Average Cost Basis Calculator
Bought the same stock at different prices? Add each lot below to find your blended average cost — the price your position needs to reach to break even.
Purchases
Lot 1@
$
Lot 2@
$
Average Cost / Share
$47.33
Total Shares
150
Total Cost Basis
$7,100.00
What this means: Across 2 purchases you own 150 shares at an average cost of $47.33.
Cost Basis by Lot
Weighted average cost
Avg Cost = Σ(Sharesᵢ × Priceᵢ) / Σ SharesᵢTotal dollars spent divided by total shares owned — a share-weighted (not simple) average of your purchase prices.
How It Works
- 1Each purchase ("lot") contributes its dollar amount to your total cost basis.
- 2The average is weighted by shares — a 500-share lot moves it far more than a 10-share lot.
- 3Averaging down lowers your breakeven but increases your exposure; make sure the thesis still holds.
Frequently Asked Questions
Is average cost how taxes are calculated?
For stocks, the IRS default is FIFO (first-in, first-out), though you can elect specific-lot identification. Average cost is the default for mutual funds.
What is averaging down?
Buying more after a price drop to lower your average cost. It cuts your breakeven but concentrates risk in a falling position.
Do reinvested dividends count?
Yes — each reinvestment is a new lot at that day’s price and adds to your cost basis (reducing future taxable gain).
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Disclaimer: These calculators are for informational and educational purposes only and do not constitute financial, tax, or investment advice. Results are estimates based on your inputs. Consult a qualified professional before making financial decisions.