Compound Interest Calculator
Compound interest is the most powerful force in personal finance — your gains generate their own gains, creating exponential growth over time. Use this calculator to project your savings or investment portfolio with monthly contributions, model your 401k or Roth IRA growth, and see what your money will actually be worth in real inflation-adjusted dollars.
Compound Interest Calculator
Final Balance after 20 years
$142,438
Total Contributed
$53,000
Interest Earned
$89,438
Growth over time
How to Use This Calculator
- 1 Enter your starting investment or savings balance.
- 2 Add a monthly contribution if you plan to invest or save regularly.
- 3 Set your expected annual return rate (S&P 500 historical average: ~10% nominal, ~7% real).
- 4 Choose how many years you'll let the money compound.
- 5 Select compounding frequency — more frequent compounding produces slightly more growth.
Wealth-Building Tips
- ✓ Starting at 25 instead of 35 can more than double your retirement balance — time in the market is your biggest advantage.
- ✓ Consistent monthly contributions often outperform a single large lump-sum — automate them so you never miss.
- ✓ Tax-advantaged accounts (401k, IRA, Roth IRA) let gains compound without annual tax drag — maximize these first.
- ✓ Always model both nominal and real (inflation-adjusted) returns — a 10% nominal return is only ~7% real after 3% inflation.
Frequently Asked Questions
What is compound interest?
Compound interest earns returns on both your original principal and your previously accumulated interest. Unlike simple interest (interest on principal only), compounding causes exponential growth — especially powerful over 20–40 year investment horizons like retirement savings.
How often does compounding frequency matter?
More frequent compounding yields slightly more: daily > monthly > quarterly > annually. However, the difference is small compared to return rate and time. For long-term investors, increasing your monthly contribution by $50 has far more impact than changing compounding frequency.
What is the Rule of 72?
Divide 72 by your annual return rate to estimate how many years it takes to double your money. At 8% return: 72 ÷ 8 = 9 years. At 6%: 12 years. At 10%: 7.2 years. It works for any compounding investment — savings accounts, stocks, or bonds.
What annual return rate should I use for retirement planning?
Common benchmarks: S&P 500 index fund = ~10% nominal, ~7% real (inflation-adjusted). Conservative balanced portfolio = 5–6%. Bonds = 3–5%. HYSA/CDs = 4–5% (current). For retirement planning, use 6–7% real return to be conservative and account for inflation.