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Stock Average Calculator

Find the weighted average cost per share across several purchases, and see what price you need to break even or hit a target on the whole position.

How to use this calculator

  1. 1List each purchase on its own line as "shares, price" — for example "100, 50.25".
  2. 2Add the current price to see what the position is worth now and whether it is above or below your average.
  3. 3Read the average cost as your break-even: selling the whole position there returns what you paid, before costs.
  4. 4Optionally enter a target price to see what the whole holding would be worth if it got there.

How the calculation works

Average cost = Σ (shares(k) × price(k)) ÷ Σ shares(k)
shares(k)
How many shares the kth purchase bought
price(k)
The price paid per share in that purchase
Σ shares(k)
The total number of shares now held

This is a weighted mean, not a plain one. Averaging the prices themselves is only correct in the special case where every purchase bought the same number of shares — otherwise a small buy at an extreme price distorts the answer badly.

The average cost is also the break-even price for the position as a whole, before trading costs. Selling the entire holding at exactly this price returns exactly what was paid.

Averaging down lowers the average cost; averaging up raises it. Neither changes the value of what is already held — they change the base against which future gains and losses are measured.

Worked example

Three buys at falling prices

  1. 1.Costs: 100 × 50 = 5,000; 50 × 40 = 2,000; 75 × 32 = 2,400. Total = 9,400.
  2. 2.Shares: 100 + 50 + 75 = 225.
  3. 3.Average cost = 9,400 ÷ 225 = 41.7778 per share.
  4. 4.A plain average of 50, 40 and 32 would give 40.67 — wrong, because the 100-share buy at 50 carries more weight than the 50-share buy at 40.

Result: 41.7778 average across 225 shares

Why the weighting matters

  1. 1.A large buy of 1,000 shares at 20 costs 20,000; a tiny buy of 10 shares at 200 costs 2,000.
  2. 2.Total: 1,010 shares costing 22,000, so the average cost is 21.78.
  3. 3.A plain average of the two prices would give (20 + 200) ÷ 2 = 110 — five times the true figure, because it treats a 10-share purchase as equal to a 1,000-share one.

Result: 21.78, not the 110 a plain average gives

Why a plain average is the wrong tool

Someone who buys a share three times naturally reaches for the average of the three prices. That answer is correct only in the special case where each purchase bought exactly the same number of shares, and wrong — sometimes dramatically — in every other case.

The reason is that the quantity is what carries the money. Buying 1,000 shares at 20 and then 10 shares at 200 gives a plain price average of 110, while the actual average cost is 21.78. The plain average treats a purchase of ten shares as equal in importance to one of a thousand. Weighting each price by the shares it bought is not a refinement of the calculation; it is the calculation.

What averaging down does and does not achieve

Buying more of a share whose price has fallen lowers the average cost of the whole position, which lowers the price at which the position returns to break-even. That much is arithmetic and is not in dispute.

What it does not do is improve the position already held. The money already lost on the original shares is lost whether or not more are bought; the new purchase is simply a fresh investment that happens to be in the same security. The honest test is therefore not "will this lower my average?" — it always will — but "is this the best available use of this money right now, given what I know today?" Framed that way, the fact of having previously bought higher is irrelevant to the decision, which is exactly the trap the technique sets.

The genuine risk is concentration. Each averaging-down purchase increases the share of the portfolio riding on one company at the moment its price is signalling difficulty. A position that was 5% of a portfolio can become 20% through a few such purchases, transforming a modest holding into the dominant determinant of the portfolio's outcome without any deliberate decision to make it so.

Average cost and the tax position

The average cost calculated here is the natural way to think about a position, but it is not necessarily the cost basis a tax authority will use when part of a holding is sold.

Jurisdictions differ, and often several methods are permitted with the choice affecting the taxable gain considerably.

  • Average cost — every share is treated as having cost the weighted average. Common for mutual funds, and the simplest to track.
  • FIFO (first in, first out) — the earliest shares bought are treated as the first sold. The default in many jurisdictions, and typically produces the largest taxable gain in a rising market.
  • Specific identification — you nominate exactly which shares are being sold, allowing the highest-cost lots to be sold first to minimise the gain. Requires records that identify each lot.

What this assumes, and where it stops

Assumptions

  • Every purchase is of the same security, and all shares are still held.
  • Prices are entered as actually paid per share, in one currency.
  • Brokerage and charges are not included unless you build them into the prices entered.
  • No corporate action — split, bonus issue, merger or consolidation — has changed the share count since purchase.

Limitations

  • The average cost here is a position-management figure, not necessarily your tax cost basis, which may be determined by FIFO or specific identification depending on jurisdiction.
  • Stock splits and bonus issues change both the share count and the per-share cost. Enter post-adjustment figures, or the average will be wrong.
  • Partial sales are not modelled — this averages purchases only.
  • Trading costs are excluded, so the true break-even sits slightly above the average cost shown.

Common questions

Why not just average the prices I paid?

Because that ignores how many shares each price bought, and it is only correct when every purchase happened to be the same size. Buy 1,000 shares at 20 and 10 shares at 200 and the plain average of the prices is 110, while the real average cost is 21.78 — the plain figure treats a ten-share purchase as equally important as a thousand-share one. Weighting by quantity is what makes the answer correct.

Does averaging down actually reduce my loss?

It reduces the price at which the whole position breaks even, which is not the same thing. The loss on the shares you already own is unchanged by buying more; you have simply made an additional investment in the same company. The useful question is whether this is the best available use of that money today, judged on what you now know — not on the fact that you paid more earlier. Averaging down also concentrates more of your portfolio in one holding at the moment its price is signalling trouble.

Is my average cost the same as my tax cost basis?

Not necessarily. Many jurisdictions default to FIFO, treating the earliest shares bought as the first sold, and some permit specific identification where you nominate which lots are being sold. Each method can produce a materially different taxable gain on a partial sale. The average cost shown here is the right figure for managing the position and judging break-even; check the rules that apply to you before relying on it for a tax return.

What happens to my average after a stock split?

A split multiplies the share count and divides the price by the same factor, so the total cost of the position is unchanged while the average cost per share falls proportionally. A two-for-one split on 100 shares averaging 50 leaves 200 shares averaging 25. Enter your holdings in post-split terms — mixing pre-split and post-split figures in the same list produces a meaningless average.

Sources

Formula and content last reviewed on .

Results are estimates for information only, not professional advice.

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