Enter your starting amount, regular contributions, return rate, and time horizon. Get future value, total interest earned, and a year-by-year growth schedule.
📅 Year-by-Year Growth Schedule
| Year | Opening Balance | Contributions | Interest Earned | Closing Balance |
|---|
See what your investment will be worth in today's purchasing power after inflation erodes value. Shows both nominal balance (account statement) and real value (what it actually buys).
Work backwards from your financial goal. Enter your target amount and timeframe, then find either the required monthly contribution or the required rate of return.
The Rule of 72 is a quick shortcut to estimate how long it takes money to double. Divide 72 by the annual interest rate. Works best for rates between 6–10%. See the full Rule of 72 guide for worked examples, retirement-planning use cases, and how inflation affects your real doubling time.
How Investment Calculators Work — Compound Interest, Real Returns & Goal Planning
Understanding compound interest is the foundation of all long-term wealth building. This guide covers the key formulas, what return rates to use, and how inflation affects your real purchasing power.
The Compound Interest Formula
For a lump sum: FV = PV × (1 + r/n)^(n×t), where PV is starting amount, r is annual rate, n is compounding periods per year, and t is years.
With monthly contributions, each payment is also compounded: FV_contrib = PMT × [(1+r/n)^(n×t) − 1] / (r/n). The total future value is the sum of both.
What Return Rate Should I Use?
| Asset / Strategy | Historical Return (Nominal) | Inflation-Adjusted |
|---|---|---|
| S&P 500 (1957–2025, div reinvested) | ~10.7% /yr | ~6.8% /yr |
| Total Stock Market (US) | ~10.0% /yr | ~6.5% /yr |
| Global Diversified (stocks) | ~8–9% /yr | ~5.5–6% /yr |
| 60/40 Stock-Bond Portfolio | ~7% /yr | ~4% /yr |
| US Government Bonds (10-yr) | ~3–5% /yr | ~1–2% /yr |
| High-Yield Savings / CDs (2026) | ~4–5.5% /yr | ~1–2% /yr |
| US Inflation (30-yr average) | ~3.1% /yr | — |
Past performance does not guarantee future results. Use 7% for conservative stock projections, 10% for optimistic.
Why Inflation Adjustment Matters
A $1,000,000 balance in 30 years sounds impressive — but at 3% annual inflation, it has the purchasing power of only $412,000 in today's dollars. The real rate formula: Real Rate = (1 + Nominal) ÷ (1 + Inflation) − 1. At 8% nominal with 3% inflation, the real rate is approximately 4.85%.
The Cost of Starting Late
Starting 10 years earlier has a dramatic effect. Investing $300/month at 7% from age 25 to 65 (40 years) = ~$798,000. Starting at 35 and investing the same amount for 30 years = ~$378,000 — less than half. The first 10 years are the most valuable because early contributions compound the longest.
Monthly vs Lump Sum Contributions
Both strategies work well. Lump sum historically outperforms if markets trend upward (money is invested earlier). Monthly contributions (DCA — Dollar Cost Averaging) reduce timing risk and work well for regular salary earners. Most investors use a combination: an initial lump sum plus regular monthly contributions.