- What is compound interest?
- Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Your money grows exponentially because you earn "interest on interest." This is why Einstein allegedly called it the eighth wonder of the world.
- Why is time the most important factor?
- The effect of compound interest grows exponentially over time. $10,000 at 7% for 10 years = $20,000. The same $10,000 for 20 years = $40,000. For 30 years = $80,000. Each decade roughly doubles your money at 7% returns.
- Are contributions at the beginning or end of the month?
- This calculator assumes contributions at the end of each month. Contributions at the beginning would yield slightly higher results (about 0.5% more with typical parameters).
- Does this include taxes?
- No. Results are before taxes. When you withdraw from brokerage accounts, you may owe capital gains taxes (typically 15-20% for long-term gains in the US). Tax-advantaged accounts like 401(k), IRA, or Roth IRA can reduce or eliminate this tax burden.
- Is the return rate guaranteed?
- No. The rate entered is an assumption for simulation. ETFs and stocks don't guarantee returns. Savings accounts and CDs have guaranteed rates but typically much lower than historical market returns.
- Savings account vs ETF - same math?
- Mathematically yes. The difference: savings accounts have guaranteed (lower) rates, ETFs have variable historical rates (higher on average at 7-10%, but with risk of short-term losses during market downturns).
- Why does inflation change the result?
- Inflation shows real purchasing power. $300,000 in 20 years at 3% inflation is worth about $166,000 in today's money. Enable the inflation option to see real gains and make better retirement planning decisions.
- What return rate is realistic?
- S&P 500 index funds: 7-10% annually (historically, before inflation). Total stock market ETFs: similar. High-yield savings: 4-5%. Bonds: 4-6%. Treasury bills: 4-5%. Always plan conservatively using 6-7% for long-term projections.
- What if the return rate is negative?
- This calculator assumes positive returns. During market crashes, portfolio values can drop significantly. Historically, stock markets have recovered within 3-7 years. Long-term investing minimizes this risk through dollar-cost averaging.
- How often is interest compounded?
- This calculator assumes monthly compounding (most common for investment accounts). Annual compounding would yield about 0.5% less, daily compounding about 0.1% more. The difference is minimal over long periods.
- Why does 1% difference matter so much?
- Over 20 years: 6% vs 7% means about 20% difference in final value. Over 30 years: about 30% difference. Compound interest amplifies small differences over time. This is why low expense ratios in index funds (0.03-0.10%) matter significantly.
- When is the best time to start investing?
- 10 years ago. The second best time is today. Every year of delay costs thousands in lost interest. Check the "Cost of delay" section above to see specific numbers for your situation.
- How to handle irregular contributions?
- This calculator assumes fixed monthly contributions. For irregular contributions: calculate your average monthly amount or run multiple simulations for different periods. Consistency matters more than perfection - even small regular amounts add up significantly.
- What is the difference between nominal and real return?
- Nominal return is the raw percentage your investment grows (e.g. 7%). Real return accounts for inflation. If your nominal return is 7% and inflation is 3%, your real return is about 3.9% (Fisher equation). Real return reflects actual purchasing power growth.
- Does compounding frequency make a big difference?
- Not significantly for typical investments. Monthly vs annual compounding at 7% over 20 years differs by about 0.5%. Daily compounding adds another 0.1%. The difference between monthly and continuous compounding is negligible for practical purposes.
- Can compound interest work against me?
- Yes. Compound interest on debt (credit cards, loans) works in reverse - you pay interest on interest. A 20% credit card rate compounds your debt rapidly. This is why paying off high-interest debt should come before investing.