Average Cost Calculator
Combine up to 5 existing lots with a new buy to get your new average cost, total shares, total invested, and break-even.
Your existing lots
Live toolNew buy
Averaged down — your average cost dropped 7.89%.
Total invested excludes fees. Break-even includes purchase-side fees only — sell-side costs (any sell commission, SEC Section 31 fee, FINRA TAF) are excluded and typically negligible at $0-commission US brokers.
Planning average only — not a tax cost-basis method. US brokers generally report cost basis lot-by-lot (FIFO or specific-share identification). Not tax advice.
How it works
Enter each batch of shares you already own — up to 5 lots, with the share count and the price you paid per share. Fractional shares are fine (up to 4 decimal places), so lots from fractional-share brokers and dividend reinvestment plans work normally. A lot row only counts when both its shares and price are filled.
Then add the purchase you are considering: how many new shares and at what price. Set new shares to 0 if you just want the average of your existing lots. Optionally add the total fees paid across all purchases and today’s market price.
The calculator blends everything into one number — your new average cost per share — along with your total shares, total invested, a fee-aware break-even price, the percent your average cost changed, and (when you supply the current price) the percent move the stock needs to reach your break-even. When you’re done, one click carries your new average cost forward as the entry price in the next tool.
The formula
Average cost = total dollars spent ÷ total shares. For each lot, dollars spent is shares × price paid; add every lot (and the new buy) together on both sides of the division:
new average cost = (existing cost + new shares × new price) ÷ (existing shares + new shares)
Break-even price = (total invested + purchase fees) ÷ total shares. To break even, the stock must recover every dollar you put in — including purchase fees — spread across every share. With $0 fees, break-even equals your average cost exactly.
Average cost change % = ((new average − old average) ÷ old average) × 100. Negative means the buy lowered your average (averaged down); positive means it raised it (averaged up).
Move to break-even %= ((break-even − current price) ÷ current price) × 100 — the percent the stock must move from today’s price to reach your break-even. A result at or below zero means the current price is already at or above break-even.
Worked example
A trader holds two lots of a stock, buys a dip at $38.00, and pays $5.00 in total fees. The current market price is $38.00.
- Existing lots — lot 1: 100 × $50.00 = $5,000.00; lot 2: 50 × $42.00 = $2,100.00. Existing shares = 100 + 50 = 150; existing cost = $5,000.00 + $2,100.00 = $7,100.00.
- Old average cost = $7,100.00 ÷ 150 = $47.33.
- New buy: 100 × $38.00 = $3,800.00. Total shares = 150 + 100 = 250; total invested = $7,100.00 + $3,800.00 = $10,900.00.
- New average cost = $10,900.00 ÷ 250 = $43.60.
- Break-even price = ($10,900.00 + $5.00) ÷ 250 = $10,905.00 ÷ 250 = $43.62.
- Average cost change = (($43.60 − $47.33) ÷ $47.33) × 100 = −7.89% — the average cost dropped 7.89%.
- Move to break-even = (($43.62 − $38.00) ÷ $38.00) × 100 = 14.79% — the stock needs to rise 14.79% from $38.00 to reach break-even.
FAQ
How do you calculate the average cost of a stock?
Divide the total dollars spent across all purchases by the total number of shares. Buying 100 shares at $50 and 100 more at $38 costs $8,800 for 200 shares — an average cost of $44.00 per share.
Is my average cost the same as my break-even price?
Only when you paid no fees. Break-even spreads total cost including purchase commissions and fees across every share, so it sits slightly above the plain average whenever fees are greater than zero. (This calculator models purchase-side fees; selling can add small regulatory fees on top.)
Does averaging down lower my break-even?
Buying shares below your current average cost lowers both the average and the break-even price — but it also increases the total capital invested in the position, which raises the dollar amount at risk. Traders often weigh both effects before adding.
Can I use fractional shares in this calculator?
Yes — share fields accept up to 4 decimal places, so lots from fractional-share brokers and dividend reinvestment plans work normally.
Is average cost how my US taxes are calculated?
Generally no for individual stocks. As of 2026, US brokers typically report cost basis lot-by-lot using FIFO or specific-share identification; the average cost basis method is generally limited to mutual funds and certain DRIP shares. This tool is a planning average, not a tax document — rules vary and change, so consult a tax professional.
What is the difference between averaging down and averaging up?
Averaging down means buying below your current average cost, pulling the average lower; averaging up means buying above it, pushing the average higher. The arithmetic in this calculator is identical for both.
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Educational disclaimer
StockTools is educational software — nothing on this page is investment, trading, legal, or tax advice, and no result is a recommendation to buy, hold, or add to any position. Averaging down commits more capital to a declining position and increases the total dollars at risk; a lower average cost is arithmetic, not a fix for a losing trade — the position-size calculator above covers the risk side of any new buy. The average cost shown is a planning figure, not a tax cost-basis method: as of 2026, US brokers typically report cost basis lot-by-lot using FIFO or specific-share identification, and the average cost basis method is generally limited to mutual funds and certain DRIP shares — rules vary and change, so consult a tax professional. Break-even here models purchase-side fees only; sell-side costs (any sell commission, SEC Section 31 fee, FINRA TAF) are excluded and typically negligible at $0-commission US brokers.