Compound Interest Calculator Guide
A compound interest calculator shows how your investment grows over time when earned interest is reinvested to generate additional interest, illustrating the exponential effect of compounding.
How Compound Interest Works
Compound interest calculates interest on both the initial principal and previously accumulated interest. The formula is A = P(1 + r/n)^(nt), where P is principal, r is annual rate, n is compounding frequency, and t is time in years. More frequent compounding produces higher returns.
Key Inputs
Principal and Contributions
Your starting amount and any regular contributions (monthly or annual). Regular contributions dramatically accelerate growth — $500/month at 8% for 30 years grows to over $745,000, while a single $10,000 deposit only reaches $100,627.
Compounding Frequency
How often interest is calculated and added to the balance. Options include annually, semi-annually, quarterly, monthly, or daily. On a $100,000 deposit at 5%, annual compounding yields $105,000 after year one, while daily compounding yields $105,127.
The Rule of 72
A quick mental shortcut: divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 8%, your investment doubles in approximately 9 years (72 ÷ 8 = 9).
Real vs Nominal Returns
The calculator typically shows nominal returns. To understand real purchasing power, subtract the inflation rate. If your investment earns 8% nominally and inflation is 3%, your real return is approximately 5%. Over long periods, this difference compounds significantly.
Related Guides
A return on investment (ROI) calculator measures the percentage gain or loss on an investment relative to its cost, providing a standardized way to compare the profitability of different investments.
DCF Valuation Calculator GuideA discounted cash flow (DCF) calculator estimates the present value of an investment based on projected future cash flows, discounted back at a rate reflecting the investment risk.