Investment Calculator FAQ

1. What's the difference between compound interest and simple interest?

Simple interest only earns on your original investment. If you invest $10,000 at 7% simple interest, you earn $700/year forever—always just 7% of $10,000.

Compound interest earns on your original investment and your previous earnings. Year 1 you earn $700 (7% of $10,000). Year 2 you earn $749 (7% of $10,700). The earnings accelerate.

Over time, compound interest grows exponentially; simple interest grows linearly. For investments, compound interest is vastly more powerful.

Try this in our calculator. Invest $10,000 at 7% for 30 years. Compound interest turns it into $76,123. Simple interest would turn it into just $31,000.

2. How often should I use this calculator?

Quick answer: Anytime your situation changes—new income, new investment amount, new financial goal.

Recommended schedule:

• Annually: Review your investment plan once/year (maybe on your birthday or New Year's)
• After major life changes: New job, inheritance, kid born, house sold
• When exploring scenarios: "What if I increase my monthly contribution by $200?"
• When market conditions change: If you're worried about a recession, model a 5% return scenario instead of 8%

Most people find it helpful to model their situation quarterly (4x/year) to stay on track.

3. Does this calculator account for taxes?

No, this calculator shows pre-tax returns. Real-world returns are reduced by taxes.

Example: If you earn a 7% return and owe 20% in capital gains tax:

• Nominal return: 7%
• After-tax return: 5.6% (7% × 80%)

Tax impact depends on account type (401k, IRA, brokerage), holding period, and tax bracket. For personalized tax planning, consult a tax professional or financial advisor.

4. What's a realistic annual return rate?

It depends on what you're investing in:

Investment	Historical Avg	Realistic Range	Risk
High-yield savings	4–5%	4–5%	Very low
Bonds	4–6%	4–6%	Low
Balanced (60/40)	7–8%	6–10%	Medium
S&P 500	10%	8–12%	Medium-high
Individual stocks	15%+	Highly variable	High

Key insight: Higher returns = higher risk and volatility. For this calculator, choose a rate that matches your investment strategy and risk tolerance. Year-by-year history, rolling 20-year windows, and a backtest against a smooth 8% path are on our historical stock market returns page.

5. How does inflation affect my investment returns?

Inflation erodes purchasing power. A 7% nominal return in a 3% inflation environment is really only a 4% real return.

Example: You invest $100,000 at 7% for 10 years = $196,715 (nominal). With 3% inflation, that $196,715 has the purchasing power of ~$145,000 in today's dollars. Your real return: 4.5% (not 7%).

Why it matters: A 5% savings account return loses money if inflation is 3%. You need investments that beat inflation to grow wealth.

Use our calculator and adjust the inflation slider to see how different inflation rates affect your real wealth.

6. Is lump sum or dollar-cost averaging better?

Short answer: It depends on market conditions and your risk tolerance.

Lump sum wins when: Markets are going up, you have limited capital, you can afford volatility.

Dollar-cost averaging wins when: Markets are volatile, money comes in regularly, you want to reduce timing risk.

Example: If you invest $100k in March 2008 (right before the crash), you'd lose 57% and take 5 years to recover. If you invested $833/month from 2008-2025, you'd capture huge gains on the recovery.

Bottom line: Both beat not investing. Try both in our calculator.

7. Can I model different investment types (stocks vs. bonds)?

This calculator doesn't distinguish between types—it just uses a return rate. But you can model any combination by choosing the right return:

• All stocks: 9–10% average return
• 60% stocks, 40% bonds: 7–8% average return
• All bonds: 4–6% average return

For detailed asset allocation modeling, use specialized tools like Personal Capital or Vanguard's portfolio planner.

8. Why doesn't this calculator include dividends or yield?

Dividends are already factored into the "annual return rate" if you use the S&P 500 average (10%). That includes both stock appreciation and dividend yield.

Example: A dividend stock returning 4% appreciation + 3% dividend yield = 7% total return.

9. What if my return varies year-to-year?

This calculator assumes a constant annual return. Real markets vary—some years +20%, some years -10%.

In reality: This calculator shows average performance. If you model 8% average over 20 years, actual returns might look like 12% average (bull market), -5% (recession), and 10% (recovery) that average to 8% overall.

Why this matters: You need emotional resilience to stay invested through down years. Dollar-cost averaging helps—you keep investing regardless of market conditions.

10. How should I adjust for market volatility?

Conservative approach: Use a lower return rate (5–6% instead of 8%).

Aggressive approach: Use a higher return rate (8–10%) and accept 20–30% drops some years.

Psychological approach: Remember that volatility cuts both ways. You might lose 20% in a crash, but also gain 20%+ in bull markets. Over 20+ years, this averages out.

Practical approach: Don't check your balance during downturns. Checking annually keeps perspective.

11. What's the difference between nominal and real return?

Nominal return = the raw percentage gain (7% on your investment)

Real return = nominal return minus inflation (7% - 3% inflation = 4% real return)

Your real return is what matters for wealth. A 5% nominal return is bad if inflation is 3%. A 10% nominal return is great if inflation is 2%.

Use our calculator's inflation adjustment to see both numbers and understand your actual purchasing power growth.

12. How do I know what return rate to assume?

Start here:

1. Decide your investment strategy (stocks, bonds, or mix)
2. Look up the historical average for that strategy
3. Use that as your baseline
4. Adjust down 1–2% if you're being conservative
5. Add a margin of error (uncertainty)

Example: 60% stocks / 40% bonds historical average = 7.5% → Use 6.5% conservatively.

Remember: Past average returns don't guarantee future results. The earlier you start, the more time to recover from underperformance.