Why your Polymarket cost basis shows more than $1.00
You won the market and the position still booked a loss, with a basis over a dollar on something that can never pay more than a dollar. The interface is not broken.
The arithmetic
An outcome share pays exactly one dollar if it resolves your way and zero if it does not. So a cost basis above 1.00 reads like an error.
It is not. What the interface shows as cost basis is total cost divided by size, and total cost includes what you paid to get filled, not just the quoted price. Once fees land on top of a fill that was already close to a dollar, the all-in cost per share crosses one.
The usual scenario
You bought late, when the outcome was nearly decided. The ask was sitting at 97 to 99 cents because the market had already worked it out. You took it anyway.
At 99 cents there is one cent of upside per share. The taker fee is more than one cent. Your all-in cost is above a dollar before the market has even resolved, so the position is guaranteed to lose money whichever way it goes, and it is guaranteed at the moment you press buy.
Winning the market and losing the position
These are separate outcomes and it is worth holding them apart. If your all-in cost was 1.02 and the position resolved in your favour, you collected 1.00 and lost two cents a share. The market went your way. The trade did not.
This is why a profile can show a winning position with a negative result, and why a win rate and a P&L can point in different directions on the same account.
Why the number changed after settlement
People often notice the loss move, say from -5.34 to -7.74, and assume something was recalculated after the fact. Usually both figures are correct and they measure different things:
- Before settlement you are seeing a live mark, your position valued against the last traded price.
- After settlement you are seeing the realised figure, valued at the actual payout with fees applied.
The gap between them is the mark converging on reality, plus the fees that were always there but were not in the live mark.
What to check on your own fill
- Look at the individual fill price, not the average. An order can fill in pieces, and one piece at 1.00 flat has no upside at any point.
- Compare the fee against your remaining upside. Buying at 98 cents leaves two cents to win, and if the fee is larger than that the trade is already decided.
- For tax purposes, keep the fee inside the position. Fees paid to open belong in cost basis, fees paid to close reduce proceeds. That way each fee reduces your gain exactly once, on the right position.
The general lesson is that on very short odds, fees are not a rounding detail, they are the entire trade. A position bought above roughly 97 cents is frequently a losing position at the moment of purchase regardless of the outcome.
Common questions
- How can a Polymarket cost basis be above $1.00?
- Because the displayed basis is total cost divided by size, and total cost includes fees. A fill near 99 cents plus a taker fee produces an all-in cost above a dollar, even though a share can only ever pay out a dollar.
- I won the market but the position shows a loss. Why?
- Your all-in cost per share exceeded the one dollar payout. You collected the maximum the share can pay and it was still less than you spent, so the market went your way and the trade did not.
- Why did my loss get bigger after the market settled?
- Before settlement you were seeing a live mark against the last traded price. After settlement you see the realised figure at the actual payout with fees applied. Both are correct, they just measure different things.
- Should I avoid buying above 95 cents?
- That is a trading decision rather than something we would advise on, but the arithmetic is worth knowing: above roughly 97 cents the remaining upside can be smaller than the fee, which makes the position a guaranteed loss at the moment of purchase whichever way it resolves.