The order book is basically a live queue of buy and sell orders at every price level. Everything that happens on an exchange shows up here: prices don’t just move on their own—someone always has to take liquidity out of the queue for the price to change.

Two Sides and the Spread

On the left (or bottom), you’ve got the bids—buy orders. On the right (or top) are the asks—sell orders. The best (highest) bid will always be lower than the best (lowest) ask. Otherwise, they’d match instantly and get filled against each other.

The gap between them is called the spread. On liquid pairs like BTC/USDT, the spread is tiny—often just a fraction of a percent. On illiquid pairs, it can be much wider, sometimes several percent. The spread is your direct cost to get in and out: if you market buy and immediately market sell, you lose the spread.

Limit Orders vs Market Orders

A limit order sits in the book and waits for the price to come to it. This adds liquidity—the order book is built from these limit orders.

A market order fills instantly at the best available prices, grabbing whatever’s in the queue. This removes liquidity from the book.

The key takeaway: market orders move the price, limit orders hold it. Heatmaps show those limit orders—the ones waiting in line.

Market Depth and Slippage

Depth is how much size is sitting at each price away from the current price. This tells you how much the price could move if a big order hits the market.

If you buy more than what’s available at the best ask, your order “eats through” that level and starts filling higher asks. The difference between your expected price and your actual average fill price is called slippage. On thin markets, a big order can move the price several percent.

That’s why whales rarely smash the market with one giant order—they build positions in pieces, often with limit orders. On the heatmap, you’ll see these as solid horizontal bands.

Why the Order Book Can Be Misleading

The order book shows intent, but intent can change fast. Orders can be pulled in a millisecond, and a lot of the book is just market makers and bots constantly moving their quotes around.

The best way to spot real interest is with time. An order that sits for hours is very different from one that blinks in and out in a second, even though they look identical in a snapshot. That’s why heatmaps are built from order book history, not just the current view—time is the missing dimension you don’t get in a regular book.

Spot vs Futures Order Books

The spot order book is for real assets—orders are backed by actual money or coins. The futures book is for contracts, where traders use leverage and there might not be any real underlying asset at all.

If you want to gauge true supply and demand, spot books are way more useful. We pull spot data from multiple exchanges—Binance, Coinbase, Bitstamp, MEXC—and aggregate it: see the Bitcoin heatmap.