The price you see on a screen is only the top of the market. Behind it sit layers of buy and sell orders at different prices, waiting to be filled. How many orders there are, and at which prices, is called market depth. It tells you how easily a market can absorb trades, and why some markets lurch while others glide.
What market depth is
Market depth describes the volume of buy orders, called bids, and sell orders, called offers or asks, at each price level away from the current price. It is shown in the order book, sometimes called Level 2 data. A deep market has lots of orders at many price levels. A thin, or shallow, market has few.
Why it matters
Depth determines how much the price moves when someone trades. In a deep market, a large order can be filled close to the current price because there are plenty of orders waiting. In a thin market, the same order eats through the available orders at the best price and has to be filled at worse and worse prices. That is called slippage, and it is why thin markets move sharply.
Depth changes throughout the day. Markets tend to be deepest when the main trading centres are open and overlapping, such as when London and New York are both active. They are often thinner overnight, around holidays and just before major news, when traders pull their orders to avoid being caught out.
A simple example
Imagine a share trading at 100p. In a deep market, there might be orders to sell 50,000 shares at 100p, another 50,000 at 100.5p and so on. A buyer wanting 20,000 shares would be filled at 100p. In a thin market, there might be only 2,000 shares offered at 100p, 3,000 at 101p and 5,000 at 103p. The same buyer would push the price up several pence just by trading, and pay an average price well above 100p.
How traders use it
Traders use depth to judge liquidity before placing larger orders and to choose when to trade. Many avoid trading less liquid shares or currency pairs at quiet times. Some short-term traders watch for large orders sitting at particular prices, which can act as temporary support or resistance, although large orders can be pulled at any moment.
Depth also helps explain sudden moves. When a market drops sharply on modest news, it is often because depth had already thinned out, leaving little to cushion the fall.
Common mistakes
A common mistake is assuming you can always trade at the price on the screen. With bigger sizes or thin markets, you may not. Another is placing market orders in illiquid shares, which can lead to poor fills. A third is reading too much into large visible orders, which may be there to influence other traders rather than to be filled.
The bottom line
Market depth shows how many orders sit behind the current price. Deep markets absorb trades smoothly; thin markets move sharply. Understanding depth helps you choose when and how to trade, and avoid paying more than you need to. If you want to test how well you understand trading mechanics, try the free Samuel & Co Trading Trader Assessment.
