Average Cost Basis Calculator
Blended cost across multiple purchases
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Need a fast answer on blended cost across multiple purchases? Average Cost Basis Calculator answers with the steps beside it. Enter First Quantity, First Price ($) and Second Quantity in any order — Average Cost Basis recalculates on every edit. Average Cost Basis displays each intermediate figure as well. Anyone comparing options in Finance can use Average Cost Basis to compare scenarios. Average Cost Basis runs in your browser. Finance data stays in the tab. Average Cost Basis is part of the Finance collection on CalcProMaster, alongside average cost basis calculator stock purchases investing taxes, free online average cost basis calculator and more. Run Average Cost Basis twice to find the driving input. Start with Average Cost Basis; the first number takes seconds.
What the defaults give
100 shares at $10 plus 100 shares at $12 is $2,200 across 200 shares, so the average cost basis is $11.00.
Selling at $15 would show a gain of $4.00 per share on whichever shares the method assigns to the sale.
The formula
Average basis = Σ (quantity × price) ÷ Σ quantity
Each purchase is a lot with its own price. The average method pools them into one figure, so every share sold carries the same basis regardless of which lot it came from. That is what makes it simple to administer and what makes it lose the ability to choose which lot to sell.
Worked examples
100 at $10 + 100 at $12 → $2,200 ÷ 200 = $11.00
50 at $10 + 100 at $12 → $1,700 ÷ 150 = $11.33
200 at $10 + 100 at $12 → $3,200 ÷ 300 = $10.67
100 at $5 + 100 at $12 → $1,700 ÷ 200 = $8.50
100 at $20 + 100 at $12 → $3,200 ÷ 200 = $16.00
100 at $10 + 200 at $12 → $3,400 ÷ 300 = $11.33
The weighting is the point. Adding 200 shares at $12 moves the average less than doubling the first lot at $10 would, because the newer, larger lot carries more of the total cost.
Average versus specific identification
With specific identification you choose the lots you sell, so you can realise the highest-basis shares first and keep the taxable gain low. With average cost the choice disappears and every sale carries the same basis. For mutual funds bought periodically, average cost is often the only practical record; for individual shares, a broker may let you elect either method.
Once a method is elected for a holding it generally has to be applied consistently, and changing it later needs care — the rules differ by jurisdiction. Reinvested dividends complicate the picture further: each reinvestment is a new lot with its own basis, so they have to be included or the average comes out too low and the gain too high.
What is missing
Commissions and fees paid to buy are usually added to the basis, which raises it slightly; this calculator uses the price alone. Splits and stock dividends change the share count without changing total cost, so the basis per share falls — enter split-adjusted quantities or the average will be wrong by the split factor.
The result is a per-share figure for reporting, not tax advice. Local rules on wash sales, holding periods and reporting thresholds apply on top of it.
Frequently Asked Questions
What is the average cost basis of 100 shares at $10 and 100 at $12?
$11.00. Total cost is $2,200 across 200 shares, so the average is $11 per share.
Does buying more shares change the average?
Yes, weighted by quantity. Adding 200 shares at $12 to 100 at $10 lowers the average to $11.33 because the cheaper lot now dominates.
Should commissions be included?
Usually yes, as part of the cost. This calculator uses price only, so add fees to total cost before dividing for a reporting figure.
How do stock splits affect the basis?
Total cost stays the same while the share count changes, so the per-share basis falls by the split factor. Enter split-adjusted quantities.
Is average cost better than specific identification?
It is simpler and often required for funds, but it removes the ability to sell high-basis lots first to reduce a taxable gain. The choice depends on your broker and local rules.
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