Compound Interest Calculator

What is Compound Interest?

Compound interest is interest earned on both the initial principal and accumulated prior interest. Formula: A = P(1 + r/n)^(nt). With monthly contributions, each deposit also compounds from the month it is made.

Rule of 72: divide 72 by your annual rate to estimate doubling time. At 7%, money doubles in ≈10 years. At 10%, in ≈7.2 years. It's a fast mental math shortcut investors use constantly.

Inflation-adjusted (real) return: the nominal balance adjusted for purchasing power loss. At 7% returns with 3% inflation, your real annual gain is only ~4%. This calculator shows both so you can see the actual vs nominal growth.

How to Use

1. Enter your initial amount, annual interest rate, and years.
2. Choose compounding frequency (monthly is most common for savings accounts and ETFs).
3. Click Calculate — see final balance, interest earned, growth multiplier, and doubling time.
4. Open Advanced to add a monthly contribution and enter an inflation rate for real return.

Tips & Best Practices

Use Ctrl+Enter to calculate without clicking. Use Ctrl+Shift+C to copy. Press Esc to clear. The Rule of 72 is a quick mental math tool — at 7%, money doubles in ~10 years. Monthly compounding is standard for most savings and ETFs; daily compounding adds marginal benefit at typical rates. Enter your expected inflation (historically ~3%) to see the real purchasing power of your future balance.

When to Use This Tool

Use for retirement planning, savings growth projections, comparing investment returns, or understanding the true impact of inflation on long-term wealth. Pair with the Loan Calculator to compare the cost of debt vs the return on savings, or the Dividend Calculator for income-focused projections.