Stock average down calculator.

Buying more of a stock that has fallen pulls your average cost down, but by less than most people expect and at the price of a bigger bet. This calculator gives the exact new average, how far the stock still has to rise before you are even, and how many shares it would take to reach an average you choose. It also keeps the tax record straight: the average is a planning number, not necessarily the basis the IRS uses.

How this is calculated

The new average cost is total money spent divided by total shares: (existing shares x existing average + new shares x new price + fees) divided by (existing shares + new shares). To reach a target average T by buying at price P, the tool solves existing shares x (existing average - T) = n x (T - P) for n, then rounds up to whole shares. It only works when the target is below your current average and above the new buying price. The breakeven rise is the new average compared with the new buying price.

The tax side is different. IRS Publication 550 says that when you cannot adequately identify the shares you sell, the basis is that of the shares you acquired first, and that except for certain mutual fund shares you cannot use the average price per share to figure gain or loss. Commissions are part of cost. Publication 550 also says a loss on a sale is disallowed as a wash sale if you buy substantially identical stock within 30 days before or after, with the disallowed loss added to the cost of the new shares. As of 2026-10-26, the cost-basis rules are taken from Publication 550 (2025).

Worked examples (hypothetical figures, real arithmetic)

Doubling down. You own 100 shares at an average of $50 and buy 100 more at $40. Total cost is $5,000 + $4,000 = $9,000.00 for 200 shares, a new average of $45.00. The average fell 10.0%, though the price fell 20%. The stock must now rise 12.5% from $40 to get you even, and at $40 you are down $1,000.00.

Reaching a target. To bring the average to $45 by buying at $40 you need 100 x ($50 - $45) / ($45 - $40) = 100 shares. For a $44 target the same formula needs 100 x 6 / 4 = 150 shares.

Smaller add with fees. Buying 50 shares at $40 with $10 of fees on top of 100 at $50 gives a cost of $7,010 over 150 shares, a new average of $46.73.

Mistakes people make

• Thinking a lower average is a gain. You still own the shares at the market price; only your breakeven moved.
• Adding to a bad idea. The calculator cannot tell whether the price fell for a reason that is still there.
• Using the average for taxes. For ordinary stock the IRS does not allow average basis; identify lots or the first shares bought are treated as sold first.
• Tripping the wash sale rule. Selling at a loss and buying back within 30 days disallows the loss for now.

Who this is for, and what it does not cover

It is for an investor adding to a single stock position. It does not decide whether you should buy, measure risk, or handle dividend reinvestment lots, stock splits, or lot-level tax reporting. Mutual fund shares have their own average-basis election under Publication 550.

Sources

Retrieved 2026-10-26. Each link was opened and checked against the claim next to it.

• IRS Publication 550 (2025), Investment Income and Expenses: basis when shares cannot be identified (first acquired), no average price for stock except certain mutual fund shares, commissions in basis and the wash sale rule.