Prediction Market Odds: Converting Share Prices With Fees

A prediction market odds converter starts from a single fact: on Polymarket, every share is priced between $0.00 and $1.00, and that price is read as the market's implied probability. A 30¢ Yes share is a 30% probability, which converts to decimal odds of 3.33, American odds of +233 and fractional odds of 7/3. The converters built for sportsbooks stop there.

Why a prediction market price is already a probability

A sportsbook quotes odds and leaves you to back out the probability. A prediction market quotes the probability directly, as a price on a $0.00 to $1.00 scale. Converting it to odds is arithmetic, not modelling.

That makes the price a cleaner input than a bookmaker line, with one catch: the price on the screen is not always a price anyone traded at. More on that below.

Prediction market odds formulas: decimal, American and fractional

Call the share price p, written as a fraction of a dollar. Every format falls out of p:

The 30¢ and 70¢ columns are mirror images: the same outcome priced from each side. Decimal odds include the returned stake, fractional odds show profit only, and American odds flip sign at the 50¢ line, where the favourite and the underdog swap places.

Bid, ask and last price: which number is the odds

Polymarket says the displayed price is the bid-ask midpoint, and when the spread is wider than $0.10 it shows the last traded price instead (as of October 2026). Neither is what a market buy fills at. A buyer pays the ask, which sits above the midpoint whenever there is a spread.

So the probability you are actually buying is the ask, not the headline. On a thin market where the screen has switched to the last trade, the headline can sit some distance from both sides of the book.

Run any conversion on the ask if you are buying and on the bid if you are selling. Converting the midpoint gives odds you cannot trade.

How fees change the odds you actually pay

A fee raises your all-in cost per share, and on a $0.00 to $1.00 scale that cost is your break-even probability. Polymarket and Kalshi price this differently.

Polymarket's taker fee formula

Polymarket publishes its taker fee formula as C × feeRate × p × (1 - p), where C is the number of shares and p is the price (as of October 2026). Makers are never charged fees; only takers pay. The fee per share is feeRate × p × (1 - p), so a taker's break-even probability is p + feeRate × p × (1 - p).

The p × (1 - p) term makes the dollar fee symmetric: Polymarket states that a 30¢ trade pays the same dollar fee as a 70¢ trade, because p × (1 - p) is 0.21 at both prices. Per dollar staked it is not symmetric. Divide by the price and the 30¢ side pays 0.7 × feeRate per dollar while the 70¢ side pays 0.3 × feeRate. The cheap side carries more than twice the fee load, a detail no sportsbook converter will show you.

Two edge cases. Fees round to five decimal places, so small trades near the extremes may pay nothing at all. And Polymarket lists geopolitical and world events markets as fee-free, where screen price and break-even match once the spread is accounted for.

Kalshi fees and resting orders

Kalshi charges a transaction fee on expected earnings of the contract, and some markets carry different fees (as of April 2026). Maker fees on resting orders apply only when the order executes; canceling a resting order costs nothing.

Kalshi also notes that a filled limit order offers the potential to avoid trading fees, with the trade-off that a limit order may never fill (as of March 2026). On either venue, the odds you get as a patient maker are closer to the screen than the odds you get crossing the spread as a taker.

Common mistakes when reading prediction market odds

• Converting the displayed midpoint instead of the ask you will pay.
• Treating the shown price as live when the spread is over $0.10 and the screen has fallen back to the last trade.
• Assuming the fee is the same share of every stake. In dollars it is symmetric around 50¢; per dollar risked, the cheap side pays more.
• Applying one venue's fee to another. Kalshi's fees vary by market, and Polymarket's world events markets carry none.

Pool questions

How do you convert a prediction market price to decimal odds?

Divide 1 by the share price. A 30¢ share converts to decimal odds of about 3.33 and a 70¢ share to about 1.43, before any spread or fee is added to the price.

What do negative American odds mean on a prediction market?

They mean the share costs more than 50¢, so the outcome is the favourite. The formula is -100 × p / (1 - p), which turns a 70¢ price into roughly -233.

Is the price shown on Polymarket the price I will pay?

Not necessarily. The displayed number is the midpoint of the bid and ask, or the last trade when the spread is wider than $0.10. A market buy fills at the ask, which sits above the midpoint.

Do prediction market fees change the implied probability?

Yes, for takers. Your break-even probability is the price plus the fee per share, so a taker fee lifts the probability you need to be right. Makers on Polymarket pay no fee, and a filled resting limit order on Kalshi can avoid trading fees.

Deep cuts nearby

• S&P 500 Prediction Market Odds: Reading Brackets Net of Fees

Chain and source links

• Polymarket Documentation, Prices & Orderbook, accessed October 2026
• Polymarket Documentation, Trading Fees, accessed October 2026
• Kalshi Help Center, Fees, April 2026
• Kalshi Help Center, Limit Orders, March 2026