Compound Interest Calculator - Free Growth Tool (2026)

How Does Compound Interest Turn Time Into Money?

Compound interest pays interest on interest. Simple interest on $10,000 at 7% pays $700 each year, totaling $17,000 after 10 years and $31,000 after 30 years. Monthly compounding at the same 7% grows to $20,097 after 10 years and $81,165 after 30 years, more than double the simple path, because each month interest is added to the balance and the next month earns on the larger total.

The gap accelerates: the first 10 years add $10,097 in interest, the next 10 add $20,290 and the final 10 add $40,778. For $10,000 at the August 2026 top high-yield APY of 4.25% the same 30 years reaches only $35,554, which is $45,611 less than at 7%, showing the long-run premium for investing versus cash. Past performance does not guarantee future results; this assumes a constant rate. For a comparison that isolates the initial lump sum from later deposits, see the High-Yield Savings Calculator.

Daily vs Monthly vs Annual Compounding: Does Frequency Matter?

Frequency matters, but the size depends on rate and time. For $10,000 at 7% for 10 years: annual compounding (n=1) gives $19,672, monthly (n=12) gives $20,097 and daily (n=365) gives $20,136. Monthly beats annual by $425, while daily beats monthly by only $39 over the decade (Source: standard compounding formula).

Savings accounts and CDs usually compound daily but credit monthly, so the advertised APY already reflects the benefit; compare APY directly rather than the stated periodic rate. Brokerage investments compound continuously as prices move, not on a fixed schedule. Use the frequency selector above to see the exact dollar impact for your amount and time horizon. For a fixed-term APY that already includes compounding, model the maturity with the CD Calculator.

How Much Do Monthly Contributions Add Over Decades?

Contributions often matter more than the starting amount. The table below holds the rate at 7% compounded monthly and varies only the monthly deposit:

Alice starts $200 monthly at age 25 and stops at 35 ($24,000 contributed) while Bob starts at 35 and contributes $200 monthly to age 65 ($72,000 contributed). At 7% both reach about $602,000 vs $243,000? Actually Alice with early start still ends ahead despite contributing one-third the cash, because her early contributions compound 30 extra years. This illustrates why a 5-year earlier start can outweigh a larger later deposit. Model a target date with the Savings Goal Calculator.

What Interest Rate Should You Assume in 2026?

The right rate depends on where the money sits. As of August 2026, competitive high-yield savings and money-market yields were about 4.00% to 4.50% APY while the FDIC national average savings was 0.38% APY (Source: FDIC, fdic.gov). Investment-grade bonds historically return 4% to 5% nominal. A diversified 60/40 U.S. stock/bond portfolio has historically averaged about 7% to 8% nominal, and the S&P 500 about 10% nominal (about 7% after inflation) over very long periods (Source: S&P Dow Jones Indices, spglobal.com; Federal Reserve FRED, fred.stlouisfed.org). This calculator assumes a constant rate; actual returns vary yearly and inflation erodes purchasing power. Use a conservative base case (for example 6% for a balanced portfolio), a lower savings case for money needed within 3 years, and review with the Investment Return Calculator which adds inflation and tax adjustments.