Midpoint

Midpoint

The midpoint is the arithmetic mean of the best bid and the best ask. On a binary or multi-outcome market it is a simple, single-number fair-value reference calculated as (best bid + best ask) / 2.

In context

You will see midpoint used across Polymarket as a quick fair-value indicator and in UI elements where a single reference price is needed (for example, as a tie-breaker when listing markets or when showing a smoothed price on small widgets). For binary markets the midpoint sits between the YES and NO best prices; for multi-outcome markets the midpoint is calculated separately per outcome using that outcome's best bid and best ask.

Why traders watch midpoint

• Fair-value shorthand: Midpoint gives a neutral, order-book-based estimate when the spread is wide or asymmetric.
• Execution reference: Traders compare their limit orders or market orders to the midpoint to estimate expected slippage relative to the current book.
• UI consistency: Polymarket sometimes uses midpoint to choose a single display price when best bid and best ask would otherwise require two numbers.

Limitations and risks

Midpoint is not an execution guarantee. It ignores depth beyond the best bid and best ask, fees, and potential rapid price moves. When spreads are large or order-book depth is thin, midpoint may be a poor predictor of the price at which an order will fill.

See also

• /glossary/spread