The Bid-Ask Spread

Before you can understand why your trades sometimes fill at unexpected prices, you need to understand the bid-ask spread — the invisible cost embedded in every single trade you make. It’s not a fee your broker charges. It’s a cost built into the structure of the market itself.

“In trading, the bid-ask spread is the toll you pay to cross the bridge between wanting to trade and actually trading.” — Larry Harris, Trading and Exchanges

What Is the Bid-Ask Spread?

At any given moment, there are two prices for every security. The bid is the highest price a buyer is currently willing to pay. The ask (also called the offer) is the lowest price a seller is currently willing to accept. The difference between these two prices is the spread.

If a stock has a bid of $49.95 and an ask of $50.00, the spread is $0.05. If you want to buy immediately (market order), you pay the ask — $50.00. If you want to sell immediately, you receive the bid — $49.95. The moment you buy at $50.00, you could only sell at $49.95, meaning you’re instantly down $0.05 per share. This is the real, built-in cost of every trade.

Key Concept: The spread is the market maker’s compensation for providing liquidity. They stand ready to buy at the bid and sell at the ask, pocketing the difference. Without market makers willing to take the other side of your trade, you’d have to wait for another retail trader who wants the exact opposite of your position — which could take hours or days.

Why Spreads Vary

Not all spreads are equal. Apple might have a spread of $0.01 while a small biotech stock might have a spread of $0.15. The factors that determine spread width are liquidity (how many shares trade daily), volatility (how fast prices move), and the number of market makers competing for order flow.

Highly liquid stocks like those in the S&P 500 have the tightest spreads because millions of shares trade daily and dozens of market makers compete to fill orders. Low-volume stocks have wider spreads because market makers need more compensation for the risk of holding less liquid positions.

Real-World Example: During normal trading hours, SPY (the S&P 500 ETF) typically has a spread of just one cent — $0.01. It’s the most liquid security in the world, trading over 80 million shares daily. Compare this to a small-cap stock trading 50,000 shares daily, which might have a spread of $0.10-$0.50. On a 100-share trade, the spread cost on SPY is $1. On the small-cap, it’s $10-$50. Over hundreds of trades, this difference compounds enormously.

Spread as a Hidden Trading Cost

Most beginners focus on broker commissions when thinking about trading costs. But for active traders, the spread often costs far more than commissions. If you make 10 round-trip trades per day on a stock with a $0.05 spread, trading 500 shares each time, your daily spread cost is 10 x 2 x 500 x $0.05 = $500. Over 250 trading days, that’s $125,000 in spread costs alone — and that’s before commissions, fees, and slippage.

Golden Insight: The single most impactful thing you can do to reduce trading costs is to trade liquid securities with tight spreads. A stock with a $0.01 spread costs you 10x less per trade than one with a $0.10 spread. Many beginners are attracted to cheap, low-volume stocks thinking they’ll get bigger moves, but the wide spreads eat their profits before the trade even starts.

Reading the Order Book (Level 2 Data)

The bid and ask you see are just the best prices — the top of the order book. Below them sits a queue of orders at various prices, called Level 2 or depth-of-market data. This shows you not just the best bid and ask, but all pending limit orders at each price level and how many shares are available.

Seeing depth is valuable because it tells you about supply and demand at different price levels. A thick stack of buy orders (bids) below the current price suggests support. A wall of sell orders (asks) above suggests resistance. The depth also tells you how much you can trade before significantly moving the price.

Key Concept: If you try to buy 10,000 shares but only 500 are offered at the ask, your order will “walk up” the order book, filling at increasingly higher prices. This is why large orders often use algorithms that break them into smaller pieces executed over time — to avoid moving the market against themselves.

When Spreads Widen

Spreads aren’t fixed — they change constantly based on market conditions. Spreads typically widen during high volatility (market makers increase their compensation for higher risk), around major news events (earnings, Fed announcements, economic data), at market open and close when order flow is chaotic, during pre-market and after-hours trading when fewer participants are active, and in market crashes when liquidity evaporates.

Common Mistake: Trading during the first and last 15 minutes of the trading day is tempting because of big moves, but spreads are often 2-5x wider during these periods. A stock with a normal $0.02 spread might have a $0.08 spread at 9:30 AM. Unless you have a specific strategy designed for the open, waiting 15-30 minutes for spreads to normalize often improves your execution significantly.

Using Spreads to Your Advantage

Instead of always paying the spread by using market orders, you can use limit orders to trade at or inside the spread. If a stock has a bid of $49.95 and ask of $50.00, placing a limit buy at $49.97 means you’re offering a better price than the current bid, which may attract sellers to fill you — and you’ve saved $0.03 per share compared to buying at the ask.

This technique, called “improving the bid,” is how professional traders operate. They rarely cross the spread unless they absolutely must have the execution immediately.

Test Your Understanding

A stock has a bid of $30.00 and ask of $30.10. You buy 1,000 shares at market. What is your immediate spread cost?

You buy at the ask ($30.10) but could only immediately sell at the bid ($30.00). That’s $0.10 per share x 1,000 shares = $100 in immediate spread cost.

Which scenario would likely have the WIDEST bid-ask spread?

Micro-cap stocks already have wide spreads due to low liquidity. After-hours trading has even fewer participants, so spreads widen further. This combination produces the widest spreads you’ll encounter.