A cross-platform arbitrage is only real if both orders can actually be placed at the prices you calculated. On Polymarket that means respecting the market's tick size and minimum order size. This article summarises what Polymarket's documentation says about them and why they matter for a two-leg trade with Kalshi. It is not trading advice, and the details can change, so check the current documentation.
What the documentation says
The Place Orders page says a price must follow the market's minimum price increment and a size must meet its minimum order size, and that min_order_size is the minimum number of shares the order book accepts. The order book endpoint returns tick_size, min_order_size and a neg_risk flag for each token; its example response shows a tick size of 0.01 and a minimum order size of 1.
Precision depends on the tick size
The documentation includes a table of how many decimals are allowed for price, size and amount at each tick size. For example, a tick of 0.01 allows 2 price decimals, 2 size decimals and 4 amount decimals, while a tick of 0.001 allows 3 price decimals. The listed tick sizes are 0.1, 0.01, 0.005, 0.0025, 0.001 and 0.0001.
The tick size can change
The market WebSocket documentation lists a tick size change event carrying the old and new tick size. So the value you read when you started is not guaranteed to hold; a tool that places orders should read it per token and react to changes.
What it means for a two-leg arbitrage
Suppose the combined cost of YES on one platform and NO on the other is 0.97 before fees. If one leg's price must be rounded to a tick, the rounding moves your cost by up to one tick on that leg, and net edges after fees are often below 1%, so a one-cent move matters. Do the calculation with the price you can actually place, not the unrounded one, and check that your size meets the minimum on both venues. The other leg has its own rules; see the Kalshi order book article for how its prices are expressed.
Where Arbitrage Agent fits
Arbitrage Agent matches markets across Kalshi and Polymarket, checks the resolution rules on both sides, and shows the result in a live feed with a step-by-step guide for both legs. You place the orders yourself, and the final check of the details is still yours. See the plans or start with the Starter plan.
Related articles
- Order Types for Two-Leg Arbitrage
- Net Edge After Fees on Polymarket
- Polymarket Fees Explained
- Prediction Market Liquidity Guide
Key terms: Spread · Complement Arbitrage