Bid, Ask, Mid and Last: Quoting Conventions

Ask someone "what's the price of this stock right now?" and there are at least four defensible answers, and they're usually not the same number. Knowing which one is being quoted, and why it matters, is basic vocabulary for reading any price screen.

The bid is the highest price a buyer is currently willing to pay; the ask (or offer) is the lowest price a seller is currently willing to accept. These two numbers sit on either side of the current market and almost never match exactly — the gap between them is the bid-ask spread, and it exists because market makers and other liquidity providers who post these standing quotes need to be compensated for the risk of being the counterparty willing to trade immediately, at any moment, in either direction. If you want to sell right now, you get the bid; if you want to buy right now, you pay the ask — the spread is effectively the cost of demanding immediate execution rather than waiting for a better price to come to you.

The mid (or mid-price) is simply the midpoint between bid and ask, computed as the average of the two. Nobody can actually trade at the mid directly in most markets — it's not a price anyone has agreed to trade at, it's a reference point, useful for things like marking a position's value for risk purposes or benchmarking how good an execution was relative to where the market "was" at that moment, without favoring either the buy side or sell side of the spread.

The last (or last trade) is the price at which the most recent actual transaction occurred. Unlike bid, ask, and mid, which describe the current state of standing orders waiting to trade, the last price describes something that already happened — and in a fast-moving or thinly traded market, the last trade can be meaningfully stale, having occurred seconds or minutes ago at a price the market has since moved away from. A stock's bid and ask can shift several times between one trade and the next, especially in an illiquid name, so relying on "last" as if it were the current price can be misleading exactly when the market is moving fastest.

Which of these four numbers is "the price" genuinely depends on the question. A trader deciding whether to hit a market order looks at bid and ask directly. A risk system marking portfolio value typically uses the mid, since it doesn't systematically overstate or understate value in either direction. A headline reporting "the stock closed at..." is almost always quoting the last trade of the day. Confusing these — especially treating last as tradable, or mid as though it were bid or ask — is a common source of confusion for anyone new to reading a market data screen.