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About Investment Projections
“If I invest $500 a month, what will I have in 20 years?” is the question that starts most investing journeys — and its honest answer is a projection, not a prediction. The math is exact; the return assumption is the guess. This calculator makes the math transparent so your attention lands on the assumption, where it belongs.
Enter a lump sum, a monthly contribution, or both, plus your assumed annual return and horizon. You get the projected value, the split between what you contributed and what compounding added, and the growth multiple — the number that explains why starting early beats starting big.
Prefer the interest-first framing with compounding frequency options? That's the Compound Interest Calculator
The Future-Value Math
Two standard formulas, added together (monthly compounding, r = annual ÷ 12):
Lump sum: FV = P × (1 + r)ⁿ Contributions: FV = PMT × ((1 + r)ⁿ − 1) ÷ r n = months · r = annual rate ÷ 12
Worked example: $10,000 start plus $500/month at 7% for 20 years → $300,851, of which only $130,000 was contributed — growth added $170,851. The same inputs at 5% give ~$240k and at 9% ~$380k: the assumption swings six figures, which is the honest headline.
Time Does the Heavy Lifting
$500/month at 7%, no starting amount — every row from this calculator's formula:
| Years | You put in | Projected value | Growth share |
|---|---|---|---|
| 10 | $60,000 | $86,542 | 31% |
| 20 | $120,000 | $260,463 | 54% |
| 30 | $180,000 | $609,985 | 70% |
| 40 | $240,000 | $1,312,407 | 82% |
Read the last column: by year 40, compounding — not contributions — is 82% of the balance. Doubling your timeline beats doubling your deposit.
Honest Notes on Return Assumptions
Historical long-run US stock averages (high single digits nominal) hide violent detours — decades exist where returns ran far below average, and sequence matters when you're withdrawing. Fees quietly compound too: a 1% annual fee on the 20-year example above costs roughly $35,000 of the final balance. Use net-of-fee assumptions, and treat anything above historical averages as fantasy-checking, not planning.
Inflation is the other silent variable: $300k in 20 years buys what roughly $180–200k buys today at typical inflation. Planning purists project with REAL (inflation-adjusted) returns — about 2–3 points below nominal — so the output reads in today's dollars. Either approach works; mixing them (nominal returns, today's expenses) is the error to avoid.
Frequently Asked Questions
What will $500 a month be worth in 20 years?
At an assumed 7%: about $260,000 on $120,000 contributed. At 5%: ~$206k; at 9%: ~$334k. The spread is the point — run your own assumptions above rather than trusting any single projection.
What annual return should I assume?
There's no correct number — only defensible ranges. Long-run US stock history averages high single digits nominal (before inflation and fees); diversified portfolios with bonds run lower. Planning with 5–7% net is common practice; planning with 12% is how disappointment gets scheduled.
Is starting early really that powerful?
Brutally: $500/month for 40 years projects ~$1.31M, but starting 10 years later cuts it to ~$610k — half the outcome for 25% fewer contributions. Years compound; catch-up contributions mostly don't.
Lump sum or monthly investing?
Mathematically, money invested earlier compounds longer, favoring the lump sum when you have one. Monthly investing (dollar-cost averaging) wins behaviorally — it automates the habit and softens bad-timing regret. This calculator handles both at once.
Does this include taxes and fees?
No — it projects gross growth at your assumed return. Account type decides taxes (401(k)/IRA defer or exempt them; taxable accounts don't), and fund fees subtract directly from returns. Model fees by lowering the assumed return; taxes by the account you'll actually use.
Why does my result differ from other calculators?
Compounding convention: monthly here (matching monthly contributions); some tools compound annually, which lands slightly lower. Over long horizons the assumption gap between 6% and 7% dwarfs the convention gap — assumptions, not conventions, deserve your scrutiny.
Methodology. This calculator uses standard financial formulas used across the industry. It is reviewed and maintained by the Vast Calculators editorial team.
Last updated · July 11, 2026
Disclaimer. This tool provides estimates for general informational purposes only and is not a substitute for professional financial advice. Always consult a qualified financial advisor before making decisions about your finances.