Dollar-Cost Averaging Calculator: Run Your Monthly Investing Plan

DCA in 30 seconds

Dollar-cost averaging is the discipline of investing a fixed amount on a fixed schedule — usually monthly — regardless of price. You buy more shares when prices are low and fewer when prices are high, which produces a lower average cost per share than if you tried to time the market.

What this calculator does: take your monthly contribution and assumed annual return, and apply the standard compound interest formula to project the account balance at the end of your horizon. It also shows the inflation-adjusted (“real”) value side by side, because the nominal number on your brokerage statement overstates what you can actually buy.

1. Enter your fixed monthly DCA amount — what you’ll actually contribute every month. Employer 401(k) match counts.
2. Set a return assumption you’d defend — 7% is the long-run real S&P 500 average. Drop it to 5% for a conservative stress test.
3. Read both the nominal and real columns — the difference is what inflation quietly extracts over decades. At 7% / 2.5%, $500/mo for 30y is $612K nominal but only ~$292K of buying power.

Want to log your actual contributions (skipped months and all), instead of assuming a constant amount? Switch the tool to Actual log mode and record each month as it happens — the same compound math runs over your real deposit dates.

How the math works

For a fixed periodic contribution (constant DCA), the closed-form future value is: FV = PMT × ((1 + r/12)12n − 1) ÷ (r/12) where PMT is your monthly contribution, r is the annual return, and n is the horizon in years. If you also have a starting lump sum, the calculator adds P × (1 + r/12)12n on top.

For inflation adjustment, we divide the nominal FV by (1 + i)n. This is the step most DCA calculators skip — and it’s the step that turns “$1M nominal” into “around $400K of actual buying power” on a 30-year horizon.

Defaults: 7% annual return (historical real return of broad US equities over a century), 2.5% inflation (long-run Fed target territory). Both sliders, both editable. Source data: SPIVA scorecard for active vs index performance, FRED / Federal Reserve for inflation series.

Math runs locally. Inputs never leave your browser. Source on github.

Where DCA doesn’t help (and what to do about it)

• You already have a windfall sitting in cash. DCA on a lump sum you already hold is, statistically, worse than investing it immediately — Vanguard’s research found lump-sum beats DCA in roughly 67% of historical 12-month windows. DCA is for what you haven’t earned yet (paychecks), not for what you already have. Use the DCA vs Lump Sum tool for that decision specifically.
• You’re DCA’ing into a single stock. The “lower average cost per share” argument assumes the asset eventually recovers. For a broad index fund that’s usually a safe assumption over 20+ years. For a single company, you can DCA the whole way down to zero and never average back up.
• Your fees are eating the math. A 1.5% expense ratio sounds small but turns the same 7% return into 5.5% over 30 years — that’s roughly a 30% smaller portfolio. Run the calculator at your real, after-fee rate, not the headline return.
• You stop during drawdowns. DCA only works mathematically if you keep buying when prices fall — that’s where the lower average cost comes from. Stopping during a 30% drawdown converts DCA from a strategy into a trap.

What to actually do

1. Set a monthly contribution you can hit every month — including months you’d rather not. The amount you’ll actually maintain > the amount you’d like to maintain.
2. Automate the transfer. Friction kills DCA over decades.
3. Run the calculator with both a 5% and a 7% rate. The honest answer lives between them.
4. Pick a horizon long enough that one bad year doesn’t dominate. 20 years is a reasonable floor for equity DCA.
5. Re-run the numbers once a year. Bump the contribution if a raise hit; the calculator will show what that extra $100/mo compounds into.