Average Cost Basis Explained (And How Averaging Down Works)
By William · Swift Penguin · Educational content · Not financial advice
“What’s my average?” is one of the most common questions after a second buy. Cost basis isn’t a vibe — it’s total dollars ÷ total shares. Here’s how it works and how to use it without digging a deeper hole.
Understand weighted average cost, work a multi-lot example, and see when averaging down is strategy vs coping. Use our free average cost calculator for the math.
Weighted average, not midpoint
Average cost = sum of (shares × price) ÷ total shares. Unequal lots mean you cannot average the prices alone.
50 @ $100 + 50 @ $80 = $90 average. 20 @ $100 + 80 @ $80 = $84 average. Same prices, different basis.
Break-even before fees
Price above average cost ≈ unrealized gain (pre-fee, pre-tax). Below ≈ unrealized loss.
When you sell, use a stock profit calculator with average cost as the buy price for a clean P&L.
Averaging down rules of thumb
Only add if the thesis is intact and the add is pre-sized as a risk % of the account — not “enough to feel green again.”
If the story broke (fraud, permanent impairment, thesis invalidated), averaging down is how small losses become account problems.
Tax lots vs average
Brokers may report average cost for display while tax lots use FIFO or specific identification. Know which method you elected.
For planning a sale’s tax hit, pair lot awareness with a capital gains tax estimate.
Next steps
Concepts stick when you apply them. Open a related calculator, run your own numbers, and — if you’re still learning execution — practice with paper trades before increasing real size. Browse more guides or the full set of free trading tools.
Free tool
Average Cost Calculator
Add buys at different prices (or model averaging down) and get weighted average cost, total shares, total invested, and unrealized P&L at a mark price. The answer to “what’s my breakeven after that add?”
Other free tools
- Stock Market Simulator — Practice trading with fake money.
- Trading 212 Portfolio Charts — Connect T212 and flex your P&L.
- Crypto Prices & Charts — Live quotes + historic charts.
Put this into practice
Run the numbers on our free tools, or paper trade before you risk real money.
This is educational content, not financial advice. Investing carries risk — you can lose money. Do your own research and consider a qualified advisor for personal decisions.