Liquidity
How easily an asset can be bought or sold without moving its price.
Also called: liquid · illiquid · depth
Written by Javier Sánchez Ros
In plain language
A liquid market has many buyers and sellers at closely spaced prices. You can put size in and take it out again without the price noticing.
An illiquid market has gaps in the order book. Your own order becomes the news: it pushes price away from you on the way in, and there is nobody to sell to on the way out.
Liquidity is not constant. The same stock is deeply liquid at midday and thin in after-hours trading. Liquidity also disappears exactly when you most want it — during a sharp sell-off.
Worked through
The same 2,000-share stop in a deep book and a thin one
- Deep book: shares near the bid
- tens of thousands
- Fill
- within a cent or two
- Thin book: shares near the bid
- a few hundred
- Fill
- several percent lower
The order is identical. What differs is how many buyers are standing underneath it. In a deep book, 2,000 shares is a fraction of what is resting at the first level and the fill is essentially the stop price. In a thin one, the order clears the first few levels and keeps going until it finds enough size, which can be a long way down.
The position size was calculated assuming the stop price. That assumption is not about your analysis — it is about the other participants, and it holds or fails for reasons entirely outside the trade.
Liquidity also disappears precisely when it is called upon. A book that looks respectable at midday empties in a panic, because the same event that triggers your stop triggers everyone else’s and removes the buyers at the same moment.
The practical test costs nothing: before sizing a position, look at the book at the level where the stop sits and ask whether your order is a rounding error there or the whole of it.
Seen on a chart
Why it matters
Liquidity determines whether your stop loss can actually be filled near your stop price. In an illiquid instrument, a stop is a hope, not a guarantee.
Common mistakes
- Sizing a position by risk math alone without checking whether the market can absorb it.
- Treating average daily volume as available liquidity. Most of that volume is not there at the moment you need it.
- Assuming a stop loss caps your loss in a market that can gap through it.
Keep exploring
These concepts are connected. Understanding one usually makes the next one easier.
The gap between the bid and the ask — the built-in cost of entering a trade.
The number of shares, contracts or units traded during a period.
The difference between the price you expected and the price you actually got.
The live list of all resting buy and sell orders at each price level.
A predefined exit that closes a losing trade before the loss becomes serious.