Project portfolio growth from your starting balance, monthly contributions, and expected return. Includes inflation-adjusted real value, year-by-year breakdown, and the contribution-vs-growth split.
Compound interest is the single most powerful concept in personal finance, and most people radically underestimate it. $500/month at 7% real return for 30 years becomes $612,000. That's $180,000 of contributions and $432,000 of pure growth. Starting 10 years earlier — $500/month at 7% for 40 years — is $1,318,000. Same monthly amount, 40% more time, more than double the balance.
This calculator runs the numbers your way. Inputs: starting balance, monthly contribution, expected annual return, years until withdrawal. Output: final balance in nominal dollars, final balance in today's dollars (inflation-adjusted real value), year-by-year breakdown showing how much of each year's gain came from contributions vs. compounding, and a chart of the trajectory.
Default assumptions: 7% nominal return (long-run US stock historical average), 2.5% inflation. You can change either. For more conservative projections, use 5% real return (5% above inflation). For accumulation phase, contributions are pre-tax; the calculator handles the conversion to after-tax projected spending power if you specify your retirement tax bracket.
Step 1 — Enter your starting balance and monthly contribution. Starting balance can be zero. Monthly contribution is what you'll put in each month going forward (the calculator assumes it stays constant in real terms).
Step 2 — Set your expected return and time horizon. 7% nominal is the US large-cap historical average since 1928. Bond-heavy portfolios are closer to 4-5%. All-stock global portfolios are also around 7%. Adjust to match your actual allocation.
Step 3 — Read the breakdown. The output shows final balance, plus the inflation-adjusted 'real' value (what it'd buy in today's dollars). The year-by-year breakdown shows the contribution-vs-growth split — in early years contributions dominate, in later years compounding dominates (the inflection point is usually around year 15-20).
Enter starting balance, monthly contribution, expected annual return, and years. The calculator returns the final balance with a year-by-year breakdown of contributions vs. compounded growth.
Open the calculator →Long-run US stock historical: ~7% real (after inflation), ~10% nominal. Long-run US bonds: ~2% real, ~5% nominal. Global stocks: ~6% real. The widely-quoted '10% return' is nominal — it includes inflation. For multi-decade projections, real return is the cleaner mental model (it lets you compare to today's spending). The calculator defaults to 7% nominal / 5% real for stock-heavy portfolios.
Divide 72 by your annual return rate to get the years to double. At 7% return, money doubles in about 10.3 years. At 10%, about 7.2 years. At 4%, about 18 years. Powerful for quick mental math: a 25-year-old's $10k Roth IRA, at 7% real, doubles to $20k by 35, $40k by 45, $80k by 55, $160k by 65 — without adding a penny. The rule overestimates slightly at high rates and underestimates at low rates, but it's accurate within 5% for normal investment ranges.
Depends on the account type. Roth IRA, Roth 401(k), HSA (qualified medical): no tax ever, use pre-tax numbers. Traditional 401(k), IRA: balance is pre-tax; multiply by (1 - retirement tax bracket) to get spendable. Taxable brokerage: dividend/interest taxed annually, capital gains at withdrawal — the calculator simplifies as ~85% of pre-tax for long-term-hold. The calculator has a tax-adjustment toggle.
Year 10 is when growth starts visibly outpacing contributions. Year 20 is when growth dominates. Year 30 is when contributions become rounding error compared to growth. The most powerful single move in personal finance is starting early — 10 years of head start at 7% real is worth about 2x your final balance vs. starting later. After that, the levers in order of impact are: savings rate, time, and (a distant third) return assumption.