Cost Per Share Calculator
Ask most investors what a stock cost them and they will quote the last price they paid, or the first. Neither is the number that decides whether they are ahead. This cost per share calculator gives you the real figure: everything you invested in a holding, divided by every share you received for it. That average is your break-even price, the line the market price has to beat before you have made anything at all.
The tool takes just two totals, so it handles single purchases, repeated buys at different prices, dividend reinvestment, and fractional shares with the same arithmetic. What changes between those cases is how carefully you build the two totals, and that is where most of the guidance on this page sits.
One holding at a time. An average blended across different stocks, funds, or coins describes nothing you actually own, so run each position separately.
Quick Answer
- Quick Answer
- Building the two totals for a multi-buy holding
- The formula, and why it is a weighted average
- Worked example: two buys, then averaging down
- Where a single average helps, and where it misleads
- What competing calculators miss
- Frequently Asked Questions
- Add This Calculator to Your Website
- References
Building the two totals for a multi-buy holding
For a single purchase there is nothing to build: the amount you paid and the shares you received go straight in. The work starts with the second purchase. List every buy for this one holding, multiply shares by price for each, then add the amounts and add the share counts separately. Fees belong in the amounts column if you want the figure your broker would recognise as basis.
List each purchase. Broker confirmations or your account history give the shares and price for every lot, including reinvested dividends.
Total the money. Shares multiplied by price for each lot, plus commissions and fees, summed into one total investment figure.
Total the shares. Every share received across all lots, including fractional shares from reinvestment, summed into one count.
Divide. Total investment divided by total shares is the weighted average cost per share. Enter the two totals above and the calculator does this step.
Once you know the average, the natural next question is what a sale returns after costs, which the stock sell calculator works through with proceeds and profit on a disposal.
The formula, and why it is a weighted average
One division produces the answer, and the weighting is hidden inside the totals.
That weighting is the whole point. A simple average of purchase prices treats a 10-share buy and a 200-share buy as equals, which they are not. Because each lot enters through its total cost, a large cheap purchase pulls the average down far more than a small cheap purchase does. This is the same average basis arithmetic the IRS describes for mutual fund shares in IRS Publication 550 on investment income and expenses: total cost of all shares owned, divided by the number of shares, gives the per-share basis. For ordinary stocks the tax rules on a partial sale usually look lot by lot instead, using first-in first-out or the specific shares you identify, a distinction the IRS overview of capital gains and losses covers. The per-share average remains the right figure for judging performance and break-even either way.
Corporate actions rewrite the inputs, not the logic. A split multiplies the share count and divides the per-share cost by the same factor. A return of capital reduces your total basis. Reinvested dividends add both cost and shares. When any of those happen, rebuild the two totals from your records rather than adjusting the old average by feel. The adjusted share price calculator handles the split side of that bookkeeping directly.
Worked example: two buys, then averaging down
The calculator is pre-filled with this case, and every figure below is what it returns for those inputs.
| Purchase | Shares | Price | Amount |
|---|---|---|---|
| First buy | 10 | $50.00 | $500 |
| Second buy | 20 | $30.00 | $600 |
| Totals entered | 30 | $1,100 | |
| Cost per share ($1,100 / 30) | $36.67 |
Notice where $36.67 sits: much closer to $30 than to $50, because two thirds of the shares arrived at the lower price. An unweighted average of the two prices would say $40.00 and overstate the break-even by $3.33 on every share, $100 across the holding.
Now extend the same holding. The price falls to $25 and you buy 20 more shares for $500. New totals: $1,600 invested, 50 shares held. Run those through the calculator and the cost per share becomes $32.00. That is averaging down shown honestly: the break-even fell by $4.67, and it cost another $500 of exposure to the same stock to get it there. Whether that trade was wise depends on why the price fell, a question no average can answer. For the execution side of repeated buying, the average execution price calculator looks at the prices your orders actually filled at.
Where a single average helps, and where it misleads
Where it helps
- Break-even checks at a glance: one comparison against the market price tells you if the position is ahead.
- Dollar-cost averaging records: each scheduled buy updates the same two totals, so the average stays current with no spreadsheet gymnastics.
- Dividend yield on cost: annual dividend per share divided by your cost per share shows the income return on what you actually paid.
- Reinvestment tracking: fractional shares from a DRIP fold into the totals instead of sitting in a separate mental account.
Where it misleads
- Partial sales for tax: stocks usually settle by lot (first-in first-out or specific identification), so the average is not the basis the tax form uses.
- Mixed holdings: one average across different companies or assets is a meaningless number, however tidy it looks.
- Decision making by itself: a falling average feels like progress even when it only records buying more of a deteriorating business.
- Ignoring selling costs: break-even here is before disposal fees and taxes, so the true exit line sits slightly higher.
Used inside those limits, the figure earns its place on a portfolio review checklist: update it after every purchase, reinvestment, and split, and compare it with the market price only for the holding it belongs to.
What competing calculators miss
Most stock average tools reviewed for this page accept row after row of individual trades. That is genuinely useful for long histories, and it is also where their guidance stops.
Miss 1: no warning about the unweighted trap. Several guides tell readers to average their purchase prices without stressing share counts. The worked example here prices that mistake exactly: $40.00 claimed against a true $36.67.
Miss 2: fees treated as an afterthought. Tools rarely say whether commissions sit inside or outside the total. This page states the rule plainly: include them in total investment if the figure is meant to be your break-even or your basis.
Miss 3: no split or reinvestment handling. Averages go stale silently after a 2-for-1 split or a year of reinvested dividends. The formula section here names each event that forces a rebuild of the totals, so the number you quote is the number you own.
One honest limit in the other direction: because this tool takes totals rather than trade rows, it will not catch a lot you forgot to add. If your broker shows a different share count than your own total, believe the share count discrepancy is yours to find before you trust the average.
Frequently Asked Questions
Cost per share is the average amount you paid for each share you own: total investment divided by number of shares. If you invested $1,100 for 30 shares, your cost per share is $36.67. It is your break-even price before selling costs: above it you are ahead, below it you are behind.
Add up what every purchase cost, add up how many shares every purchase delivered, and divide the first total by the second. Ten shares at $50 ($500) plus twenty shares at $30 ($600) is $1,100 for 30 shares, or $36.67 per share. Never average the two prices directly: $50 and $30 averaged as prices would give $40.00, which ignores that you bought twice as many shares at the lower price.
Only if you include them in the total investment figure, and for a true break-even you should. Commissions, platform fees, and transaction charges are part of what the shares cost you. Add them to the purchase amounts before dividing, and your cost per share will match the basis your broker reports more closely.
Yes. The share count accepts decimals, so 5.25 shares works exactly the same way: divide your total investment by 5.25. Fractional purchases from dividend reinvestment or automatic investing plans are one of the main reasons investors need this calculation at all, because no single trade price describes the holding.
Cost per share is the per-share form of your cost basis. Your total basis is everything you paid, including reinvested amounts and fees; dividing it by the shares you hold gives the per-share basis. For tax, the method matters: according to IRS Publication 550, stocks generally use first-in first-out or specific identification when you sell part of a holding, while mutual fund shares can elect the average basis method, which is this same division applied to the whole fund holding.
It moves toward the price of the new purchase, weighted by size. Buying more below your current average lowers it, which investors call averaging down. Buying above it raises it. In the worked example on this page, adding 20 shares at $25 to a $36.67 average produces a new average of $32.00 across 50 shares.
Yes, mechanically, but it does not change what you invested. In a 2-for-1 split your share count doubles and your cost per share halves: $36.67 across 30 shares becomes $18.33 across 60 shares, with the same $1,100 total. Recalculate with the new share count so the per-share figure stays comparable with the post-split market price.
No. Market price is what the share trades for now and it moves all day. Cost per share is your own historical average and it only changes when you buy, receive reinvested shares, or go through a split or similar corporate action. The gap between the two figures is your unrealized gain or loss per share.
The division works for any holding where you know the total spent and the units received: ETF shares, mutual fund shares, or units of a cryptocurrency. Keep each holding separate, because an average blended across different assets has no meaning. For mutual funds, check whether your fund account already reports an average basis, since that is the figure used at tax time.
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References
- Internal Revenue Service, "Publication 550: Investment Income and Expenses": basis rules for stocks and funds, including the average basis method for mutual fund shares. IRS: Publication 550. Accessed October 2026.
- Internal Revenue Service, "Topic No. 409: Capital Gains and Losses": how gains and losses are classified and reported when shares are sold. IRS: Topic No. 409 Capital Gains and Losses. Accessed October 2026.
- Internal Revenue Service, "Publication 551: Basis of Assets": what counts toward basis and the adjustments that raise or lower it. IRS: Publication 551 Basis of Assets. Accessed October 2026.
- Internal Revenue Service, "Topic No. 703: Basis of Assets": a short statement of the basis concept used across capital asset reporting. IRS: Topic No. 703 Basis of Assets. Accessed October 2026.
- Internal Revenue Service, "Instructions for Form 8949": how each sale of shares is listed with its basis before totals reach Schedule D. IRS: Instructions for Form 8949. Accessed October 2026.
- Internal Revenue Service, "Instructions for Schedule D": netting capital gains and losses across a tax year. IRS: Instructions for Schedule D. Accessed October 2026.
- US Securities and Exchange Commission, Investor.gov, "Cost Basis" glossary entry: a plain-language definition of basis for investors. Investor.gov: Cost Basis Glossary. Accessed October 2026.
- US Securities and Exchange Commission, Investor.gov, "Dollar Cost Averaging" glossary entry: the scheduled-buying approach whose average this calculator tracks. Investor.gov: Dollar Cost Averaging Glossary. Accessed October 2026.
- US Securities and Exchange Commission, Investor.gov, "Stocks" investment products guide: what a share represents and how stock ownership works. Investor.gov: Stocks. Accessed October 2026.
- FINRA, "Cost Basis: You Figure" investor guidance: how basis is tracked, reported by brokers, and why records matter at sale time. FINRA: Cost Basis, You Figure. Accessed October 2026.