Level 1, Level 2 And Level 3 Market Data

Ask a broker's basic app for a stock's price and you get a single number: the best bid, the best ask, the last trade. Ask a professional trading desk and you can get, order by order, the identity and exact size of every single resting order in the book. These are different levels of market data, and which one a strategy needs depends entirely on what it's trying to do.

Level 1 is the top of book: the best bid price and size, the best ask price and size, and the last trade. It's what a retail trading app shows and it's enough to know "what would I pay right now," but nothing about how deep liquidity goes beyond the very top.

Level 2 adds depth: the full ladder of bid and ask prices below the touch, aggregated by price level (so you see "500 shares bid at 99.98, 300 at 99.97," etc., but not which individual orders make up that 500). This is what most professional and semi-professional trading terminals show, and it's the minimum needed to estimate things like queue imbalance or the microprice, which require knowing size at multiple price levels.

Level 3 goes further: every individual order at every price level, with a unique order ID, so you can track a specific order from the moment it's posted through partial fills, modifications, and cancellation. This is also called market-by-order (MBO) data, and it's what's needed to reconstruct exact queue position — knowing not just that there's 500 shares bid at 99.98, but that your specific order is the 3rd one in that queue with 200 shares ahead of it.

Worked example. A stock's book at a moment in time: Level 1 shows "Bid: 99.98 x 500, Ask: 100.00 x 300." Level 2 adds the next few levels: "99.98 x 500, 99.97 x 1200, 99.96 x 800" on the bid side and "100.00 x 300, 100.01 x 600, 100.02 x 950" on the ask. Level 3 breaks the 99.98 x 500 bid level down further: "Order #4471: 200 shares (posted 09:31:02.114), Order #4483: 150 shares (09:31:04.887), Order #4501: 150 shares (09:31:09.220)" — showing that a trader whose order is #4501 has 350 shares of queue ahead of them at that price, information Level 2 alone cannot provide.

What this means in practice

Level 1 is enough for most retail and even many systematic strategies operating on slower timescales. Level 2 is the baseline for anything that reasons about liquidity or order-flow imbalance. Level 3/MBO data is expensive, high-bandwidth, and mostly the domain of market makers and HFT firms whose edge specifically depends on knowing exact queue position, because that determines whether a resting order will actually get filled before the price moves away.

See Market-By-Order Versus Market-By-Price Feeds for the deeper technical distinction between how Level 2 and Level 3 feeds are actually constructed and transmitted, and Queue Position and Priority for why exact queue position is worth paying for Level 3 data to know.