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Free · runs in your browser · no signup · live order book September 25, 2026
A market buy on Polymarket costs more than the price it displays: you pay the ask, half a spread above the midpoint; a large order climbs through worse asks (slippage); and taker orders pay a fee. Enter the bid, the ask, your order size and, if you have it, the depth, and the calculator shows your average fill price, what the spread and slippage cost, and the break-even probability — the chance you need for the trade to pay off. Example: $1,000 into a 52¢ bid / 54¢ ask market with the depth below fills at 54.79¢ and breaks even at 55.79%, 2.79 points above the 53¢ midpoint.
Fee assumption: Polymarket's taker fee, shares × rate × p × (1 − p) on each fill, at the rate you pick; a limit order that rests on the book (maker) pays no fee. Live rates by category: Polymarket fees.
The book has a 52¢ best bid and these asks: 800 shares at 54¢, 600 shares at 55¢, 1,200 shares at 56¢, 3,000 shares at 58¢. You spend $1,000 with a market buy in a market with a 4% taker fee.
On September 25, 2026 the best bid was 66.0¢ and the best ask 67.0¢: a 1.0¢ spread around a 66.50¢ midpoint. What a market buy would cost against the visible asks:
| Spend | Shares | Average price | Slippage | Spread cost | Fee | Break-even |
|---|---|---|---|---|---|---|
| $100 | 149 | 67.00¢ | 0.00¢ | $0.75 | $1.65 | 68.11% |
| $1,000 | 1,493 | 67.00¢ | 0.00¢ | $7.46 | $16.50 | 68.11% |
| $10,000 | 14,925 | 67.00¢ | 0.00¢ | $74.63 | $165.00 | 68.11% |
| $50,000 | 74,058 | 67.52¢ | 0.52¢ | $370.29 | $812.03 | 68.61% |
Fees at this market's 5% taker rate. Snapshot from Polymarket's order book, refreshed twice a day; the full book is on how to read a Polymarket order book.
Midpoint = (bid + ask) ÷ 2, the price Polymarket shows as the probability (it shows the last trade instead when the spread is wider than 10¢, per the Polymarket Help Center). A market buy pays the ask, so every share costs ask − midpoint more than the displayed price: half the spread. Buying and selling straight back pays the full spread.
Your dollars then walk the asks from the cheapest up: shares = what fills at each level, average price = dollars ÷ shares, and slippage = average price − best ask. The taker fee is shares × rate × p × (1 − p) at each level's price p. A share pays $1 if the outcome happens, so the break-even probability is your all-in cost per share: (dollars + fee) ÷ shares. Selling works the same way against the bids.
Related tools: how to read a Polymarket order book (with a live book), Polymarket fees by category, and the expected value calculator to check the edge once you know the break-even. Terms: bid-ask spread, slippage.
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A market buy pays the ask instead of the midpoint, so it costs half the spread per share. With a 52¢ bid and a 54¢ ask that is 1.0¢ a share, or 1.85% of the stake at 54¢. Buying and selling straight back costs the whole spread.
The extra you pay when your order is larger than the shares offered at the best ask, so part of it fills at worse prices. The calculator walks the ask levels you enter from the cheapest up and reports the average price and how far it is above the best ask.
Your all-in cost per share: what you spend plus fees, divided by the shares you get. A share pays $1 if the outcome happens, so the trade has positive expected value only if you believe the chance is higher than that cost.
A limit order that rests on the book waits for a seller to come to your price, so it does not cross the spread, and as a maker order it pays no taker fee — but it may never fill. The taker fee is shares × rate × p × (1 − p), with the rate set by the market's category.
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