Polymarket Arbitrage Calculator

How to use this calculator

Some Polymarket events are a group of binary markets that together cover every outcome, such as "Who will win?" with one market per candidate. Paste the YES price of each market, in cents, and a total stake. The calculator checks two baskets:

• Buy every YES. Exactly one market resolves YES and pays $1.00 per share, so if the YES prices add up to less than 100¢, buying them all in the right proportions is a guaranteed profit.
• Buy every NO. All markets but one resolve NO, so with n markets the NO shares pay n − 1 dollars. If the NO prices add up to less than n − 1 dollars, buying them all is the arbitrage.

The stakes are sized so every possible result pays the same, and the share counts are the stake divided by the price. Fees are applied to profit at the rate you choose.

Two kinds of Polymarket arbitrage

Within one market. A binary market has a YES and a NO share that together always pay $1.00. If the best asks sum to less than 100¢ (say YES at 48¢ and NO at 50¢), buying one of each costs 98¢ for a certain $1.00. This is rare and short-lived on liquid markets, because it is the simplest trade there is.

Across a group. When an event is split into one market per outcome, the YES prices should sum to about 100¢, since exactly one resolves YES. They are set by different order books that move at different speeds, so the sum drifts. Below 100¢, buying every YES is an arbitrage; above, buying every NO may be. This calculator handles the group case, and the group case is where arbitrages actually appear.

A worked example

Three markets in a group trade at 30¢, 30¢ and 35¢. The YES prices sum to 95¢, so the YES basket is an arbitrage. With 100 to stake, the calculator puts 31.58 on each of the first two markets and 36.84 on the third, which buys 105.3 shares of each. Whichever market resolves YES pays 105.26, a profit of 5.26 on 100, or 5.26%. Open this example to see the outcome table.

The NO basket on the same prices costs 205¢ for shares that pay $2.00, so it loses 2.44% and the calculator says so. Now move the prices to 40¢, 35¢ and 28¢. The YES sum is 103¢, no arbitrage there, but the NO prices sum to 197¢ against a $2.00 payout, and buying every NO returns 1.52% on the stake. Check both baskets every time; the mispricing can sit on either side.

Fees come off the profit. With an illustrative 2% fee the 5.26% on the first example shrinks to 3.83%, and a basket that was marginal without fees can turn into a loss with them. Enter the fee for the market category you are trading.

Why arbitrages disappear

The sum of YES prices rarely stays below 100¢ for long, and the reasons are practical rather than mathematical:

• Liquidity. The best ask may hold only a few hundred shares. The calculator's share counts assume you can fill the whole size at the quoted price; on a thin book the average fill is worse.
• Slippage. Buying each leg moves that leg's price. By the time the third leg is filled, the first may have repriced, and other traders watching the same group are filling the same gap.
• Fees. A 1–2% gap is exactly the size that fees remove.
• Resolution risk. The arbitrage only holds if exactly one market in the group resolves YES. Read the rules for ties, withdrawn candidates and "other" markets before assuming the group is complete.
• Capital lock-up. Long-dated groups tie money up until resolution. A 3% return over nine months is a different proposition from 3% over a weekend.

Polymarket against a bookmaker

The same logic works across venues. A YES share at 64¢ is decimal 1.5625; if a bookmaker offers 2.90 on the opposite outcome, the implied probabilities sum to 98.48% and backing both sides locks in 1.54% before fees. That is a two-leg job for the arbitrage calculator, which takes a bookmaker price on one leg and the Polymarket preset on the other. The Polymarket vs sportsbook arbitrage guide covers the sizing and the settlement risks.

Frequently asked questions

Do the prices have to sum to exactly 100¢?

In an efficient market the YES prices sum to a little more than 100¢, because of the spread. An arbitrage appears when they briefly sum to less, which happens when the markets are thin or move at different speeds.

Why is the NO basket usually not an arbitrage?

Because when YES prices sum to more than 100¢, NO prices sum to less than n − 1 only by the same excess, and the spread on each leg tends to swallow it. Check both anyway; sometimes one side is mispriced.

Are fees included?

Fees are modelled as a share of profit at the rate you select. The preset rate is a placeholder; check the fee for the market category you are trading and enter your own.

What if one market in the group can also resolve NO for everyone?

Then the group does not cover every outcome and the YES basket can lose on every leg. Add the missing market (often an "other" or "none of the above") or treat the group as incomplete. The Polymarket converter is the tool for a single market's YES and NO prices, and the commission calculator shows what a fee on profit does to one leg.