Day-One P&L And The Spread You Captured

The instant a trade fills, before the market has moved at all, marking the position to the current midpoint already produces a P&L number. If you buy at the bid, you're immediately marked up to the midpoint, a small paper gain. If you buy at the ask (crossing the spread to get filled fast), you're immediately marked down. This "day-one" or "spread-capture" P&L isn't about forecasting skill at all; it's a direct readout of how favorably a trade was executed relative to the prevailing spread.

Why traders watch it

A market maker who consistently earns positive spread-capture P&L is getting filled passively, at prices better than the midpoint, which is the whole business model. A directional trader who consistently books negative spread-capture P&L (because they cross the spread to trade urgently) isn't doing anything wrong necessarily, urgency has a cost, but tracking the number separately from the strategy's longer-horizon P&L keeps execution cost visible instead of buried inside the total return.

Worked example

A market maker buys 1,000 shares at the bid of $50.00 when the midpoint is $50.02. The instant the fill is marked to midpoint, the position shows a day-one gain of $0.02 × 1,000 = $20, pure spread capture, before the market has moved a cent. If the same trader instead had to cross the spread and buy at $50.04 to get filled urgently, the same 1,000 shares would show a day-one loss of $20 relative to the $50.02 midpoint, reflecting the cost of demanding immediacy.