S&P 500 DCA Calculator

The simulation

• Employer match is a flat dollar amount added to every purchase, like a 401(k) match. Total invested, ROI and annualised return count it as money invested, so they show what the account earned, not the return on your own contributions alone.
• “Monthly” means every 30 days, not the same date each calendar month — about 12.2 buys a year, and the date drifts earlier over time. Weekly is 7 days, bi-weekly is 14.
• Each purchase fills at that UTC day’s closing price — the most recent close on or before the date, never a later one. Intraday highs and lows are ignored.
• No fees, spreads, or taxes. A platform charging ~1% a buy would leave you ~1% fewer shares every time, so on costs alone every figure here is optimistic. Nothing is ever sold — profit is unrealised.
• Buying continues through today, so an earlier start date also means more total dollars invested. Compare runs on ROI, not on profit.

The price data

• Purchases fill at SPY’s actual closing price — what the SPDR S&P 500 ETF really traded at, adjusted for share splits.
• Dividends are a switch on the plan. With Reinvest dividends ticked — the default — every distribution buys more shares at the closing price on its ex-dividend date. A real fund reinvests on the later pay date, so this is an approximation of one, not a replica. Switched off, dividends are left out entirely, neither reinvested nor counted as cash: SPY pays roughly 1–2% a year, so every figure then understates what a dividend-reinvesting fund would have earned.
• SPY’s 0.09% expense ratio is already reflected in its price, so there is nothing further to deduct for it.

Reading the results

• Average Cost per share lands below the average price you actually paid — a fixed dollar amount buys more shares when the price is low. That gap is dollar-cost averaging doing its job.
• Return on Investment is the total across the whole duration. Annualised Return is the money-weighted rate (XIRR) — it accounts for when each contribution actually went in, so it is the figure to compare runs of different lengths on. Don’t divide ROI by the number of years.
• Portfolio Value prices your whole stack at the most recent daily close. In the history table, Value on Date prices it at that row’s date instead — which is why the last row and the summary differ.