Slippage

The difference between the price you saw and the average price you actually filled at.

Slippage has two sources. One is walking the ladder: any order larger than the size resting at the best price fills partly at worse prices, so the average is worse than the quote. The other is time — between reading a price and submitting an order, someone else may have taken it.

The site models it as a cost rather than ignoring it. The Polymarket per-share charge of roughly two tenths of a cent is explicitly gas plus a slippage allowance, since Polymarket levies no trading fee of its own. Every hypothetical result on the site is labeled with what it does not model: the tail simulation is retroactive and ignores slippage, timing and liquidity, and is described that way wherever it appears rather than in a footnote.

Two habits follow. Size against depth, not against the top-of-book quote. And treat any lock showing less than about a cent of net edge as noise, because a cent is inside the range that ordinary execution friction can erase between the moment you see it and the moment both legs are filled.

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Definitions describe how Polymarket and Kalshi behave and how WhaleTracks models them; venue rules and fee schedules can change, so confirm anything you size a position on with the venue itself. WhaleTracks is informational analytics, not financial advice. Past performance does not guarantee future results.