Future Value Calculator
Grow a starting balance plus regular monthly contributions over time.
Input sheet
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See what a starting balance plus steady monthly contributions can grow into over years of compound returns — the core engine behind retirement accounts, brokerage saving, and any long-term investing plan.
How it works
Lump sum grows by (1+r)ⁿ; level monthly deposits use the future-value-of-annuity factor ((1+r)ⁿ − 1) ÷ r. r is the monthly rate.
Two things grow at once. Your lump sum compounds on its own, and each monthly deposit becomes its own little lump sum that compounds from the day it lands. The calculator adds both streams and separates how much you contributed from how much was pure growth.
Deposits are treated as end-of-month (an ordinary annuity), the standard convention. Because earlier dollars have more time to compound, the growth portion accelerates the longer your horizon — which is why starting early beats contributing more later.
Future value = starting amount × (1 + monthly rate) raised to the number of months, plus monthly contribution × (((1 + monthly rate) raised to the number of months, minus 1) ÷ monthly rate). Monthly rate is the annual return ÷ 12; months is years × 12.
Worked examples
$5,000 start, $200/month, 7% annual return, 20 years. → $124,379 future value; $53,000 contributed; $71,379 from growth.
You put in $5,000 plus 240 × $200 = $48,000, for $53,000 total contributed. Compounding at roughly 0.583% per month turns that into about $124,379 — meaning more than half the ending balance is growth, not your own deposits.
Tips & gotchas
- The annual return is an assumption, not a promise. Run a conservative case (5-6%) and an optimistic one (8-9%) to see the range you might land in.
- Time is the strongest lever here. Adding five years to the horizon usually beats adding a large chunk to the monthly contribution.
- Returns shown are nominal. If you want today's-dollars purchasing power, subtract your inflation assumption from the return before entering it.
- Inside a tax-advantaged account (401k, IRA) the growth compounds untaxed; in a taxable account, taxes on dividends and gains will trim the real result.
FAQ
Are contributions made at the start or end of the month?
This assumes end-of-month (ordinary annuity), the common default.
Does this account for taxes or fees?
No. It models gross compound growth. Investment fees and taxes in a taxable account would reduce the final figure, so treat the output as a pre-tax, pre-fee ceiling.
What return should I assume for stocks?
Long-run diversified stock returns have historically averaged in the high single digits before inflation, but any given 20-year stretch can differ. Pick a rate you can defend and stress-test it.
What if I can only contribute occasionally?
This assumes level monthly deposits. For lumpy contributions, approximate by averaging your yearly total into a monthly figure, or run separate scenarios for each lump.
How does increasing my contribution compare to a higher return?
Early on, contributions dominate the balance; later, growth dominates. Over long horizons a higher return compounds harder, but it is the one variable you cannot control — so most savers focus on contributions and time.
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