Calculator Wizard
How Investment Growth Compounds
This calculator combines two things: your initial investment growing on its own through compound returns, and regular monthly contributions that each get their own time to compound. The formula extends the compound interest math from our Compound Interest guide to include ongoing contributions, not just a single lump sum.
Where P is your initial investment, PMT is your monthly contribution, r is the annual return rate, n is 12 (monthly compounding), and t is years.
Worked Example
Example: $5,000 initial, $300/month, 7% annual return, 20 years
Total contributed over 20 years: $5,000 + ($300 × 240 months) = $77,000
Future value after compounding: $176,471.69
That means $99,471.69 — well over half the final total — came purely from investment growth, not from money you actually put in.
Why Starting Early Matters More Than the Amount
Because each contribution compounds for however many years remain until your target date, a dollar contributed early has far more time to grow than the same dollar contributed later — meaning time in the market, not just the amount contributed, is one of the biggest levers in long-term investment growth.
What return rate should I use?
That depends entirely on what you're investing in and your own assumptions — this calculator doesn't recommend a rate. Historical long-term averages for different asset classes vary widely, and past performance never guarantees future results.
Does this account for taxes or fees?
No — this shows gross growth before any taxes, investment fees, or expense ratios, which can meaningfully reduce actual returns depending on the account type and investments involved.
Want the underlying compound interest math explained?
Read the Compound Interest Guide