Stock Cost-Basis / Averaging Calculator

How to use this tool

1. Add each stock purchase with the number of shares and price per share.
2. Enter the current market price to see unrealized gain/loss.
3. Read the weighted average cost basis.

What is averaging down?

Averaging down means buying additional shares of a stock you already own after the price has fallen, with the intention of lowering your average cost per share. If the stock eventually recovers to a price above your new average, you profit (or break even) sooner than you would have based on your original purchase price.

Worked example

Average cost per share = $1,100 ÷ 30 = $36.67. The stock only needs to recover to $36.67 to break even, rather than to the original $50.00.

Averaging down vs. dollar-cost averaging

Averaging down is a reactive strategy: you buy more specifically because the price has fallen. Dollar-cost averaging (DCA) is a systematic strategy: you invest a fixed dollar amount at regular intervals regardless of price. DCA reduces timing risk and emotional decision-making; averaging down requires a judgment call about whether the price decline is temporary or reflects deteriorating fundamentals.

When averaging down makes sense - and when it doesn’t

Potentially appropriate: the company’s fundamentals are unchanged, the decline is due to broad market conditions or temporary sentiment, and you have conviction about the long-term value.

Dangerous: the price is falling because of genuine deterioration in the business (a “value trap”). Adding to a losing position in a structurally impaired company amplifies your loss. A key discipline: only average down on positions you would buy fresh at today’s price given what you know now.

Break-even price with transaction fees

The simple average cost per share formula ignores transaction fees (commissions, spreads). Including fees, your true break-even is:

Break-even price = (total cost including all fees) ÷ total shares

For example, if you paid $5 commission on each of two purchases in the worked example above ($500 + $600 + $10 fees = $1,110 total for 30 shares), your true break-even is $1,110 ÷ 30 = $37.00 - slightly higher than the unadjusted $36.67. At low commission rates ($0–$1 per trade with modern brokers), this difference is small but is worth factoring in for smaller positions.

Position size warning

Averaging down increases the concentration of your portfolio in a single position, which amplifies both potential gains and losses. Key risk management guidelines:

• Many professional portfolio managers limit any single position to 5–10% of portfolio value. Averaging down into a falling position can breach this limit quickly.
• Before adding to a losing position, calculate what percentage of your total portfolio it will represent after the new purchase.
• Consider setting a maximum additional investment rule (e.g., "I will average down at most once, and never add more than my original position size") to limit downside exposure.