What it is

A stock does not trade at a single price. At any moment there are two live prices:

• The bid price - the highest price a buyer is currently willing to pay.
• The ask price (or offer) - the lowest price a seller is currently willing to accept.

The bid-ask spread is the gap between them. If the bid is 100.00 and the ask is 100.05, the spread is 5 cents. To buy right now you pay the ask; to sell right now you receive the bid. Crossing that gap is a real, if hidden, cost of trading that most beginners overlook because no separate fee line ever shows it.

How it works

Think of a marketplace. Buyers post the most they will pay; sellers post the least they will take. The two best prices face each other across the spread, and only when a buyer agrees to meet a seller's ask (or a seller agrees to meet a buyer's bid) does a trade actually print.

When you place a market order, you accept whatever the best available price is - the ask if buying, the bid if selling. So the wider the spread, the more it costs you simply to enter and exit. Buy at the ask and immediately sell at the bid and you are already down by the full spread before the price has moved at all.

The size of the spread is driven by liquidity - how easily an asset trades without moving its price.

• A heavily traded large company has huge volume and many competing orders, so the spread is tiny (a penny or less).
• A thinly traded small stock has few orders, so the spread can be wide (several cents or more).
• Spreads also widen at times of stress or low activity - overnight, in extended hours, or during fast news - because fewer participants are willing to quote.

Closely related is depth: how many shares are waiting at each price level. A deep market can absorb a large order without the price slipping; a shallow market cannot. Two stocks can show the same narrow spread, yet one may have thousands of shares behind the quote and the other only a few hundred - the second will move far more when a big order arrives.

How to read it

When you look at a quote, read both sides:

1. Note the bid and the ask - your real entry and exit prices.
2. Subtract them to see the spread - your built-in round-trip cost.
3. Compare the spread to the size of the move you expect. A 10-cent spread is trivial on a $2 swing, but ruinous on a 5-cent scalp.
4. Glance at the depth behind the best prices. Thin depth warns that a large order will suffer slippage - a worse fill than the quoted price.

For example, imagine a stock quoted 50.00 bid / 50.10 ask - a 10-cent spread. Buy 100 shares with a market order and you pay 50.10; sell them back instantly and you receive 50.00. You have lost 10 dollars on the round trip with no price move at all. If you trade this stock ten times a day, the spread alone is a steady drain. On a deeply liquid stock quoted 50.00 / 50.01, the same round trips cost a tenth as much.

Key takeaway: The spread is a cost you pay every round trip; liquidity and depth decide how large that cost is.

Strengths & limits

The spread is a fast, honest read on liquidity: narrow means liquid, wide means thin. It needs no calculation and updates in real time, which makes it one of the first things experienced traders glance at before committing to a name. But it only shows the very top of the order book - the best bid and ask. It says nothing about how the price will behave once your order is bigger than the visible depth. In fast markets the quote can also change between the moment you see it and the moment you trade, so the spread you pay may differ from the spread you read. Treat a tight spread as necessary but not sufficient evidence of good liquidity, and always check depth before sizing up.