Reading an Order Book: Depth, Slippage and Why Your Fill Price Moves

The book is a queue, not a price

An order book is a list of intentions: buyers waiting at their prices, sellers waiting at theirs. The number displayed as ‘the price’ is simply the last completed trade — it says nothing about how much you can buy at that level.

The useful figure is depth: how much volume sits within one percent of the current price. Two markets can show the same price and behave completely differently once you place a real order.

Slippage is arithmetic, not bad luck

A market order walks the book: it fills against the best price, then the next, then the next, until the requested size is complete. If the first level holds a fraction of your order, the rest fills higher — and the average price you get is worse than the one you saw.

This is why the same order can cost noticeably more during quiet hours. The book thins out, and each level holds less.

Limit orders trade certainty of price for certainty of fill

A limit order guarantees the price and risks not being filled at all. A market order guarantees the fill and risks the price. There is no third option, and choosing between them is most of practical execution.

For larger sizes, splitting an order into parts across several minutes usually beats both — it gives the book time to refill between fills.

Spread tells you the cost of changing your mind

A wide spread means entering and leaving immediately costs you the difference. On thin pairs that round trip can exceed a week of expected gains.

Comparing spreads across pairs is a faster reality check than comparing headline fees.

A short checklist before pressing buy

Look at depth within one percent, the current spread, and recent volume during this hour of the day. Size the order so it consumes no more than a small share of visible depth.

Two minutes of this saves more than most fee discounts ever will.