Understand bid, ask, and spread basics

Core Concept

WHY these exist: Markets are continuous auctions. At any moment, there's a queue of unfilled buy orders (bids) and unfilled sell orders (asks). The best bid and best ask sit at the top of the order book. When they match, a trade executes and those orders disappear. The spread exists because the best buyer and best seller haven't agreed yet.

The Order Book Model

WHAT is an order book? A real-time list of all pending limit orders, sorted by price. Buy orders (bids) descend from highest to lowest; sell orders (asks) ascend from lowest to highest.

Deriving the spread from first principles:

1. Assume $N$ buyers submit limit orders at prices $b_1 \geq b_2 \geq \ldots \geq b_N$.
2. Assume $M$ sellers submit limit orders at prices $a_1 \leq a_2 \leq \ldots \leq a_M$.
3. The best bid is $B = b_1$ (highest buyer).
4. The best ask is $A = a_1$ (lowest seller).
5. The spread is $S = A - B$.

Constraint for no instant trade: $A > B$. If $A \leq B$, the orders would have already matched (a market-clearing event). So the order book persists only when $A > B$, hence $S > 0$ in a stable snapshot.

WHY the mid-price? It's the market's best guess of "fair value" when no trade has occurred. Many algorithms and charts reference the mid-price, though you can't trade at it (you pay the ask to buy, get the bid to sell).

Worked Examples

Calculate the spread:

Calculate the mid-price:

Calculate percentage spread:

WHY this matters: If you buy at $50.10 and immediately sell at \50.00, you lose \0.10 per share (the spread). This is the implicit transaction cost. High-frequency traders care about even1-cent spreads because they trade millions of shares.

Spread: $5.50 - 5.00 = 0.50$0.50$

Percentage spread: $\frac{0.50}{5.25} \times 100\% \approx 9.5\%$

WHY so wide? Few buyers and sellers. Market makers demand a large spread to compensate for inventory risk (holding shares they may not offload easily). If you pay $5.50 to buy, you're instantly down9.5% relative to mid-price—before the stock moves at all.

WHAT does this tell you? High spread = low liquidity. Avoid market orders here; use limit orders near the mid-price and wait for fills.

New event: A buyer submits a market order to buy 100 shares.

WHAT happens?

1. The market order "crosses the spread" and executes at the ask price ($100.05).
2. The seller at $100.05 is filled.
3. If that seller's order is exhausted, the next ask (say $100.06) becomes the new best ask.

WHY the buyer pays the ask: A market order prioritizes speed over price. The buyer says "I'll take the best available offer right now," which is the ask. Conversely, a market sell order hits the bid.

Spread after trade: Suppose the next ask is $100.06 and bid stays100.00. New spread = \0.06. The spread fluctuates as orders are added and removed.

Common Mistakes

Steel-man (why it feels right): Charts often show the mid-price as "the price," and it's the average of bid and ask—seems fair!

Why it's wrong: The mid-price is not a tradable level. To buy, you must pay the ask (someone has to be willing to sell to you at that price). To sell, you receive the bid. The mid is a reference, not an execution point.

The fix: When estimating costs, always add half the spread to the mid-price for buys (you pay the ask) and subtract half for sells (you get the bid). Use limit orders if you want to target a price between bid and ask, but know you might not get filled immediately.

Steel-man: 10 cents on a $50 stock is 0.2%, less than typical price moves.

Why it's wrong: If you trade frequently (day trading, rebalancing), spread costs compound. 10 trades at 0.2% each = 2% loss before the stock moves. Also, wide spreads indicate illiquidity; your10-share order might move the price if the order book is thin.

The fix: Multiply spread cost by your expected trade frequency. For buy-and-hold investors, spread is one-time overhead. For active traders, it's a recurring drag. Always check average daily volume and order book depth before trading.

The Spread as Liquidity Signal

HOW to interpret spread size:

• Tight spread (< 0.1% on large-cap stocks): High liquidity, many market makers, low risk for them. Easy to enter/exit positions.
• Wide spread (> 1%): Low liquidity, few participants, high risk. Market makers demand premium. Price can gap unpredictably.

Derivation of spread's economic role:

1. Market makers provide liquidity by placing both bid and ask orders.
2. They profit from the spread: buy at bid, sell at ask, pocket $S$ per share.
3. Their risk: the stock moves against them before they offload inventory.
4. In equilibrium, $S$ must compensate for adverse selection risk (informed traders) and inventory risk (volatility).

Formula (simplified market-maker model):

where $\sigma^2$ is variance (volatility). Higher vol or lower volume → wider spread.

WHY this matters: The spread is not arbitrary. It reflects the market's uncertainty and liquidity. A widening spread can signal upcoming news or declining interest (volume drying up).

Active Recall Practice

If the kid really wants the game right now, they'll pay your $20 (they "cross the spread" like a market order). If you need cash right now, you'll accept their15 (you cross the spread the other way). But if both of you wait, maybe you drop to \18 \text{ and they raise to } \backslash$17, and you meet in the middle eventually.

In the stock market, thousands of people are doing this at once with shares. The "bid" is the best price any buyer is offering, the "ask" is the best price any seller wants, and the spread is the gap. Tight spread = lots of people buying and selling (easy to trade). Wide spread = not many people interested (harder to trade without losing money).

Or: Bid is Buyers' best offer. Ask is Sellers' best offer. The spread is the gap between them.

Connections

• Order Book Structure – Bid/ask are the top of the order book; understanding depth matters for large orders.
• Market Orders vs. Limit Orders – Market orders pay the spread instantly; limit orders try to avoid it.
• Liquidity and Volume – High volume usually means tight spreads; low volume means wide spreads.
• Market Makers – They profit from the spread and provide continuous bid/ask quotes.
• Transaction Costs – Spread is an implicit cost, alongside commissions and slippage.
• Price Discovery – The mid-price aggregates bid/ask info to estimate fair value.
• High-Frequency Trading – Exploits tiny spreads at microsecond speeds.

Flashcards

#flashcards/stock-market

What is the bid price? :: The highest price a buyer is currently willing to pay for a security (what you get if you sell with a market order).

What is the ask price? :: The lowest price a seller is currently willing to accept for a security (what you pay if you buy with a market order).

Concept Map

Hinglish (regional understanding)