The bid ask spread. The hidden cost of every trade.

Every trade pays the bid ask spread to enter and the spread again to exit. On liquid large caps the cost is pennies per share and barely noticed. On illiquid stocks and options the spread is the largest single cost of trading, often dwarfing commissions. Most retail traders ignore the spread until they look at their net P and L and realize a meaningful portion of their edge has been quietly eaten by it.

What the bid ask spread actually is

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 spread is the difference between them.

SPY current quote. Bid $500.00, ask $500.01. Spread $0.01.

To buy SPY immediately, you pay the ask ($500.01). To sell immediately, you receive the bid ($500.00). The $0.01 difference is the spread cost.

If you buy and immediately sell, you pay the spread. $500.01 to buy, $500.00 to sell. Net loss $0.01 per share before considering any market movement.

Why the spread exists

Buyers and sellers do not arrive at the market at the same moment. Someone wants to buy at 10:15 AM. Someone else wants to sell at 2:35 PM. Without a matching system, the buyer would have to wait hours for a seller to show up.

Market makers provide the matching. They post both a bid (where they will buy from sellers) and an ask (where they will sell to buyers). They earn the spread as compensation for warehousing the risk of holding the inventory between when the buyer and seller actually arrive.

Modern markets have many competing market makers and high frequency trading firms that compete on tightness of spread. The competition drives spreads tighter over time on the most liquid names.

The spread by ticker type

Mega cap stocks. Penny spreads.

SPY, QQQ, AAPL, NVDA, MSFT typically have spreads of $0.01 per share. The cost as a percentage of price is negligible for most retail position sizes.

Large cap stocks. One to five cent spreads.

Most S and P 500 stocks have spreads of $0.01 to $0.05. Still small in percentage terms.

Mid cap stocks. Five to twenty cent spreads.

Less liquid names have wider spreads. The cost as a percentage of price starts becoming meaningful.

Small cap stocks. Twenty cent to several dollar spreads.

Illiquid names can have spreads of 1 to 5 percent of the share price. Trading these names costs meaningful percentages just to enter and exit.

Penny stocks. Spreads sometimes 10 to 20 percent.

The most illiquid stocks have spreads that are a meaningful fraction of the share price. The trader who buys and immediately sells loses a large portion of capital just to the spread.

Options. Wider than the underlying.

Options spreads are typically wider than the underlying stock spread. SPY options on the most active strikes have $0.01 to $0.05 spreads. Less active strikes (deep out of the money, far dated) can have spreads of $0.20 to $1.00 or more.

How spreads silently eat retail edge

A scalper targeting $0.20 per share profit on a stock with a $0.02 spread is giving up 10 percent of the profit to the spread on every trade. Over 100 trades a month, the spread cost is 10 percent of the gross P and L.

An options trader buying at $1.20 ask on a contract that bids at $1.10 has paid the $0.10 spread on entry. To break even at the same instant, the contract would need to rise to $1.20 bid. The 8 percent gain just covers the spread.

The spread is invisible in the trade ticker (the trader sees only the price they paid, not the discount available on the other side). The cost only shows in the cumulative P and L over time.

The fix is twofold. Use limit orders inside the spread when possible. Avoid trading instruments with very wide spreads relative to the targeted move.

How to read the spread

Most platforms show the bid and ask separately on the order entry screen. The displayed "last" price is the most recent trade, which may have been at the bid, the ask, or somewhere in between.

Level 2 data shows the full depth of bids and asks at each price level. The "best bid" and "best ask" are the highest bid and lowest ask. The "size" at each level shows how many shares are available at that price.

For options, the chain shows bid and ask for every strike and expiration. The spread varies dramatically across the chain. At the money front month options have tight spreads. Deep out of the money or far dated options have wider spreads.

The cost calculation

Spread cost per trade = spread in dollars times share count.

Spread cost as percentage of trade = spread divided by midpoint price times 100.

Round trip spread cost = spread times 2 (you pay on entry and on exit).

Example. AAPL trade. Buy 100 shares at $200 ask. Sell 100 shares at $199.99 bid. Spread $0.01. Round trip cost $0.02 per share times 100 shares = $2.00 total. On a $20,000 position, $2.00 is 0.01 percent. Negligible.

Example. Small cap trade. Buy 1,000 shares at $5.20 ask. Sell at $5.00 bid. Spread $0.20. Round trip $0.40 per share times 1,000 shares = $400 total. On a $5,200 position, $400 is 7.7 percent. The spread alone is a major cost.

The strategies for managing spread cost

Trade liquid names.

The simplest mitigation. Stick to SPY, QQQ, AAPL, NVDA, MSFT and similar. The penny spreads do not materially affect P and L.

Use limit orders inside the spread.

A limit at the bid (or one penny above) for buys gives the market maker no automatic improvement but sometimes a seller arrives and the order fills at the bid.

Limits at the midpoint sometimes get price improvement on liquid names. Limits at the ask just fill immediately like a market order.

Time your trades to high volume windows.

Spreads are tightest during high volume periods (9:30 to 11:00 AM, 3:00 to 4:00 PM). Spreads widen during lunch and in pre market / after hours.

For options, prefer monthly expiries on active strikes.

The most active strikes (within a few percent of the current price) and the standard monthly expirations have the tightest options spreads. Weeklies on the most liquid underlyings (SPY, QQQ) also have tight spreads.

Avoid market orders on anything that is not liquid.

The slippage on market orders compounds the spread cost. Limit orders give the trader control over the maximum price paid.

The spread tells you about liquidity

A tight spread (penny on the underlying, small percentage on options) signals high liquidity. The market maker is confident in finding the other side quickly.

A wide spread (cents or dollars on the underlying, large percentage on options) signals low liquidity. The market maker is demanding more compensation because the inventory risk is higher.

Spread width is a real time read on liquidity that the trader can use to size positions. Wide spread, smaller position size. Tight spread, normal position size.

Where the audit fits

The audit reads the actual trades and calculates the spread cost embedded in the entries and exits. For most retail traders the pattern is trading instruments where spread cost is eating 5 to 15 percent of gross edge. The plan locks the rule that no trade happens on instruments with spread cost exceeding the planned profit target by a defined percentage. Five to seven pages.