PRICE MECHANICS
How Polymarket BTC 5-Minute Prices Work: Bid, Ask, Midpoint and Last Trade
Why the price displayed on screen can differ from the price available for an actual BTC 5-minute order—and how to read bid, ask, midpoint and last trade correctly.
The short answer
Bid, ask, midpoint and last trade are four different Polymarket price observations. The bid is the best visible buying offer, the ask is the best visible selling offer, the midpoint is halfway between them, and the last trade records the most recent match. A user buying immediately interacts with asks, so the displayed midpoint or last trade may not be executable.
Evidence boundary: official documentation establishes platform mechanics and source behavior; calculations, examples and interpretations are PolyCerno Research analysis. They do not guarantee an outcome or profit.
Terms used in this guide
- Best bid
- The highest visible price currently offered by a buyer.
- Best ask
- The lowest visible price currently offered by a seller.
- Midpoint
- The arithmetic midpoint between the current best bid and best ask.
- Last trade
- The price of the most recently matched transaction, whether or not that liquidity remains available.
Four prices that answer four different questions
A Polymarket outcome token trades between zero and one dollar. That makes every visible number look like a probability, but the numbers around an order book do not describe the same thing.
| Observation | Question it answers |
|---|---|
| Best bid | What is the highest visible price a buyer is currently offering? |
| Best ask | What is the lowest visible price a seller is currently offering? |
| Midpoint | What lies halfway between the best bid and ask? |
| Last trade | At what price did the most recent match occur? |
A trader buying immediately normally interacts with asks. A trader selling immediately normally interacts with bids. The midpoint is not itself a resting order, and a last trade can describe liquidity that is no longer available.
Why the displayed price may not be executable
Polymarket documents a display convention based on the order-book midpoint, with the last traded price used in specified wide-spread conditions. That display helps summarize the market, but it does not guarantee a fill at the displayed value.
A buyer who wants immediate execution starts at 62¢, not 59¢. A seller starts at 56¢. The six-cent spread is part of the decision.
Price changes with intended order size
The best ask describes only the first available level. If 10 shares are offered at 62¢ and the next 90 are offered at 65¢, a 100-share buy cannot be evaluated using 62¢ alone. The relevant value is the size-weighted average price across the levels the order would consume.
This is why PolyCerno keeps displayed prices and modeled execution costs separate. Direction can remain unchanged while a larger order becomes materially less attractive.
UP and DOWN must be read as separate books
Binary outcomes are economically related, but their visible order books can have different spreads, depth and update timing. Adding two displayed prices and treating the result as a guaranteed executable relationship is unsafe.
Compare executable bids and asks on both sides. If UP appears cheap while its ask depth is thin, or DOWN appears expensive because of a stale last trade, the apparent discrepancy may disappear when the books are walked.
A correct price without a timestamp is incomplete
BTC, UP, DOWN and the settlement target can update on different clocks. A contract price captured even a few seconds before a BTC move may look irrational when placed next to a newer underlying price.
Before calling a lag or mispricing, verify source age, book freshness and whether the last trade occurred before or after the underlying move. Missing observations should remain missing rather than being carried forward without disclosure.
A practical price-reading sequence
- Confirm the exact market and outcome tokens.
- Read the best bid and ask for both UP and DOWN.
- Note the spread and the timestamp of the book.
- Identify whether the displayed value is a midpoint or recent trade.
- Walk enough depth for the intended size.
- Add current fee treatment and verify fill completeness.
- Only then compare price with the directional evidence.
Sources and further reading
Primary documentation was checked on . Source interfaces and market rules can change; verify the current market before relying on a field.
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