest Formula

For a lump sum: A = P × (1 + r/n)^(n×t) — where P = principal, r = annual rate, n = compounding periods per year, t = years. With regular contributions, each contribution compounds from the date it's added. This compound interest calculator simulates this month-by-month, which is why results may differ slightly from simplified online estimates that use approximate formulas.

Future Value: Where Your Savings Will Be

The final balance is the future value of your money. When each contribution matches the compounding period, it has a closed form: FV = P × (1 + i)^N + PMT × ((1 + i)^N − 1) ÷ i, where i is the rate per period, N the number of periods, and PMT the contribution per period. The first term is the "Starting Amount Alone Grows To" result; the second is what your contributions grow to. Present value runs the same formula backwards — PV = FV ÷ (1 + i)^N — to tell you what a future sum is worth today.

Timing matters a little. Contributions made at the end of each month (an ordinary annuity, the usual assumption) start earning the following month; contributions at the start of each month (an annuity due) earn one extra month of interest each. On $10,000 plus $200 a month at 7% for 30 years, that is about $325,160 versus $326,580. The bigger adjustment is inflation: at 3% a year, $326,580 in 30 years buys what roughly $134,500 buys today, which is why the calculator can show your balance in today's dollars.

5 Real Compound Growth Scenarios

For real-world scenarios with step-by-step numbers, explore our compound interest examples — including early vs. late investing, high vs. low rates, and lump sum vs. monthly contributions.

Best Accounts for Compound Growth

High-Yield Savings (HYSA): Currently 4–5% APY, FDIC insured, compounding daily — best for emergency funds and short-term goals under 3 years. Roth IRA/401(k): Tax-advantaged compounding — contributions grow and withdrawals in retirement are completely tax-free in a Roth. The most powerful long-term vehicle for most earners. S&P 500 Index Funds: ~10% historical nominal return with expense ratios as low as 0.03%, meaning nearly all the return compounds for you. I-Bonds: Inflation-protected savings bonds — currently 3–5%, no federal income tax until cashed.

4 Habits That Kill Compound Growth

• Withdrawing early — Each withdrawal removes not just that money but all its future compound growth. Early 401(k) withdrawals also trigger a 10% penalty plus ordinary income tax.
• High expense ratios — A 1% annual fund fee vs. a 0.03% index fund costs you nearly 1% of your total balance every year. On $200,000 over 20 years at 7%, that's over $80,000 in lost compounding.
• Pausing contributions — Even 3 years of skipped contributions in your 30s can cost $150,000+ at retirement. Automate contributions so they happen regardless of life events.
• Ignoring inflation — A savings account at 2% during 3% inflation is a -1% real return. Your money must outpace inflation to truly grow in purchasing power.