How we model prediction-market fees

The calculator models the trading fee — what the venue charges to open or close a position — for five prediction-market platforms: Kalshi, Polymarket, DraftKings Predictions, Robinhood Predictions and Gemini Predictions. A contract settles at $1.00 if it resolves in your favour and $0.00 otherwise, so its price in cents (1–99¢) doubles as its implied chance. Everything is computed from the number of contracts, the contract price and the order type. It does not cover casino or sportsbook deposit and withdrawal fees; that is a different product and search intent.

Each platform’s formula is taken from its own published schedule and re-checked against the source:

• Kalshi: Kalshi fee schedule (PDF) — effective 2026-07-07 — checked 2026-07-16.
• Polymarket: Polymarket trading-fees documentation — checked 2026-07-16.
• DraftKings Predictions: DraftKings Predictions fee disclosure — checked 2026-07-16.
• Robinhood Predictions: Robinhood event-contracts overview — checked 2026-07-16.
• Gemini Predictions: Gemini Titan fees notice (PDF) — checked 2026-07-16.

• Kalshi: ceil_to_cent(coeff × contracts × price × (1 − price)), with a standard taker coefficient of 0.07. The July 2026 schedule defaults the maker coefficient to 0 and lists specific series that carry a 0.0175 maker coefficient; fee-free series use 0.
• Polymarket: contracts × categoryRate × price × (1 − price), taker only (makers pay 0), rounded to five decimals. Category rates include crypto 0.07; sports, economics, culture, weather and general 0.05; finance, politics, mentions and tech 0.04; geopolitics 0.
• DraftKings Predictions: flat per contract, per side. DKeX/CDNA markets charge $0.01 at 1–19¢ and 97–99¢ and $0.02 at 20–96¢; CME markets charge a flat $0.02.
• Robinhood Predictions: min(ceil_to_cent(k × price × (1 − price) × contracts), $0.01 × contracts), where k is 0.10 (Standard) or 0.05 (Gold). A venue exchange fee of 0–1¢ per contract, copied from the order preview, is added on top.
• Gemini Predictions: min(price, 1 − price) × contracts × feeRate, with the current Titan notice rates of 0.00 maker and 0.05 taker.

• Kalshi, 100 @ 50¢, standard taker: 0.07 × 100 × 0.5 × 0.5 = $1.75. Total paid $51.75; break-even 51.75%.
• Polymarket, 100 @ 50¢, sports taker: 100 × 0.05 × 0.5 × 0.5 = $1.25. Crypto would be $1.75; geopolitics $0.
• DraftKings, 100 @ 10¢, DKeX/CDNA: 100 × $0.01 = $1.00. At 50¢ it is 100 × $0.02 = $2.00.
• Robinhood, 100 @ 10¢, Standard: ceil(0.10 × 0.1 × 0.9 × 100) = $0.90; at 50¢ the $0.01-per-contract cap holds it at $1.00.
• Gemini, 100 @ 10¢, taker: min(0.1, 0.9) × 100 × 0.05 = $0.50; at 50¢ it is $2.50.

Kalshi and Robinhood commissions round upto the nearest cent; Robinhood then caps the commission at $0.01 per contract. Polymarket and Gemini use five-decimal precision, and Polymarket applies a 0.00001 USDC floor where a positive fee would otherwise round to zero. DraftKings is exact per contract by price band. The calculator assumes a single fill at one price; a real order that fills in parts across several prices is the sum of each fill’s fee, which can differ slightly from a single-price estimate.

Order type and market classification change the fee materially. Kalshi and Polymarket separate maker and taker, and Polymarket’s rate depends on the market category. DraftKings depends on whether the market is a DKeX/CDNA or CME product. Robinhood depends on Standard versus Gold account tier and a venue exchange fee. Gemini keeps maker and taker as the meaningful differentiator. DraftKings and Robinhood do not expose a maker/taker choice, so the calculator marks that control unavailable rather than coercing it.

• Funding costs. Card, bank and crypto-network charges to move money are a separate funding cost, shown on their own line and never added to the trading fee.
• Bid–ask spread and slippage, unless you enter executable entry and exit prices. We do not silently add a spread to a hold-to-settlement result.
• A moving market price. Sell mode is arithmetic on the exit price you enter, not a forecast of where the market will trade.
• User-specific rebates and incentives. Temporary maker rebates, liquidity rewards and referral discounts change independently and are not part of the base fee.
• Taxes and unpublished schedules. Where a platform does not publish inputs that support a reproducible calculation, we omit it rather than guess.

Each formula carries its source URL, effective date where one is published, and the date we last verified it against the source. Fee schedules are re-checked on the platform review cycle and out of cycle when a platform announces a change or a user reports a discrepancy. The current snapshot was verified on 2026-07-16; the notable recent change is Kalshi’s schedule effective 7 July 2026 and Polymarket’s move to category-based taker fees.