📈 Investment Calculator
Project investment growth with compound interest and monthly contributions over time
How This Calculator Works
This calculator uses the compound interest formula: FV = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) - 1) / (r/n)] where FV is future value, P is initial principal, r is annual interest rate, n is compounding frequency (12 for monthly), t is years, and PMT is the regular monthly contribution.
The magic of compound interest is that your money earns returns, then those returns earn their own returns. Over decades, this exponential growth creates wealth. A 7% average return (historical S&P 500 average adjusted for inflation) means your money doubles approximately every 10 years. Monthly contributions accelerate growth because you're consistently buying assets regardless of market conditions (dollar-cost averaging).
This calculator shows the breakdown between your contributions (money you put in) and investment gains (money your money earned). The longer your timeline, the more gains dominate your final balance.
Example Calculation
Real-World Example: Retirement Investing at Age 25
Scenario: Start with $5,000, contribute $500/month, 7% average annual return, 40 years until age 65
Your Contributions: $5,000 initial + ($500 × 12 months × 40 years) = $245,000 total contributed
Investment Growth: Through compound interest, your $245k grows to approximately $1,310,000
Breakdown: You contributed $245k (19%), compound interest added $1,065k (81%)
The Power of Time: If you wait until age 35 to start (same $500/month, 30 years): Final balance $612,000. By starting 10 years earlier, you gain an extra $698,000 despite only contributing $60k more. Time is more powerful than contribution amount.
Key Lesson: Start investing ASAP, even with small amounts. A 25-year-old investing $200/month beats a 35-year-old investing $500/month every time.
Common Questions
What's a realistic investment return to expect?
Historical S&P 500 returns average 10% nominal (before inflation) and 7% real (after inflation) over long periods. Conservative planning uses 6-7%. Bonds return 3-5%. Cash savings accounts return 0.5-2%. For retirement planning 30+ years out, 7% is reasonable for stock-heavy portfolios. For 10-year goals, use 5-6% to be conservative. Past performance doesn't guarantee future results, but provides reasonable estimates.
Should I invest monthly or make lump-sum contributions?
Monthly contributions (dollar-cost averaging) reduce risk by spreading purchases across market highs and lows. You buy more shares when prices are low, fewer when high. Lump-sum investing performs slightly better statistically (~2/3 of the time) because markets trend upward, but requires perfect timing discipline. For most people, automatic monthly contributions from paycheck work best - you never see the money, so you don't miss it, and you avoid emotional investing decisions.
How much should I invest monthly for retirement?
General rule: Save 15-20% of gross income for retirement. For $60k salary, that's $750-1,000/month. Start with employer 401(k) match (free money), max Roth IRA ($6,500/year = $542/month for 2024), then back to 401(k). If you start at 25 and save 15% with 7% returns, you'll replace 80-100% of your pre-retirement income. Start at 35? You need 20-25%. Start at 45? Need 30-35%. The earlier you start, the less you need to save monthly.