Stock average calculator (average down)

What your average price is

If you bought the same stock, fund or coin more than once at different prices, you no longer have a single entry price. You have an average price (also called average cost or cost basis per share): the price at which the whole position would break even, before costs.

It is a weighted average. Buying 100 shares at $50 and 300 at $40 does not give an average of $45. It gives $42.50, because three quarters of the shares were bought at the lower price. The simple average of the prices is only correct when every purchase was the same quantity.

Averaging down means buying more after the price has fallen, which lowers your average. Averaging up means adding after a rise, which raises it. Dollar-cost averaging is the scheduled version: a fixed amount at fixed intervals whatever the price, which automatically buys more units when the price is low. All three are just repeated purchases; this calculator works out where they leave you.

The formulas

Average price = (P₁ × Q₁ + P₂ × Q₂ + … ) ÷ (Q₁ + Q₂ + …)

where P is the price and Q the quantity of each purchase. Unrealised profit or loss at the current price C is then (C − average) × total quantity.

To find how many extra units N to buy at the current price C so that the new average equals a target T, set the new average equal to T and solve:

(cost + N × C) ÷ (quantity + N) = T  →  N = quantity × (average − T) ÷ (T − C)

Two details change the answer slightly in real life. Commission: if your broker charges a fee per purchase, add it to that purchase's cost (or enter a fractionally higher price) and the average will reflect the true cost basis. Partial sales: selling some of the position does not change the average price of the units you keep, under the average-cost method most brokers display. It only reduces the quantity. So if you sell, enter the remaining quantity at the same average as a single row and carry on from there.

The formula only gives a positive answer when T lies strictly between the current price and your average. That is the arithmetic behind the rule above: each extra unit bought at C pulls the average towards C, ever more slowly, and never past it. If you ask for a target at or below the current price (when averaging down) or at or above it (when averaging up), the calculator tells you it cannot be reached rather than returning a negative or infinite number.

A worked example

A hypothetical position: you bought 100 shares at $50, then another 100 at $40 after a fall. The stock now trades at $38.

1. Total cost: 100 × $50 + 100 × $40 = $9,000 for 200 shares.
2. Average price: $9,000 ÷ 200 = $45.00.
3. Position now: 200 × $38 = $7,600, an unrealised loss of $1,400 (−15.56%).
4. Target average $42: N = 200 × ($45 − $42) ÷ ($42 − $38) = 200 × 3 ÷ 4 = 150 shares, costing $5,700.
5. After the buy: 350 shares for $14,700, which is exactly $42 each.

Notice what happened to the risk. Before the extra buy, $7,600 was exposed to the stock. After it, $13,300 is. The break-even moved $3 closer, and the amount you lose for every further dollar the stock falls rose from $200 to $350.

A target of $40 would need 200 × 5 ÷ 2 = 500 shares, and $39 would need 1,200. A target of $37 is impossible at $38: no number of shares bought at $38 brings the average below $38. Try those in the calculator to see the requirement climb.

When averaging down helps, and when it hurts

Averaging down is a tool, not a rescue plan. It makes sense when buying in stages was the plan from the start: a long-term investor adding to a broad index fund on a fixed schedule, or a trader who deliberately splits an entry across a price zone with the total size and the stop decided in advance. The lesson on scaling in and out covers that approach.

It becomes dangerous when it is a reaction to a loss. Adding to a losing trade to make the average "look better" increases the size of a position the market is already telling you is wrong, and a stock can keep falling long after it looks cheap. Single shares can and do go to zero; leveraged positions can be closed out by the broker well before that.

Practical rules that keep it honest:

• Decide the maximum total position, in money, before the first buy, and never let averaging push you past it.
• Keep a stop or an exit condition for the whole position, and size it so the loss at that point fits your per-trade limit. The drawdown calculator shows how much harder a deep loss is to recover from.
• Count commission. Each extra purchase adds a fee, and some tax rules require it to be included in your cost basis.
• Ask whether you would open the position fresh at today's price. If not, a lower average is not a reason to add.

For crypto the maths is identical, and quantities can be fractional: enter 0.25 for a quarter of a coin. The calculator rounds nothing except the "whole shares" suggestion, which rounds up so the target is reached.

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