硬核拆解Polymarket手續費公式：90+%的極端費率是怎麼蹦出來的？
- 核心觀點：預測市場平台Polymarket因更新手續費計算公式時出現技術錯誤，導致部分交易（尤其是低價交易）被收取異常高昂的手續費，最高費率一度達到94.8%，平台發現問題後已緊急修復並調整了公式。
- 關鍵要素：
- 問題直接原因是平台更新公式時，錯誤地移除了一個關鍵的價格乘數（× p），導致手續費計算值異常升高，價格越低的交易受影響越嚴重。
- 在「天氣」和「經濟」類市場，錯誤公式還引入了exponent參數（設為0.5），進一步放大了低價交易的手續費，有用戶下單100份價值0.1美分的份額，潛在收益因手續費被侵蝕94.8%。
- Polymarket發現問題後，迅速將公式修正為fee = C × feeRate × p × (1 - p)，相當於將exponent參數設為1，此舉大幅降低了整體費用，尤其抑制了極端高價費率。
- 修正後，官方文件顯示「天氣」和「經濟」市場的極端費率已降至5%左右，平台回應速度較快。
- 平台建議用戶透過掛限價單（做市）來避免手續費，甚至可獲得20%-25%的做市返利，或使用Split功能間接建倉。
Original | Odaily (@OdailyChina)
Author｜Azuma (@azuma_eth)
Polymarket suddenly found itself embroiled in a fee controversy.
Last night, multiple community users discovered that they were charged unusually high fees when trading on Polymarket, resulting in significantly reduced shares received or profits compared to before.
An overseas user, Frosen (@frosen), even posted a screenshot showing that when they tried to place an order for 100 shares at a price of 0.1 cent in the "Economics" market, the Polymarket frontend displayed a predicted correct payout of only $5.2 (normally it should be $100) — corresponding to an outrageously high fee rate of 94.8%!
What's going on? Is Polymarket desperate for money? Based on Polymarket's official disclosures and community investigation, Odaily found that the direct cause of this unexpected situation was that Polymarket modified the platform's fee formula last night, and there were three versions of changes:
- First, the "old formula" introduced starting March 30: fee = C × p × feeRate × (p × (1 - p))^exponent;
- Then came the first change, the formula that caused the unexpected situation (referred to as the "abnormal formula"): fee = C × feeRate × (p × (1 - p))^exponent;
- After Polymarket realized the issue, they made a correction, resulting in the current version's "new formula": fee = C × feeRate × p × (1 - p);
- It's important to note that in all three formulas, C refers to the number of shares traded, p refers to the price of shares traded, and feeRate and exponent are variables.
Deconstructing the Abnormal Formula: How Did the Outrageous 94.8% Fee Rate Happen?
You don't need to worry too much about the mathematical details. By comparing the "old formula" and the "abnormal formula," you can easily see that the latter simply removed one " × p" (this is a multiplication symbol, not a lowercase X) compared to the former, meaning it ultimately multiplied by the share price one less time.
Since the price of all shares on Polymarket is always less than $1, this would inevitably cause the overall fee to increase. Furthermore, the lower the share price, the more pronounced the fee increase due to missing that multiplication becomes. When the share price is close to 0, it could lead to extremely outrageous fee rates — because the total order value is also very low at that point, making the fee rate appear particularly exaggerated.
As for how outrageous this fee could become, it also depends on the same variable ^exponent present in both the old and abnormal formulas. ^exponent directly translates to "raised to the power of exponent." This variable is primarily used to control the steepness of the fee curve.
According to Polymarket official Mustafa, last night's abnormal formula only introduced the exponent in the "Weather" and "Economics" markets (other markets had the parameter set to 1, effectively ignoring this variable). Furthermore, based on disclosures by overseas KOL Quant Chad (@Autonomous_Chad), the exponent parameter set for these two markets at the time was 0.5.
Now, back to Frosen's case and plug the corresponding numbers into the abnormal formula: fee = C × feeRate × (p × (1 - p))^exponent. It is known that C equals 100, meaning Frosen wanted to place an order for 100 shares; p equals 0.001, which is $0.001 (0.1 cent); exponent equals 0.5, meaning raising (p × (1 - p)) to the power of 0.5; and the final fee rate was 94.8%.
Feeding this to AI allows us to inversely deduce that the feeRate level at the time was approximately 0.03, while also reconstructing the detailed formula calculation Polymarket performed for this order.
In simple terms, based on the abnormal formula, Polymarket calculated that the fee for this order should be $0.0948. Since Polymarket's fee deduction method for buy orders is to directly deduct shares of equivalent value, and the share price at the time was only $0.001, it meant deducting 94.8 shares. Therefore, Frosen ultimately received only 5.2 shares, and even if the prediction was correct, the potential profit would only be $5.2.
Polymarket's Remedial Measures
Shortly after the abnormal fee issue emerged, Polymarket quickly responded by modifying the formula to the current version: fee = C × feeRate × p × (1 - p). Compared to the abnormal formula, the new formula removed the "^exponent" — essentially increasing the exponent parameter in the abnormal formula fee = C × feeRate × (p × (1 - p))^exponent from 0.5 to 1.
In the abnormal formula, the effect of ^exponent was to raise the data set p × (1-p) to a power. In Polymarket's actual operational conditions, the theoretical result range for p × (1 - p) is between "0.000999 - 0.25" — the closer p is to 0.5 (share price closer to $0.5), the closer this data set is to 0.25; when p is closer to 0 or 1 (share price closer to $0 or $1, with extreme quotes being $0.001 and $0.999), the closer this data set is to 0.000999.
Within the range of "0.000999 - 0.25," regardless of the value taken, when the exponent parameter increases from 0.5 to 1, it directly reduces the final fee result in the formula calculation, thereby lowering the overall fee.
More importantly, this reduction has a more pronounced inhibitory effect on the abnormally high fee rates near extreme low prices — when p × (1-p)=0.000999, the fee under the new formula is only about 3.16% of the fee under the abnormal formula, equivalent to a decrease of about 96.84%; whereas when p × (1-p)=0.25, the fee under the new formula is 50% of the fee under the abnormal formula.
As shown in Polymarket's official documentation, after the new formula was implemented, the fee rate at extremes in the "Weather" and "Economics" markets has now been reduced to 5%.
How Can Retail Users Avoid Fees?
I know most users can't be bothered to look at the formulas above but are still concerned about Polymarket's current fee issue.
Regarding this, Mustafa mentioned in the official Discord: "If you're worried about fees, you can place limit orders for free, and after this new update, you can also get a 20%-25% maker rebate — meaning when your limit order is filled, you'll receive 20%-25% of the taker fee from the counterparty. So not only are you trading for free, you can even get paid for trading and providing competitive liquidity."
So change your habits. Try to avoid taking orders directly as much as possible. Instead, use limit orders more often. You can also try using Polymarket's Split function more to indirectly build positions by placing reverse limit orders to sell shares on the other side.
