Core terms
Cost drag
How much of a process's gross edge its trading costs remove, usually measured as average costs per trade (fees, commissions, slippage) divided by planned risk, in R.
Cost drag puts costs on the same scale as results. If a plan earns +0.15R per trade before costs and its fees and slippage average 0.12R, it keeps +0.03R (Illustrative). The drag is often the largest number in a scalper's review.
Two multipliers make it larger for scalpers. Each round trip pays commissions, fees and usually part of the spread, so more trades mean more drag. And a tighter stop needs more shares for the same dollar risk, so per-share costs cost more per R: in one Illustrative comparison, the same per-share costs came to 0.10R on a 5-cent stop and 0.02R on a 25-cent stop.
Cost drag also rises when extra trades go into wider-spread names or smaller targets, which is one way overtrading shows up in a log.
What traders should verify
Record fees and slippage per trade, divide by the planned 1R, and average by setup tag and by trade number of the day. Compare the result with gross expectancy for the same group, with the trade count beside it.
Do not conflate
Trader usage; there is no official definition. Logs differ on whether the spread paid is included or left inside the fill prices, so state the formula. A small drag figure from a few trades can shift once more fills are logged.
Primary source
Trader usage: there is no official definition, and platforms and tools may calculate it differently.