What a Polymarket trade really costs
The price on the screen is not what a Polymarket trade costs. The real cost adds the spread, the taker fee where a market charges one, and slippage when an order is bigger than the best offer, and that all-in price is the win rate you need to break even.
What is the spread?
The spread is the gap between the best price to buy (the ask) and the best price to sell (the bid). Buy at 63 cents and sell straight back at 61 cents, and the spread has cost you 2 cents a share.
What is slippage?
An order book only holds so many shares at each price. A bigger order uses up the best price and moves on to worse ones, so the average price paid is above the price on the screen.
For example, if 100 shares are offered at 62 cents and 200 more at 64 cents, buying all 300 costs about 63.3 cents a share on average.
What fees does Polymarket charge?
Polymarket’s own fee page, read on 3 October 2026, says only takers pay a fee. A taker’s order fills at once against one already on the book; a maker’s order waits on the book until someone takes it, and makers pay nothing. Geopolitical and world events markets charge no fee at all, and Polymarket charges nothing to deposit or withdraw, though a service used to move the money may.
Where a fee applies, it is the number of shares times the market’s fee rate times p × (1 − p), where p is the share price. So it is largest at 50 cents and shrinks toward both ends: 100 shares at 30 cents pay the same fee as 100 shares at 70 cents. That day most rates ran from 0.04 to 0.07, set by category, and some markets charged less or nothing. At a rate of 0.05, 100 shares bought at 50 cents pay $1.25 in fees, which lifts the all-in price from 50 cents to 51.25 cents a share.
Kahyna includes this fee in every Polymarket cost estimate, at the rate Polymarket publishes for that market. For other venues, check the venue’s own fee rules before you trade: Kahyna’s cost estimate does not include their fees.
What win rate do I need to break even?
Your all-in price per share. A winning share pays $1, so paying 63.3 cents means you need to win more than 63.3% of the time to come out ahead over many bets.
A trade only makes sense when your estimate of the chance is above that all-in price.
How does Kahyna show the cost?
Where a venue’s order book is available, Kahyna estimates what a $100 order would really pay, with the spread and slippage included, and on Polymarket the market’s fee too. It is part of the free plan.