How this calculator works
This calculator uses monthly compounding to model your investment growth. Each month, your current balance grows by your annual return rate divided by 12, and your monthly contribution is added. This closely mirrors how index funds and retirement accounts grow over time.
The 7% historical return benchmark
The S&P 500 has returned roughly 10% per year on average historically — about 7% after adjusting for inflation. Using 7% as a default gives a reasonable, inflation-adjusted estimate for a diversified stock portfolio held long-term. More conservative portfolios (bonds + stocks) might use 4–5%.
Monthly contributions matter more than timing
Trying to time the market rarely beats consistent monthly investing. A person who contributes $500/month for 30 years at 7% ends up with significantly more than someone who invested a lump sum and stopped. Consistency compounds. See also our compound interest calculator for savings-focused scenarios.
Paying off debt vs. investing
If you have high-interest debt, paying it off first is often the better "investment." A 20% credit card rate is a guaranteed 20% return when you pay it off. Use our debt payoff calculator to plan your path to debt freedom before investing heavily.
Example: $10,000 plus $500 a month for 30 years at 7%
These are the calculator's default inputs. After 30 years the portfolio would be about $691,000. You would have put in $190,000 ($10,000 plus $500 for 360 months), so about $501,000 of the final value is growth. The same inputs after 10 years give about $106,600, and after 20 years about $300,900. Most of the growth comes late, because the balance earning returns is larger in the later years. That is why staying invested matters more than any single year.
How the return rate changes the result
Same $10,000 and $500 a month for 30 years, with different assumed average returns (calculated, not guaranteed):
| Annual return | Final value |
|---|---|
| 5% | about $460,800 |
| 7% | about $691,200 |
| 9% | about $1,062,700 |
A two-point change in the assumed return moves the 30-year result by hundreds of thousands of dollars. Plan with a cautious number and treat anything above it as a bonus. Our guide to what return to expect from the stock market explains why.
Starting early vs. contributing more
Investing $500 a month at 7% from age 25 to 65 (40 years) grows to about $1,312,000, from $240,000 contributed. Starting at 35 (30 years) gives about $610,000, from $180,000 contributed. Ten extra years of contributions more than doubles the result. In another comparison, $100 a month for 30 years gives about $122,000, while $500 a month gives about $610,000, which is five times as much for five times the contribution. Time and amount both matter, but the extra years do the most work.
The effect of fees
Investment fees come out of your return. If a fund charges 1% a year, a 7% return becomes about 6% for you. On the default example, that is about $562,500 instead of $691,200 after 30 years, roughly $128,700 less. Check the expense ratio of any fund you use, since low-cost index funds often charge a small fraction of 1%.
What this calculator does not include
- Inflation. Results are in future dollars. If prices rise about 3% a year, $691,000 in 30 years has much less buying power than it does today.
- Taxes. Taxable accounts may owe tax on dividends and gains, while 401(k)s and IRAs have their own rules.
- Market swings. Real returns vary year to year. An average of 7% does not mean 7% every year.
- Changing contributions. Many people raise their monthly amount as income grows, which this calculator does not model.
Common mistakes
- Using an optimistic return rate. This makes a plan look better than it is.
- Waiting for the perfect time to start. Time in the market usually matters more than timing.
- Investing before building a cash cushion. Without savings, an emergency can force you to sell at a bad time. See our emergency fund guide.
- Stopping contributions after a drop. Pausing during a downturn can reduce your long-term result.