Bid Size

The bid size is the quantity of a security that buyers are willing to purchase at a specified bid price. It indicates how many shares (or contracts) are available to be bought at the current best bid and is a key indicator of demand and liquidity in the market.

Key takeaways

• Bid size shows the number of shares buyers want at a given bid price.
• In many markets, bid size is expressed in board lots (commonly 100 shares per lot). A bid size of 4 therefore means 400 shares.
• Level 1 quotes show the bid size for the best available bid only. Level 2 quotes show additional bid prices and sizes (depth of market).
• Comparing bid size and ask size helps gauge supply, demand, and likely price impact for large orders.

How bid size works

Bid size is typically shown alongside the best bid price in a quote. For example, a bid price of $50 with a bid size of 5 means buyers are willing to purchase 500 shares at $50 each. The ask size is the converse: how many shares sellers are offering at the ask price.

Beyond the best bid, there are often multiple lower bid prices, each with its own bid size. That fuller picture is available via level 2 market data or “depth of market” (DOM) feeds, which show multiple bid and ask layers and their sizes.

Real-world example

Suppose the order book shows:
* Bid: $50 — size 5 (500 shares)
* Next bid: $49 — size 10 (1,000 shares)

If you want to sell 1,500 shares immediately:
* 500 shares would fill at $50 = $25,000
* 1,000 shares would fill at $49 = $49,000
Total proceeds = $74,000

If you sell more than 1,500 shares, the next available bid might be significantly lower (for example, $40), so selling a larger block would drive the market price down and reduce average execution price.

Why bid size matters

• Liquidity assessment: larger bid sizes at the top of the book indicate greater immediate buying interest and easier execution for sellers.
• Execution planning: traders use bid/ask sizes and depth to decide whether to place market orders, use limit orders, or break large orders into smaller slices to minimize market impact.
• Price impact and risk: low bid sizes relative to an order size increase the chance of moving the market and getting worse average prices.

Practical notes

• Level 1 quotes are sufficient for small retail trades; active or large traders often subscribe to level 2 or DOM data to see full depth.
• Board lot conventions (e.g., 100 shares) are common but can vary by market; odd-lot orders also exist and may be treated differently.

Understanding bid size helps evaluate current demand, estimate execution risk, and plan order strategies to limit adverse price movement.