What a Market Order Taught Me About Liquidity

Your order does not pay what you think it pays. The quote on your screen is one price attached to one quantity. The quantity is the half almost nobody reads.

Folklore says the spread is your execution cost. The spread is the cost of the first few hundred shares. After that you are into territory nobody quoted you.

A quote is a price and a size

The ask says 20.10. What it means is that somebody is willing to sell a specific number of shares at 20.10. Four hundred, say. Once those four hundred are gone, 20.10 stops existing.

You want three thousand shares and you want them now. The rest of your order takes whatever sits above.

Shares offered	Price
400	20.10
700	20.14
900	20.19
1,000	20.28

Your fill is the weighted average of that walk. (400 x 20.10) + (700 x 20.14) + (900 x 20.19) + (1,000 x 20.28) = 60,589, and 60,589 / 3,000 = 20.196.

You were quoted 20.10 and you paid a touch over 20.19. The bid was 20.06. The spread you thought you were crossing was four cents. The bill came to nearly ten. In dollars, 60,589 - (3,000 x 20.10) = 289. Later you will describe that position to somebody as bought at twenty ten.

Nobody sends you an invoice

This is the part that keeps the mistake alive. The commission line is small and honest. The platform shows you one tidy average fill. Nowhere on the confirmation is there a field that says you paid $289 for immediacy in a name that could not supply it, and there never will be, because the charge was collected by the people who sold to you, and they are under no obligation to itemize it.

The only way to see the charge is to write down the quote before you press the button and compare it with the fill afterwards. Almost nobody does this after a winning trade. I started doing it after every trade, winner or loser. The pattern came out fast. In the largest names the difference rounded to nothing. Below a certain average daily volume it ran to several times the spread I thought I had seen. Every time.

The same ticker is two different stocks at different hours

Depth belongs to whoever happens to be quoting. The company has nothing to do with it.

At eleven in the morning the book in a mid cap can be four deep and civilized. At 9:31 the same ticker will show you a hundred shares a side and a spread three times as wide, because the people who normally supply the depth are still working out what the overnight session meant. Ten seconds after a headline the depth goes away entirely. Which is precisely when everybody wants to trade.

So the question is never whether a stock is liquid. It is whether it is liquid for your size, at the minute you intend to act.

What a limit order actually buys you

I use marketable limits now. A limit set a cent or two through the offer fills most of the time, and it puts a ceiling on how far the price can run away from me while the order works.

When the book is too thin to fill me there, the order sits. That silence is the useful part. A name that will not give me my size at a sensible price is telling me something I would otherwise have found out from the confirmation screen.

The other change was giving up market orders as a way of expressing urgency. Urgency is a feeling. The book prices size and time. It has no field for how you feel. When you genuinely need out of a position right now, a market order is the correct tool and you pay the bill without complaining, and when you merely feel like you need out, the same order costs you the same money and buys nothing.

None of this is exotic. The plumbing, including who is quoting and why the spread exists at all, is in market makers and liquidity, and the order types themselves, with the specific ways each one hurts you, are in stock order types explained. If you want the version with the theory attached, market microstructure walks through how a price forms out of these collisions.

Check the size on the bid and the ask against the size you intend to trade. Do it before you choose the order type. If your order is larger than the top of the book, you are about to buy the next few layers too, and the only question left is whether that was a decision or an accident.