What your average cost actually is
Buy 100 shares at $50, then 100 more at $30, and your average cost is not $40 in any way that matters — it is $40 only because the two purchases happened to be the same size. Average cost is a weighted average: total money spent divided by total shares held. Change the number of shares in either purchase and the average moves somewhere else entirely. This guide shows how the arithmetic works, what “averaging down” does and does not achieve, and the three mistakes that make people think a position has recovered when it has not.
The formula
average cost = total amount invested ÷ total shares, where the total includes commissions if you paid them. Worked through a three-purchase position:
| Purchase | Shares | Price | Cost | Running shares | Running average |
|---|---|---|---|---|---|
| 1 | 100 | $50.00 | $5,000 | 100 | $50.00 |
| 2 | 100 | $30.00 | $3,000 | 200 | $40.00 |
| 3 | 300 | $20.00 | $6,000 | 500 | $28.00 |
Notice what the third purchase did. It was at a price 33% below the second, but because it was three times the size, it pulled the average down by $12 — more than the second purchase moved it. The size of a purchase matters as much as its price, which is precisely the thing people estimate wrongly in their heads. The Stock Average Calculator does this for any number of buys and also shows the unrealised profit or loss at a price you enter.
How far the price has to recover
The asymmetry of percentages is the part that surprises people. A 50% fall needs a 100% rise to get back to even, because the rise is measured from the lower base.
| Fall from your cost | Rise needed to break even |
|---|---|
| −10% | +11.1% |
| −20% | +25% |
| −33% | +50% |
| −50% | +100% |
| −67% | +200% |
| −90% | +900% |
Averaging down changes the reference point of this table. In the example above, the position needs the price to reach $28 to break even instead of $50 — a far easier recovery. That is the genuine mathematical benefit, and it is the only one.
What averaging down does not do
- It does not reduce your loss. At $20 a share, the 100-share position was down $3,000. After buying 400 more, the 500-share position is down $4,000. The average cost fell; the money at risk rose. Both statements are true at once, and the first one feels better than the second deserves.
- It does not make the investment better. Price alone tells you nothing about whether the business improved. Adding to a position because it fell, rather than because your assessment of the company still holds at the new price, is a decision about your feelings rather than the asset.
- It concentrates risk. A position you keep adding to becomes a larger share of the portfolio precisely as it performs worst. Check what percentage of your total the position has become before the next purchase, not after.
- It is not the same as dollar-cost averaging. DCA is a fixed amount on a schedule, decided in advance, regardless of price. Averaging down is a discretionary reaction to a fall. They produce different behaviour under stress — see the guide on DCA versus lump sum.
Three arithmetic mistakes
- Averaging the prices instead of weighting them. Buying $1,000 at $50 and $1,000 at $10 does not give an average of $30. You bought 20 shares then 100 shares, so the average is $2,000 ÷ 120 = $16.67. Fixed-dollar buying always lands nearer the low price, because your money buys more shares there.
- Forgetting commissions and taxes. Five $10 commissions on a $2,000 position is 2.5% — the whole of a good year's dividend. Include fees in the cost basis, and remember that your break-even sell price is above your average cost once selling fees and capital-gains tax are counted. The Stock Profit Calculator works out that real break-even.
- Confusing average cost with cost basis for tax. Some jurisdictions require FIFO (first shares bought are first sold) rather than average cost, so the gain reported on a partial sale may differ from what the average suggests. Check the rule where you file before assuming.
A checklist before adding to a losing position
- Would you open this position today at this price, knowing nothing about what you paid before? If not, the purchase is about the old decision, not the new one.
- What has changed in the business, not the chart? A falling price on unchanged fundamentals and a falling price on deteriorating ones look identical on screen.
- What will the position weigh in your portfolio afterwards, and are you comfortable with that if it keeps falling?
- What is the total you are prepared to lose here, and does this purchase stay inside it? The Position Size Calculator turns that number into a share count.
- Do you have a level at which you stop adding? Deciding it afterwards never works.