Betting Mechanics
Polymarket isn't a traditional betting platform. There's no bookmaker, no 'odds' in the Betfair sense, and no matched betting against a specific counterparty. Instead, you buy and sell outcome tokens against an automated market maker. Understanding the conditional token construction and the effective cost formula is the key to reasoning about your position.
? The Conditional Token AMM - How Shares Get Priced
When you deposit collateral into a Polymarket market, the protocol mints equal quantities of YES and NO outcome tokens. This is the conditional token (?U) framework: your YES token is redeemable for $1 if YES wins, worthless if NO wins. The NO token is the opposite. Together they sum to exactly $1 - one wins, one loses, net zero sum.
The AMM receives your collateral. The protocol mints two tokens: YES and NO. You receive both - 100 YES tokens and 100 NO tokens.
The AMM prices YES shares at P(YES) = Q_yes / (Q_yes + Q_no). The current pool has 60M YES and 40M NO tokens, so P(YES) = 60%. Your YES tokens are worth $0.60 each right now - not yet settled, but the market says 60% chance of $1.
If you want to bet on YES, you hold your YES tokens and sell your NO tokens on the market (or to the AMM). The net cost of your YES position = $0.60 per share. If you want to bet on NO, you do the opposite.
When the market resolves YES, YES tokens redeem at $1, NO tokens are worthless. When NO resolves, NO redeems at $1 and YES is worthless. You can't double-dip - only one outcome pays.
The Effective Cost Formula
The cost of buying 1 YES share is simply the AMM's price: P(YES). But the full accounting matters because when you buy YES, you also receive NO tokens (or equivalently, the protocol mints them for you). Here's how to think about the effective cost precisely.
? Share Purchase Simulator
Adjust the market probability, your stake, and whether you're buying YES or NO. See the exact cost, share count, maximum payout, and the implied odds ratio.
? Polymarket vs Betfair vs PredictionBook
The key architectural difference is how orders get matched. Understanding this will help you reason about pricing, slippage, and where your trade actually goes.
Fee Breakdown at Different Position Sizes
The fees on Polymarket look small in percentage terms but compound significantly on large positions or high-frequency trading. Here's how the effective cost changes at different stakes and probabilities.
Conditional token framework
The conditional token (?U) construction is the architectural foundation of Polymarket's binary market design. When a trader deposits collateral into a market, the protocol mints equal quantities of YES and NO outcome tokens. This is mathematically similar to the conditional claim primitive in financial engineering - each token is a claim that pays $1 if a specific condition is met, and $0 otherwise.
The key property is that one YES token and one NO token always sum to at most $1 in aggregate value: after resolution, one is worth $1 and the other is worth $0, so their sum is exactly $1. Before resolution, the market prices them so that P(YES) + P(NO) = 1 - because the AMM's constant-product invariant forces the prices to be complementary probabilities. This is what makes the pricing clean and the AMM mathematically simple.
In practice, this means: if you want to bet on YES, you buy YES tokens (or equivalently, receive them when you deposit and sell the NO tokens). If you want to bet on NO, you buy NO tokens. You never need to find a counterparty who wants the opposite position - the AMM creates the opposite token when you take a position. This is the key difference from Betfair's P2P model and what allows Polymarket to offer markets on any resolvable question.
The effective cost formula
The effective cost of a YES share is simply P(YES). This is not an approximation - it's exactly what you pay per share on the AMM. The formula: Cost = P(YES) Quantity. If P(YES) = $0.35 and you stake $100, you receive 285.7 YES shares costing $35 each, with total cost of $100 before fees.
The AMM enforces this through the constant-product invariant. When you buy YES, the protocol burns YES tokens from the pool and mints NO tokens. This shifts the ratio Q_yes/(Q_yes + Q_no) - which changes P(YES). You paid exactly P(YES) for each share because the AMM rebalanced to make that the price.
The only cost above the AMM price is the fee layer: 0.5% platform fee plus 0-2% creator fee, applied as a small slippage in the trade. On a $100 trade at $0.60 probability, that's $0.60 platform + $1.20 creator = $1.80 total. On small trades (under $50), fees can be 3-5% of the stake. On large trades ($10K+), fees compress to under 1% of the stake. This is a meaningful difference and rewards large position sizing.
How Polymarket differs from traditional prediction markets
Betfair operates as a P2P exchange. When you back a horse at 4:1, you're betting against someone who laid it at 4:1. Betfair takes commission (2-5%) on your winnings. The odds you get depend entirely on who else is willing to trade at that moment. If no one wants to lay the bet, your order sits unmatched and you get nothing.
PredictionBook uses a simpler model: the market creator sets fixed odds, and traders bet against the creator's balance. It's more like a traditional bookmaker than an exchange. The creator takes the other side of every bet. There's no spread or dynamic pricing - the odds are whatever the creator set. This means illiquid or controversial questions may not have any market at all.
Polymarket's AMM solves both problems. The AMM always provides liquidity at a deterministic price - no counterparty search required. The price is always a valid probability (between 0 and 1). And the cross-venue arbitrage with Betfair and other prediction markets keeps Polymarket's prices connected to the broader information market ecosystem, so you're not operating in a vacuum. The 0.5% platform fee on settled volume is also lower than Betfair's effective commission on winnings for large traders.
Fee structure in detail
Polymarket's fee structure has two components: the platform fee (0.5% of the trade value, charged at settlement) and the creator fee (0-2%, set by the market creator, charged similarly as slippage at trade time). The platform fee is paid by the taker (the trader), not the maker. Market makers posting limit orders to the CLOB typically receive maker rebates or?? platform fees.
For a $1,000 trade at $0.50 probability: share cost = $500. Platform fee = $5. Creator fee at 2% = $10. Total cost = $515. Payout if your side wins = $1,000. Net profit = $485. Effective "vigorish" ? 3% on the stake (3% of $1,000 = $30). At $0.30 probability, the math is more interesting: $300 stake for 333.3 shares paying $333.3 - effective vig ? 3% still, but the payout ratio is much more favorable relative to your probability estimate.
The fee structure creates an interesting dynamic for LPs: the AMM earns the spread between what traders pay and the AMM's price. LPs who provide liquidity to the AMM pool earn a proportional share of the platform fee revenue (which comes from the taker fees on trades that fill against the AMM). LPs don't earn the creator fee - that goes to the market creator. The creator fee is an incentive to seed markets, not an LP revenue stream.
Frequently asked questions
- How is a Polymarket share different from a Betfair bet?
- On Betfair, your bet is matched directly against another user who took the opposite side. The exchange takes a commission (typically 2-5%) on winnings. On Polymarket, your trade fills against the AMM or a market maker's limit order - you're not matched with a specific counterparty, you're trading against the liquidity pool. The price you pay per share always reflects the AMM's current probability. There's no 'lay' vs 'back' distinction - the same share type is used for both buying and selling (you sell by buying the opposite outcome). Commission structure is 0.5% platform fee on settled volume, not on winnings.
- What does 'conditional token' mean in practice?
- When you deposit USDC into a Polymarket market, the protocol mints equal quantities of YES and NO outcome tokens. You receive both. If you want to hold YES, you hold onto your YES tokens and the NO tokens effectively sit in your wallet (or you can sell them). The aggregate value of one YES and one NO token is always exactly $1 (before fees) - because one of them will be worth $1 after resolution and the other will be worth $0. This is the conditional token construction: each token's redemption value is conditional on the market's outcome. The upshot is you can buy YES without anyone needing to sell it to you - the AMM creates it from nothing.
- Why does buying YES at 30% probability cost $0.30, not $0.30 plus fees?
- The AMM prices YES shares at P(YES). When you buy one YES share, the AMM burns one YES token and mints one NO token - keeping the constant product invariant balanced. The cost to you is exactly the AMM price at that moment. The platform fee (0.5%) and creator fee (2%) are applied to the trade, but they show as a slippage cost on the trade confirmation rather than an explicit fee line. So buying $100 of YES at 30% effectively costs slightly more than $100 - about $102.50 total including fees.
- Is Polymarket legal in the United States?
- This is contested. Polymarket has faced CFTC enforcement action - the CFTC argued that offering prediction markets to US persons without proper registration is illegal. Polymarket restricted US persons in 2022 and again in some subsequent enforcement discussions. As of 2026, the platform is accessible globally and has structured its market creation rules to fall outside the Commission's definition of 'gaming' contracts. However, prediction markets remain in a regulatory gray area and US traders should seek their own legal advice before trading. The platform itself is non-custodial and operates on Polygon, which creates additional jurisdictional complexity.
- Why can't I buy partial shares on Polymarket?
- You can and do - the share purchase is fractional. If the price is $0.35, a $100 stake buys 285.7 shares. The AMM is continuous, not discrete. What you can't do is buy a 'partial outcome' - each share is a binary claim that either pays $1 or $0. This is fundamentally different from betting on a multi-outcome event where your payout could be fractional (e.g., a 3:1 payout on a horse race). On Polymarket, every share is always worth either $1 or $0 at resolution - there's no middle value.
- How does the 'effective cost' formula work and why does it matter?
- The effective cost of 1 YES share = P(YES) $1. That's it. The AMM charges you exactly the probability of the outcome. But because the AMM also has to balance YES and NO token quantities, when you buy YES you implicitly mint NO tokens (which you can sell). So the net cost of a YES position is actually: Cost = (collateral P(YES)) ? (collateral P(NO) proceeds from selling NO). Since P(YES) + P(NO) = 1, this simplifies to just the P(YES) collateral amount. The AMM makes this calculation invisible to the trader - you see one price, you pay one price. Understanding the conditional token construction behind it explains why the price is always between $0 and $1.