Midpoint Price

Unlike Last Price, which looks backward at completed trades, the midpoint price is forward-looking: it reflects current supply and demand right now. When the Bid-Ask Spread is narrow, the midpoint closely approximates the actual Market Price at which you could trade. For this reason, portfolio tools and Prediction Market Aggregator services use midpoint as the primary basis for Mark-to-Market valuation of open positions. On platforms like Kalshi and Polymarket, midpoint pricing lets analysts compare implied probabilities across markets without being biased by a single stale last trade. As a proxy for Implied Probability, a midpoint of $0.72 suggests the market currently prices the event at roughly a 72% chance of occurring.

Active traders who want to avoid paying the full Bid-Ask Spread often post Limit Order bids or asks at or near the midpoint, hoping to get filled between the two sides rather than crossing the spread. This is a core tactic for Liquidity Provider market makers. When the Depth of Market is shallow, the midpoint can shift dramatically with a single large order, so treating it as a fixed fair value is risky. Comparing midpoints across platforms is a key step in Line Shopping: if Kalshi shows a midpoint of $0.58 and Polymarket shows $0.62 for identical event contracts, there may be an Arbitrage opportunity or simply a difference in perceived probability driven by each platform's trader base.