Direct answer
A forex order book is a display of outstanding orders to buy and sell currencies at different price levels. To read it, you focus on two sides (bids and asks), the prices each side is willing to trade at, and the size (depth) available at those prices. Because order books update as orders are added, canceled, or partially filled, what you see at one moment may not persist.
How it works (and what each field means)
Most order books show:
- Bid side: price levels where buyers are willing to purchase.
- Ask side: price levels where sellers are willing to sell.
- Price levels: discrete prices arranged from best to worse (exact spacing depends on the venue).
- Size or depth: how much quantity is currently represented at each price level.
A common way to interpret the book is to identify the best bid (highest bid) and best ask (lowest ask). The difference between these is often referred to as the spread. Reading the book also involves looking “down the ladder” on each side to see how much liquidity sits at higher/lower prices.
What changes and what stays readable
- The top of book (best bid/ask) is usually most relevant for near-term trading conditions.
- The shape of depth (how thick or thin the book is at levels away from the top) can suggest where large resting liquidity currently sits.
Example reading workflow (plus checks you can do)
- Locate bid and ask ladders: confirm which rows correspond to buying vs selling.
- Find best bid and best ask: compare the highest bid to the lowest ask.
- Check depth at a few levels: read the size next to each visible price level near the spread.
- Validate units and aggregation: ensure you understand whether sizes are shown per level, aggregated over time/venue, or expressed in base/quote terms.
- Re-check shortly after: if the book is live, observe whether the top levels or nearby depth move between readings.
These checks help you independently verify that you are interpreting the display correctly for the specific platform or data feed, since formatting and units can differ.
Relevant limitations and risks
- No guaranteed outcomes: an order book reflects current resting orders, not future execution.
- Hidden intent: you typically cannot see why orders were placed or whether they will be maintained.
- Rapid updates: cancellations and partial fills can change depth quickly, so any snapshot may become outdated.
- Incomplete picture: some trading activity may occur through mechanisms that are not fully visible in a given book view.
- Venue differences: the meaning of “one level,” size units, and how updates are aggregated can vary across providers.
For verification, treat order book data as an evolving snapshot and confirm the platform’s definitions for side, size units, and update behavior before drawing conclusions.