Mark Price vs. Last Price: The Number on Your Option Chain That Can Lie
Two numbers, one row, two different meanings
Pull up almost any option chain and you'll find a row with a bid, an ask, a last price, and a mark. Traders glance at the mark and treat it as "the price" — the number their P&L is measured against, the number the platform shows as the option's current value. But the mark isn't a trade. Nobody bought or sold at it. It's a calculated number, and understanding how it's calculated is the difference between reading the chain correctly and being quietly misled by it.
Where the mark actually comes from
The mark price is, in its simplest form, the midpoint of the National Best Bid and Offer (NBBO): (bid + ask) / 2. If a contract is quoted $2.10 bid / $2.30 ask, the mark is $2.20. Some brokers apply small adjustments — rounding to the nearest cent or half-cent, or nudging the mark slightly toward the side with more size — but the core idea is the same everywhere: mark is a snapshot of where the quote currently sits, not a record of where a trade happened.
That's exactly why brokers use it for account valuation. Unrealized P&L, margin calculations, and the equity figure at the top of your account all need a number that updates continuously throughout the session, even on contracts that haven't traded in hours. The bid and ask update essentially in real time as market makers adjust their quotes; the mark rides along with them. Last price does not — it only changes when an actual trade prints.
Why last price can be the stale one
On a liquid, near-the-money weekly option, last price and mark usually sit close together because trades happen constantly and keep the last print fresh. On a thin, far-dated, or deep-OTM contract, that breaks down fast. A strike that last traded at 10:15am can still be showing that same last price at 2:00pm, even though the underlying has moved and the bid/ask has repriced substantially around it. In that scenario, the mark is the number reflecting reality; last price is a fossil.
The reverse can also happen. A single outsized print — a large institutional order that got worked through at a price away from the prevailing quote, or a trade reported with a delay — can leave last price temporarily out of step with a bid/ask that has already moved on. Reading last price in isolation, without checking it against the current bid/ask, is a common way to misjudge where a contract actually sits.
The gap between mark and what you'll actually pay
The bigger trap is treating the mark as an executable price. It isn't. It's the midpoint of a two-sided market, and market makers are under no obligation to trade with you there. On a tight market — a penny or two wide — the mark is a reasonable estimate of a fair fill. On a wide market, the gap between mark and what you'll actually pay to buy, or receive to sell, can be substantial. A contract quoted $1.00 bid / $1.60 ask has a mark of $1.30, but a market order to buy fills at $1.60 — 23% above the number the chain displayed as "the price." That gap is exactly the cost explored in how bid-ask spreads quietly tax every entry and exit, and it's the reason wide-market contracts deserve more caution than the mark alone suggests.
This is also why the mark is a reasonable starting point for a limit order, not a target. A common approach is to place a limit at or near the mark first, then "walk" the price toward the bid or ask in small increments if it doesn't fill within a few seconds — rather than crossing the full spread immediately with a market order.
Reading the row correctly
Put together, the three numbers answer three different questions. Last price answers "when did this contract most recently trade, and at what price?" Bid and ask answer "what could I actually transact at right now?" Mark answers "what is this position worth on paper at this instant?" None of them alone tells the full story, and on any contract wide enough to matter, the honest read of the row comes from comparing all three — not defaulting to whichever one loads first. For a fuller column-by-column walkthrough of everything else on the row, see how to read an option chain without getting lost, and for how these numbers can shift between the moment you look and the moment you click, see why your option chain might not match what your broker shows.