What bid and ask size means
Binance futures · BTCUSDT · FlowMap Chart
Bid size and ask size are how much is waiting on each side of the Binance BTCUSDT book. Not volume. Not a candle. Waiting size.
The order book note is the layout. This note is the number on each row: what it counts, what it does not, and how that number shows up on FlowMap.
Bid size
Bid size is how much buy-limit interest sits at a price (or at the best bid). Traders named a price at or below the ask and left size there. They want to buy there, not higher.
On BTCUSDT the unit is contracts in BTC, not dollars. A larger number means more size is willing to buy at that price right now. It does not mean that size will still be there when a sell hits.
Ask size
Ask size is the same idea on the sell side. Sell limits waiting at a price (or at the best ask). They want to sell there, not lower.
Market buys lift the ask and take ask size. Market sells hit the bid and take bid size. The size that traded becomes a print. The size that stays is still bid or ask size.
Best bid size and best ask size
The best bid is the highest buy price still waiting. Best bid size is how much sits at that price. The best ask is the lowest sell price still waiting. Best ask size is how much sits there.
Those two sizes are the first size a market order meets. If the print is larger than that first row, it walks to the next price. That walk is how one order can clear several rows.
On FlowMap Chart the best prices are the BBO lines. The live ladder is COB — current order book. Bar length is size at each price right now.
What the number is not
Not traded volume. Volume already printed. Bid and ask size have not printed yet.
Not a wall that must hold. The trader who left the size can pull it. A thick best ask can go thin before a buy arrives.
Not every Bitcoin book. This is Binance futures BTCUSDT. Other venues keep their own bids and asks.
Limits vs market orders are in market orders vs limit orders. For learning only — not financial advice.
Watch bid and ask size live on FlowMap Chart · BTCUSDT · course: limit vs market.