Best Bid & Ask (Level 1)
The highest price a buyer will pay and the lowest a seller will take — the only two truly tradable prices.
What it means
The best bid and best ask (together, the NBBO in US equities) are the top of the book. A market buy fills at the ask; a market sell fills at the bid. The last price is history — these two are the present.
Their sizes matter as much as their prices: a 100-share ask breaks with a breath, a 50,000-share ask is a wall. Watching how size refreshes after each trade reveals persistent buyers and sellers.
Why traders care
- Defines your instant execution price and cost on both sides.
- Thin top-of-book warns that a small order will move the price.
In a TraderWe strategy
if AskSize1 < BidSize1 and Price >= Ask1:
Buy()Thin offer against a heavy bid, and price is already lifting it.
Related terms
Educational content, not investment advice. Engine details describe how TraderWe computes this value; other platforms may define it differently.