Stock Average Calculator
↻ Updated 2026Add each buy lot with its shares and price to compute your weighted average cost per share and total cost basis.
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Runs entirely in your browser — nothing you enter is sent to us.How this works
How to read your average cost per share
$47.33 is your break-even, not your tax cost basis — and for individual stocks those are two different things the IRS treats differently. The average is weighted, so the 100-share lot at $50 pulls it twice as hard as the 50-share lot at $42. That's why $47.33 sits well above the $46.00 midpoint of the two prices.
How weighted average cost per share is calculated
Every lot contributes to the average in proportion to how many shares it holds, not how many rows it occupies. Multiply shares by price to get what each lot cost, add the lots together, and divide by the total shares. That's the whole calculation, and the weighting is the only part people get wrong.
The instinct is to average the prices. With two lots that produces the midpoint, and the midpoint is right only in the rare case where both lots hold identical share counts. Buy 100 at $50 and 50 at $42 and the true average is $47.33 — $1.33 above the midpoint, because two-thirds of your shares came at the higher price.
lot cost = shares × price per share total cost = lot cost₁ + lot cost₂ + … + lot costₙ total shares = shares₁ + shares₂ + … + sharesₙ average cost per share = total cost ÷ total shares (this is a weighted mean — NOT the average of the prices)
- shares
- The share count in one buy lot — the weight; a lot with twice the shares moves the average twice as much
- price per share
- What you paid per share in that lot — include the commission per share if your broker charges one — most large US brokers now charge $0 for stock and ETF trades
- lot cost
- What one purchase cost you in total — $5,000 for 100 shares at $50; $2,100 for 50 at $42
- total cost
- Every dollar spent across all lots — your cost basis — $7,100 on the defaults
- total shares
- Every share you hold across all lots — 150 on the defaults
- average cost per share
- Total cost ÷ total shares — your break-even price — $47.33; the stock must trade above this for the whole position to be in profit
The average is what the position needs to clear to break even, which makes it a useful reference and a poor decision input. It's a fact about your history, not about the stock — the market has no memory of what you paid, and a share worth $42 is worth $42 whether your average is $47.33 or $12. Anchoring on the average is how positions get held for years waiting to 'get back to even'.
Averaging down moves the number fast, and that's exactly why it deserves suspicion. Add a third lot of 150 shares at $30 and the average drops from $47.33 to $38.67 — a $8.66 improvement that feels like progress while your total invested rises from $7,100 to $11,600. The average fell because you bought more of something that got cheaper, which is either an opportunity or an escalation depending on facts this page can't see. Note what the dollar-cost averaging calculator does differently: it buys on a schedule regardless of price, which is not the same behaviour as buying because the price dropped.
Worked examples
Example: 100 shares at $50, then 50 more at $42
The calculator's default lots. A position opened at $50 and added to after the stock fell 16%.
| Lot 1100 shares × $50 | $5,000 |
| Lot 250 shares × $42 | $2,100 |
| Total shares | 150 |
| Total cost basis$5,000 + $2,100 | $7,100 |
| Average cost per share$7,100 ÷ 150 | $47.33 |
| Midpoint of the two pricesthe wrong answer — ignores share counts | $46.00 |
| Position value at $42150 × $42 | $6,300 |
| Unrealized loss$6,300 − $7,100 | −$800 |
$47.33, not $46.00. The $1.33 gap is the weighting: 100 of your 150 shares were bought at $50, so the average has to lean that way. The second purchase lowered your average by $2.67 a share — from $50.00 to $47.33 — and simultaneously increased the money at risk from $5,000 to $7,100. Both of those are true at once, which is the thing averaging down makes easy to forget.
Example: adding a third lot of 150 shares at $30
The stock keeps falling. Click 'Add buy lot' and enter 150 shares at $30 — doubling your share count at a price 40% below your first purchase.
| Lot 3150 shares × $30 | $4,500 |
| Total sharesdoubled from 150 | 300 |
| Total cost basis$7,100 + $4,500 | $11,600 |
| New average cost$11,600 ÷ 300 | $38.67 |
| Previous average | $47.33 |
| Improvement in average18% lower | $8.66 |
| Increase in money at risk63% more capital committed | +$4,500 |
The average improves 18% and the exposure grows 63%. Half your shares now sit in the newest, cheapest lot, which is why it drags the average so hard — weight follows share count. The break-even fell to $38.67, but the stock has to reach $38.67 across 300 shares instead of $47.33 across 150. A lower average is not a smaller problem; it's the same problem with more money in it.
Frequently asked questions
Can I use average cost basis for individual stocks on my taxes?
No — which surprises people who've just used a stock average calculator. The average cost basis method is permitted only for mutual funds and other regulated investment companies (certain ETFs and unit investment trusts included), and you must elect it. For individual stocks the IRS does not allow it (IRS, Mutual Funds FAQ; Publication 550).
For individual stocks the default is FIFO — first in, first out. Sell 50 shares of this position and the IRS assumes they came from the $50 lot unless you say otherwise, giving a $400 loss at $42. The alternative is specific identification: you tell your broker which lot you're selling, at or before the trade. That's where the tax leverage is, since the $50 lot realises a loss while the $42 lot realises nothing. So $47.33 is your break-even — not a number that appears on your Form 8949.
Should I average down on a losing stock?
This page won't advise you, but it will say what the arithmetic does and doesn't. Lowering your average from $47.33 to $38.67 is not, by itself, evidence of anything — you can lower your average on any stock at any price by buying more of it, including one on its way to zero.
The commonly stated distinction is whether your original reason for owning it still holds. What's worth noticing is the position-sizing consequence this calculator makes visible: adding that third lot took the position from $7,100 to $11,600, so a stock that disappointed you is now a bigger share of your portfolio than when you liked it more. Averaging down is a decision to concentrate, and the improving average tends to obscure that.
What is the wash sale rule and how does it affect my cost basis?
It stops you claiming a tax loss while staying effectively invested. If you sell at a loss and buy the same or a 'substantially identical' security within 30 days before or after the sale — a 61-day window centred on the trade — the loss is disallowed for that year (IRC §1091; IRS Publication 550).
The loss isn't destroyed, it's moved. The disallowed amount is added to the cost basis of the replacement shares and the holding period carries over, so you get it back when you finally sell without repurchasing. It matters directly to this page: sell your $50 lot at $42 for a $800 loss, then buy more shares two weeks later because the average looked attractive, and you've just triggered a wash sale on a purchase you made for entirely unrelated reasons. Averaging down within 30 days of realising a loss is the most common accidental way in.
Do commissions and fees count in my cost basis?
Yes. Purchase commissions are added to basis; selling commissions reduce your proceeds. Both cut your taxable gain, so leaving them out means overpaying tax on a gain you didn't make.
This calculator has no fee field, so fold them into the lot price yourself: 100 shares at $50 with a $10 commission is $5,010 for 100 shares — a true price of $50.10. This feels obscure now only because most large US brokers moved to $0 stock and ETF commissions around 2019, but it still bites on older lots, on options, and anywhere a per-contract or transfer fee applies.
How do I calculate average cost with more than five lots?
The page caps at five, but the arithmetic doesn't care — total cost divided by total shares, however many rows feed it. For more lots, combine them in batches: total the cost and shares for a group, then enter that group as one line with its own weighted price.
Anyone tracking many small lots — typically from a DRIP or an automatic monthly purchase — is in the situation US brokers are required to handle for you. Since 2011 they've had to report cost basis to the IRS for covered securities, so your broker's records are authoritative at tax time. This page is for reasoning about a position, not reconstructing what the 1099-B already knows.
What the average cost number doesn't tell you
One weighted mean, no context. Five things it leaves on the table.
- It isn't your tax cost basis for stocks — The IRS permits average cost only for mutual funds and RICs. Individual stocks default to FIFO, with specific identification available if you designate lots at the time of sale. The $47.33 here is a break-even, not a figure for Form 8949.
- No fees — Commissions, transfer fees and SEC fees are all part of basis and none of them have a field. Add them into the lot price manually or the average is slightly too low — and your eventual taxable gain slightly too high.
- No wash sale adjustment — If a prior loss was disallowed, its amount is bolted onto the basis of your replacement shares — so your real basis is higher than what you paid. This tool has no way to know that happened, and neither does the average it shows you.
- No corporate actions — Stock splits, reverse splits, spin-offs and return-of-capital distributions all change your share count or your basis per share retroactively. A 2-for-1 split halves your average. Every lot entered here is assumed untouched since purchase.
- It says nothing about the stock — The average is a fact about your buying history. It doesn't know the company, the price today, or whether more shares is a good idea — and treating a break-even price as a target is how a position becomes a hostage.
- ·Weighted-average cost basis method
Rates, brackets and limits here are checked against primary sources. If a number still looks off, email support@realmoneyiq.com and we'll review and fix it.
RealMoneyIQ provides free educational calculators, not financial, tax, investment or legal advice. Results are estimates based on the assumptions you enter and publicly published rates; your actual outcome will differ. Always confirm decisions with a licensed professional who knows your full situation.