Partial-Sell Calculator
Diluted cost vs break-even cost, both in one pass
Position & this sale
Net proceeds of this sale
CNY 5,991.94
Break-even cost ⓘ
8.04 CNY
If the remaining 500 shares sell at this price (sell fees included), the whole trade is flat.
Diluted cost ⓘ
10.00 CNY
The per-share cost of the remainder never changes (what you paid is what you paid — selling does not change it).
Why does my math differ from my broker's?
Three "costs" answer three different questions: break-even cost answers "at what price must the remainder sell for the whole trade to be flat" (this tool's convention, future sell fees included); diluted cost answers "what is the buy cost of the remainder" (unchanged); profit-spread folds realized gains into the cost (what brokers commonly show — that is why you see the cost "drop"). All are correct — just do not compare them with each other.
How does the cost basis change after a partial sell?
It depends on which "cost" you mean. Under the diluted-cost convention, a partial sell does not change the per-share cost of the remaining position — buy at 10, sell 500 shares, and the remaining 500 still cost 10. Under the break-even convention, realized profit or loss is spread over the remaining shares: after selling 500 shares at 12, the remainder only needs to reach about 8 (fees included) for the whole trade to be flat; after selling 500 shares at 8, the remainder must reach above 12 for the whole trade to break even.
Why does my calculation differ from my broker's?
Because most broker apps use the "profit-spreading" convention: realized gains are folded into the remaining position's cost, so you see the cost "drop" after a profitable sell. That is a display convention, not a change in what you actually paid. This tool shows all three conventions side by side (break-even cost, diluted cost, profit-spread reference) — find which one your broker uses and you will be able to derive any cost it ever shows you.
What does a negative cost mean?
When realized profit is large enough, it can exceed the total cost of the remaining position, and the break-even convention (and some brokers' displays) goes negative. The meaning is clear: even if you gave the remaining shares away at zero, the whole trade is already in profit. No need to panic at a negative cost — it is the math of "profit already locked in".