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What it costs to open a position on Aster
Verified against Aster's official docs on 2026-09-29. Trading fees are separate; see Aster fees.
The margin an order needs on Aster is not just notional ÷ leverage. The docs add an open loss: the amount you would be down the moment the order fills, measured against the mark price. The cost of opening is initial margin plus open loss.
The open-loss rule
Open loss = contracts × |min[0, direction × (mark price − order price)]|, where direction is 1 for a long and −1 for a short. In words: a buy priced above the mark, or a sell priced below it, pays the gap up front. An order at or better than the mark has no open loss.
Limit orders: the docs' example
A 1 BTC long limit at 102,990.0 with 20x leverage and a mark of 102,988.4. Initial margin is 102,990.0 ÷ 20 = 5,149.50. The buy is 1.6 above the mark, so open loss is 1.6 and the cost is 5,151.1. The same order as a short has no open loss and costs 5,149.50.
Market orders: the 0.05% buffer
For a market order Aster first estimates the entry price: best ask × (1 + 0.05%) for a long, and the higher of best bid and mark price for a short. In the docs' example (ask 102,946.8, bid 102,946.9, mark 102,941.0) the long is estimated at 102,998.27. That is 57.27 above the mark, so the long needs 5,207.1835 against 5,147.345 for the short, about 1.2% more.
The docs call this an estimated entry price, used to size the margin. Because a market short is estimated at no lower than the mark price, it never carries open loss in this calculation; a market long nearly always does.
FAQ
Is open loss a fee?
The docs present it as extra margin needed to open, on top of initial margin, not as a trading fee.
Do shorts ever pay open loss?
A short limit order priced below the mark does, by the same formula. Market shorts do not, as explained above.
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