Polymarket Raised Its Fees — How Much More Does a Trade Cost?
We compare the cost of trading the same games on Polymarket and Kalshi, and also check whether the gap between buy and sell prices, and how accurately the odds predict results, have changed.
Study details
On this page
Key findings
From MLB games in summer 2026: about 1.2¢ per share on average across 120 games before the change, and about 1.7¢ across 167 games after; most of the increase is the fee itself. Each share is worth at most $1. This is a direct before-and-after comparison. Over the same period, another platform, Kalshi, also rose slightly; the formal comparison that takes this out is the first point below.
- Compared with another platform, Kalshi, over the same period (taking out market-wide changes), Polymarket’s cost rose about 0.3¢ more. This is the formal comparison in our pre-registered plan, and the result holds. On Kalshi, the same games only went from about 1.4¢ to 1.6¢.
- The gap between buy and sell prices stayed at almost exactly the minimum of 1¢ on both platforms. Different methods give different results, so we can’t tell whether it got bigger.
- Compared with Kalshi, there was no clear change in how accurately Polymarket’s odds predict game results before and after the fee change. This doesn’t mean the odds themselves are always accurate.
- During the NFL’s no-fee period, Polymarket’s trading volume relative to Kalshi was higher, but the data are incomplete, so treat this as reference only.
In plain words
What do we want to know?
On 2026/7/10, Polymarket (a prediction platform where people buy and sell shares in game results) raised its fees for sports games. We mainly want to know: does one trade now cost more? Has the accuracy of the odds at predicting game results changed? For trading volume, we only have a reference analysis for the NFL, so we can’t yet say whether trading dropped overall.
How do we study it?
We compare buy prices, sell prices and trade data for the same games on both platforms, before and after the fee change. Another prediction platform, Kalshi, did not change its fees in this period. So if both change together, the cause is more likely the overall environment; if only Polymarket changes, it is more likely linked to the fee change.
What have we found?
After the fee change, the cost of trading on Polymarket went up, and most of the increase is the fee itself. Over the same period, Kalshi’s cost rose only a little; the difference between the two increases is about 0.3¢. The spread results disagree depending on the method, so we can’t yet say whether the spread widened. Compared with Kalshi, there is not yet enough evidence to say whether the odds got better or worse at predicting results.
How much did fees go up?
Polymarket’s fee is worked out as “rate × price × (1 − price)” (the price is between $0 and $1), and the rate went from 0.03 to 0.05. For a share priced at $0.50 (roughly an even chance), the fee per share went from about $0.0075 to about $0.0125 — roughly 1.5% to 2.5% of the price. The closer the price is to $0 or $1, the smaller the fee.
What can’t we say yet?
On 7/10, Polymarket didn’t just raise the fee; it also cut the reward for people who post orders, so what we see is the combined effect. The comparison period is shorter because the free archive stopped (15 days before the change, 26 days after), and the World Cup was happening at the same time. The spread was almost always at its minimum, so changes are hard to see; not seeing it widen doesn’t mean there was no effect at all. The NFL data are not complete enough, so they are reference only.
Terms used on this page
- Polymarket
- A prediction platform where people buy and sell shares in game results. Prices run from $0 to $1 and roughly show the chance the traders think an outcome has — not necessarily the true chance. A price converts directly to odds: odds = 1 ÷ price.
- Kalshi
- Another US prediction platform where the same games can be traded. It did not change its fees in this period, so it is used for comparison.
- Order book
- The list of buy and sell orders on a platform that are still waiting to be filled.
- Spread
- The gap between the highest buy price and the lowest sell price. For example, if someone will buy at $0.48 and someone will sell at $0.52, the spread is $0.04; to buy right away, you have to pay $0.52. The bigger the gap, the more it costs to trade right away.
- Taker / maker
- A taker accepts someone else’s existing price and trades right away; a maker posts their own price and waits for someone to trade with it.
- Maker reward
- The platform gives part of the fees back to makers, to encourage people to post prices.
- Midpoint
- The value halfway between the highest buy price and the lowest sell price, often used as the current reference price.
Detailed results
Costs rose on both platforms, but more on Polymarket
Scaling the average cost to 100 shares makes the amounts easier to read. All amounts below are US dollars.
Polymarket
The platform that raised its fee
- Before, per 100 shares
- 1.22 USD
- After, per 100 shares
- 1.74 USD
An increase of
0.52 USD / 100 shares
Kalshi
The comparison platform
- Before, per 100 shares
- 1.38 USD
- After, per 100 shares
- 1.57 USD
An increase of
0.19 USD / 100 shares
How much larger was Polymarket’s increase?
0.52 − 0.19 ＝ 0.33 USD / 100 shares
In short: per 100 shares, Polymarket’s average cost rose about $0.3 more. This compares increases, not the gap between the final costs.
Cost means the extra paid relative to the reference price, plus fees; it excludes the share price itself. Scaling to 100 shares is a unit conversion, not a simulation of buying 100 shares.
Original units and calculation details
Original units are cents per share. Since $1 = 100 cents, 1.22 cents per share scales to $1.22 per 100 shares.
Total cost = half the spread + fee. The calculation above uses rounded display values. The precise analysis estimates about 0.32 cents per share, or 0.3 cents to one decimal place.
The same 287 MLB games (120 before the fee change, 167 after; for all of them, the fee can be confirmed on the blockchain); each share is worth at most $1.
Main analysis (v1.1 plan, corrected after external code review) | fees determined only from on-chain charges before each quote time | the switch actually happened around 03:00 UTC on July 10.
HypothesesFor deeper reading
- H1After the fee change, the gap between Polymarket’s buy and sell prices gets bigger → not seen with the method in our plan; a recalculation done after seeing the results shows a small increase; the two disagree, so we can’t tellInsufficient evidence
- H2(Reference only) After the fee change, the share of trades made by accepting an existing price falls → it rose slightly instead; this share is of limited use as a referenceExploratory
- H3After the fee change, the cost of buying or selling one share rises more than on Kalshi → holds: about 0.3¢ more (287 games)Supported in this sample
- H4After the fee change, how accurately the odds predict game results (compared with Kalshi) changes → not seenInsufficient evidence
- H5(Reference only) During the NFL’s no-fee period, the gap between buy and sell prices is smaller and there is more trading → the gap was always the minimum of 1¢; in the no-fee period, Polymarket’s trading volume was higher relative to Kalshi; the data are incomplete, so this is reference onlyExploratory
Background and why it matters
People who accept an existing price usually pay a fee. How trading costs, spreads and the accuracy of the odds change around a fee change affects whether trading on Polymarket is still worth it. For the model, before using a prediction platform’s odds as a reference, we need to know whether rule changes affect the quality of those odds.
Latest progress
Latest：Initial analysis done; the start time of the fee change and the calculation method corrected after an external code review
Next step：Write up the results and confirm data licensing, then publish
Study timeline
- Research direction set2026-10-20
- Writing the plan and timestamping it2026-10-20
- First data checkpoint failed; plan revised to v1.1 and re-timestamped2026-10-20
- Collecting data and analyzing2026-10-21
- Publishing results and tracking
- External code review and publicationIn progress
Methods
What we want to know
- After the 2026/7/10 fee change, did the gap between the highest buy price and the lowest sell price (the spread) get bigger?
- Did trading volume and the number of posted orders drop? How much did the real cost of one trade change for traders?
- Do the odds still predict game results as accurately? In the NFL, do the effects of “fees → no fees → fees again” mirror each other?
Method summary
- The fee change: from 2026/7/10, the fee rate for sports games went from 0.03 to 0.05, and the reward for people who post orders dropped from 25% to 15%. Polymarket said it took effect at midnight, but the actual charges on the blockchain switched about 3 hours later, which doesn’t change which games fall before or after. Fee = rate × price × (1 − price), worked out per share.
- Comparison group: buy and sell prices for the same games on another platform, Kalshi (Kalshi didn’t change its fees in this period), used to take out changes that hit both platforms together.
- Days compared: only days where the free archive has Polymarket data (15 days before the fee change, 26 days after).
- What we measure: the gap between buy and sell prices, the cost of one trade (estimated from the buy and sell prices; what people actually pay depends on the order), and how accurate the odds are 1 hour before the game. We also check that the prices are recent enough.
- NFL: games are split into three periods — fees charged, no fees, and reduced fees — based on each game’s own fee setting. The data are not complete enough, so this is reference only.
Data and limitations
Data we use (all public and free)
- Polymarket hourly order-book archive (PMXT; MLB) and Pendulum Flow archive (NFL)
- Kalshi official public data (buy prices, sell prices and trades, minute by minute)
- Polymarket’s official fee explanation, market data, and the actual fees charged on the blockchain
Limitations
- The free archive stopped partway (Kalshi after 6/11, Polymarket after 8/10). After switching to other sources, fewer days can be compared.
- On 7/10, Polymarket both raised the fee and cut the reward for posting orders, so we can only see the combined effect.
- The fee rate shown on the website is not always what is actually charged, so we check it against the actual charges on the blockchain. During the World Cup, the users on the two platforms may be quite different.
Update log
Version history
- v1.1-fix2026-10-21
Fixed after another AI tool checked the code: corrected when the fee change took effect; when working out the fee, we no longer use data from later on (the result changed from 0.40 to 0.32¢); NFL trading volume now uses the same set of games
- v1.1-results2026-10-21
Main result: the trading cost rose about 0.4¢ more than on Kalshi (after seeing the results, we changed to using only regular trades to work out the fee; both methods are kept)
- v1.12026-10-20
The free archive stopped, so Kalshi switched to official public data, and the comparison only uses days with Polymarket data; guess 2 was changed to a reference-only result. Re-timestamped
- v1.02026-10-20
Study plan finalized and timestamped; data collection started
Study record
459f124a326bf093e16f6f332f70db9c765e8d4c49aa005a5033711541f55049effb078814f65d16c6b15949a9cf45edb93dddf4a8f203d76b76e5092b59a33e91cc6a8cdf3157be482fd2ca5b037a6b529f6fd8336a6c3262d0a83d109da7495360c662804dd03c3f81c35c0da25248e16de053e8b330ae8ce500be04c9571eThis page is a research log for reference only and is not betting advice or a guarantee of profit.