Compound interest is the process of earning interest on both your initial principal and the accumulated interest from previous periods — often called "interest on interest." It is the fundamental force behind long-term investment growth and the reason Albert Einstein is often (apocryphally) quoted c...
FINAL BALANCE
$144,572.72
Interest earned: $86,572.72 · APY: 7.23%
FINAL VALUE
$144,572.72
INTEREST
$86,572.72
APY
7.23%
GROWTH OVER TIME
RULE OF 72
Doubles in ~10.3 years at 7%
72 ÷ 7 = 10.3
Live diagram · updates as you type
Enter your starting principal — the initial amount you are investing or have saved. Then enter the annual interest rate as a percentage (e.g. 7 for 7%).
Select your compounding frequency. Monthly is most common for savings accounts and CDs. Daily is used by many online banks. Annually is used for bonds and some investments.
Enter the investment period in years. Optionally add a regular monthly contribution to see how consistent investing accelerates growth.
The calculator shows your final balance, total interest earned, and a year-by-year growth table. Use the Rule of 72 estimate to quickly see when your money doubles.
Sarah invests $10,000 at 7% annual interest, compounded monthly, and adds $200 per month for 20 years. Principal: $10,000. Monthly contributions: $200 × 240 months = $48,000. Total invested: $58,000. Final balance: A = $10,000 × (1 + 0.07/12)^240 + $200 × [((1.005833)^240 - 1) / 0.005833] = $130,252. Total interest earned: $72,252. The power of compounding turned $58,000 of contributions into $130,252 — an extra $72,252 from interest alone.
| FORMULA | EXPRESSION | USE WHEN |
|---|---|---|
| Standard | A = P(1 + r/n)^(nt) | Regular compounding, no contributions |
| With PMT | A = P(1+r/n)^nt + PMT×[((1+r/n)^nt−1)/(r/n)] | Regular contributions added each period |
| Continuous | A = Pe^(rt) | Continuous compounding (theoretical max) |
| APY | APY = (1 + r/n)^n − 1 | True annual yield accounting for frequency |
| Rule of 72 | Years to double ≈ 72 / r | Quick doubling time estimate (r in %) |
P = principal · r = annual rate (decimal) · n = compounding periods/year · t = years · PMT = periodic contribution
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Formula: A = P(1 + r/n)^(nt) · Continuous: A = Pe^(rt) · Last updated: April 25, 2026 · Formula verified by EagleCalculator team · For informational purposes only — consult a financial advisor · Eagle-eyed accuracy for every calculation.