Stock Average Calculator — Calculate Average Cost Per Share

Stock averaging is the practice of calculating your average cost per share when you have bought the same stock at different prices and in different quantities. Every time you purchase additional shares, your overall cost basis shifts — and knowing your true average cost is essential for evaluating whether you are in profit, sizing positions, and making informed sell decisions.

There are two main contexts where stock averaging matters. The first is dollar-cost averaging, an intentional strategy where you invest a fixed dollar amount at regular intervals. Because you buy more shares when the price is low and fewer when it is high, your average cost per share tends to be lower than the average of the prices you paid. The second is averaging down, where you deliberately buy more shares after a price decline to reduce your break-even point. Both approaches change your cost basis, and this calculator computes the result instantly.

Understanding your average cost is also critical for tax purposes. When you sell shares, your capital gain or loss is measured against your average cost basis, not the price of any single purchase. Our cost basis calculator provides a deeper dive into tracking your total investment cost for tax and reporting purposes.

This calculator is straightforward to use. For each purchase you have made, enter the price per share and the number of shares you bought. You can add as many purchases as you need — the calculator handles two, five, or twenty transactions with equal ease.

Step by Step

Start by entering your first purchase: the price you paid per share and how many shares you bought. Click "Add Purchase" to enter your next buy. Repeat for every purchase you have made. The calculator automatically computes your total shares, total cost, and weighted average cost per share as you go.

Reading the Results

The average cost per share is your break-even price — the price at which you would neither gain nor lose money if you sold all your shares today (excluding commissions and taxes). If the current market price is above your average cost, you are in profit. If it is below, you are at an unrealized loss. The total cost shows how much capital you have deployed, and the total shares shows your complete position size.

Fractional Shares

You can enter fractional share quantities such as 2.5 or 0.75. This is useful for brokerages that support fractional share trading or when you are calculating averages for mutual fund purchases where you invest a dollar amount rather than whole shares.

The stock average formula is a weighted average: Average Cost = Total Cost / Total Shares. In expanded form: Average Cost = (Price₁ × Shares₁ + Price₂ × Shares₂ + ... + Priceₙ × Sharesₙ) / (Shares₁ + Shares₂ + ... + Sharesₙ).

For example, suppose you made three purchases of a stock: 100 shares at $50, 50 shares at $40, and 75 shares at $45. Your total cost is (100 × $50) + (50 × $40) + (75 × $45) = $5,000 + $2,000 + $3,375 = $10,375. Your total shares are 100 + 50 + 75 = 225. Your average cost per share is $10,375 / 225 = $46.11. Notice that this is not the simple average of $50, $40, and $45 (which would be $45.00) — the weighted average is higher because you bought more shares at the higher $50 price.

This same formula applies whether you are averaging two purchases or twenty. The key insight is that each purchase contributes proportionally to the final average based on how many shares it represents, not just what price was paid. See our ROI calculator to evaluate your return once you know your average cost.

Example 1: Averaging Down After a Drop

You bought 200 shares of Company X at $60 per share (total: $12,000). The stock dropped to $45, so you bought 100 more shares (total: $4,500). Your total cost is now $16,500 for 300 shares. Average cost: $16,500 / 300 = $55.00. You lowered your break-even from $60 to $55 — the stock now only needs to recover to $55 for you to break even, rather than $60.

Example 2: Dollar-Cost Averaging Into an ETF

You invest $500 every month into an S&P 500 ETF. In January the price is $400 so you buy 1.25 shares. In February it rises to $500 so you buy 1.00 share. In March it drops to $450 so you buy 1.11 shares. Over three months you spent $1,500 and own 3.36 shares. Average cost: $1,500 / 3.36 = $446.43 — lower than the simple average of $450 because you bought more shares when the price was low.

Example 3: Multiple Buys at Rising Prices

You bought 50 shares at $30, then 50 shares at $35, then 50 shares at $40. Total cost: $1,500 + $1,750 + $2,000 = $5,250. Total shares: 150. Average cost: $5,250 / 150 = $35.00. Even though you paid as high as $40, your equal-sized purchases produce an average that sits exactly in the middle of the price range.

Averaging Down Is Not Always Wise

Averaging down only makes sense if the stock's decline is temporary and its long-term thesis is intact. If the company is facing fundamental problems — declining revenue, regulatory risk, or loss of competitive advantage — averaging down means allocating more capital to a deteriorating position. Always ask why the stock fell before adding more.

Watch Your Position Sizing

Repeatedly averaging down can cause a single stock to grow into an outsized portion of your portfolio. A position that was 5% of your portfolio can quietly become 15% or 20% after several averaging-down purchases. Diversification matters — check that no single holding dominates your portfolio.

Commissions and Fees Are Excluded

This calculator computes the raw average cost per share. Brokerage commissions, SEC fees, and other transaction costs are not included. For most investors these are small relative to the share price, but they do affect your true cost basis — factor them in manually if precision matters.

Selling Does Not Change Your Average

If you sell some of your shares, the average cost per share of the remaining shares stays the same. The average only changes when you make new purchases. This is important for tax-lot accounting — understand whether your broker uses FIFO, LIFO, or specific identification when determining which shares you sold.

All content on this page is educational and does not constitute financial advice. Stock averaging is a calculation method, not a guarantee of profits. Past price movements do not predict future results. Consult a qualified financial advisor before making investment decisions.