Compound Interest Calculator

Compound Interest Calculator

Model how a starting balance grows when interest compounds and you add money each month.

Capabilities

• End state: Final balance, total amount you contributed (initial plus all monthly deposits), and total interest earned.
• Effective annual rate: Nominal rate compounded at the chosen frequency, expressed as the equivalent yearly return (1 + r/n)^n - 1.
• Rule of 72: Rough years to double at the stated annual percentage rate (only when the rate is positive).
• Year-by-year schedule: Starting balance, contributions in that year, interest earned that year, and ending balance. Long horizons return the first 30 years plus the final year to keep payloads bounded.

Gotchas

• Rate format: annual_rate is a percentage (e.g. 7 for 7%), not a decimal like 0.07.
• Contributions: monthly_contribution is applied in equal parts across compounding periods within each year so totals match twelve monthly deposits per year.
• Compounding: Choose daily, monthly, quarterly, or annually; growth uses the standard per-period factor (1 + annual_rate/100/n) for n periods per year.
• Zero rate: With a 0% rate, balances only increase by contributions; effective annual rate is 0 and Rule of 72 is omitted.

Test scenario

Growth: $10,000 principal, 7% annual, 10 years, $500 monthly contributions — compare final balance and schedule to a spreadsheet or trusted calculator.