What Is the Bid-Ask Spread?
The bid-ask spread is the gap between the highest price anyone will pay and the lowest price anyone will accept. Crossing it is what a market order does, which makes the spread a cost charged on entry and again on exit, without ever appearing as a line item.
How it works
A quoted price is not one number. At any moment there is a highest price someone is willing to buy at — the bid — and a lowest price someone is willing to sell at — the ask. The two are never the same, because if they met they would trade.
The chart on a screen usually shows one line. That line is the last trade, or the midpoint, and neither is a price you can transact at. To buy you pay the ask; to sell you receive the bid.
On the site’s shared price history that gap measures 2% of a typical bar’s range. That number is the whole page in one figure, and it is the reason costs are discussed before strategy.
Where the two prices come from
The spread is the distance between the best bid and the best ask in the order book. Nothing more exotic than that. It exists because the two sides of a market are separate populations of resting orders, and the gap is what separates them.
You cross it twice. Buying at the ask and later selling at the bid means the spread is charged on entry and again on exit. A position that closes at exactly the price it opened at is a loss of one full spread, before commission.
The quote comes with a quantity attached. A bid of 99.98 for 200 shares is a promise about 200 shares. Sell 2,000 and the first 200 fill there while the rest fill lower, walking down the book. That is why the spread you see and the spread you pay diverge as size grows.
In practice: when the spread widens
The spread is a function of how many people are competing to quote. Where there are many, it is narrow; where there are few, it is wide. Nothing about the company changes between those two states — only the number of participants.
It also varies through the day. It is widest at the open, while the overnight news is being repriced, and around the close. In the quiet middle of a session it is at its narrowest, which is the opposite of when most people want to trade.
Now put a target next to it. A 10-cent target against a 4-cent spread hands 40% of the move to the cost before the trade begins. The same 4-cent spread against a 2-dollar target is 2%. The spread has not changed; the trade has.
This is why scalping is a costs problem before it is a skill problem. The smaller the target, the larger the proportion the spread takes, and no amount of accuracy recovers it.
Commission is visible and the spread is not. A zero-commission broker has removed the line item, not the cost. The spread is still charged, twice, and it never appears on a statement.
That invisibility is what makes it worth measuring deliberately. A trader who counts commissions and ignores spreads is counting the smaller of the two numbers on almost every retail account opened in the last five years. The exercise takes a minute: take the current bid and ask, double the gap, and compare it to the move you are aiming at.
When it fails
In a quiet range the spread can exceed the opportunity. The chart above has plenty of bars whose entire range is a small multiple of the spread. Trading it actively means paying the cost repeatedly for a move that barely covers it once.
The other failure is assuming the spread is stable. It widens exactly when conditions are difficult — on news, at halts, in the first minute — which is when a market order is most likely to be sent and least likely to be well filled.
A third failure is trusting the quoted spread as the cost you paid. The quoted spread is the gap between the two best prices. The effective spread is what your fill actually cost against the midpoint at the moment you sent the order, and on a fast-moving symbol the two can differ substantially. Only the second one is a number about your trade.
And an odd lot — fewer than 100 shares — is treated differently. Odd-lot orders do not set the National best bid and offer, so a screen quote can look tight while the size behind it is not available to you at all. Checking the size next to the price is as important as checking the price.
The fourth failure is comparing spreads across instruments as though they were comparable. Two cents on a hundred-dollar share and two cents on a five-dollar share are the same absolute number and very different costs. The only version of the figure worth carrying between symbols is the one expressed against that symbol’s own typical range, which is why every cost quoted on this site is given as a percentage of a median bar rather than in currency.
The original data
Only 3 of the 24,971 videos measured for this site cover the bid-ask spread, at a median of 26 views. That is the lowest median of any topic in the market-mechanics group, and the mismatch is striking: it is the most universal cost in trading and close to the least discussed.
The 2% figure used throughout this page is measured, not assumed. It is the site’s standard round trip expressed against the median bar range of the shared history, and it is the same number used on every costs discussion here so that two pages never quote two answers.
Related
The order book is where the bid and ask actually live. Market makers are the participants whose business is quoting both sides of it. And the order types page explains which instructions cross the spread and which ones wait on the far side of it.
The spread is the number I check before I check anything else on an unfamiliar symbol, because it sets the smallest trade that can possibly make sense. If the spread is a third of the move I am aiming at, I am not trading it — there is nothing to think about.
— Michael Whitman
This page is educational, not financial advice. Test every idea on your own charts before risking money.