Who this is for — Anyone comparing chart price to real fills and not understanding the gap. Charts often show mid; you pay the ask.
The mid price is the average of the best bid and best ask on the order book:
mid = (bid + ask) / 2In plain terms — Bid 99.90, ask 100.10 → mid 100.00. It is the centre, not what you pay on a market buy.
Mid vs price you actually get
| Action | Typical execution price |
|---|---|
| You pay the ask | |
| Sell at market | You receive the bid |
| Ticker / aggregated chart | Often mid or last trade |
The spread is already an implicit cost if you expect mid but fill at ask — part of slippage.
Typical mistake — Backtest or alerts on mid, live execution at ask/bid — simulated edge inflated.
Example — Mid 100.00, spread 0.08. Market buy → fill ~100.04. Without half-spread, real result is worse than the chart.
Summary card
- What it is: midpoint between bid and ask.
- What changes: separates indicative quote from real fill.
- Quick check: does my platform show mid or last?