If You Can’t Measure Slippage, You Can’t Size

Your risk model is usually built on a clean stop: entry, stop price, fixed loss.

In live markets, the stop is a request. In fast moves or thin liquidity, you may be filled several ticks/pips worse. The loss becomes larger than planned, and the risk of the next trade compounds because you now trade from a damaged base.

Sizing without slippage is like building a bridge while ignoring wind load.

Execution reality: the market you backtested is not the market you trade. Spreads are stateful, liquidity is time-of-day dependent, and fills degrade exactly when your stop becomes most sensitive.

• Track spread-to-ATR (or spread-to-range) as a live risk input, not a “cost”.
• When spreads widen, your effective stop tightens and your R:R collapses.
• If your edge needs perfect fills, your edge is mostly fictional.

Normal day: -1R planned. Fast day: -1.6R realized. You keep sizing for -1R → drawdowns deepen → you tighten stops to “fix it” → stop-out frequency rises → account death via friction and oversized losses.

How it kills accounts:

1. Edge looks fine in backtest.
2. Live spreads widen at the exact wrong moments.
3. Stops trigger inside noise, so you widen stops.
4. Same size + wider stop = silent leverage increase.
5. A normal spike becomes structural damage.

Track slippage per instrument and session.

Define a “slippage budget” (e.g., 0.2R). If expected slippage can exceed budget, trade smaller or don’t trade.

Your max size is the size that still survives worst-case slippage days.

Rule that survives:

• Spread is a gate, not a footnote. If it’s abnormal, you don’t trade or you trade smaller.
• Assume worst-case fills in fast markets.
• Size is the adapter: reduce size before changing the stop model.

You risk 1% with a 10-pip stop. You assume a clean fill.

On news, your stop fills 6 pips worse. That’s 60% more loss than planned. Do that twice in a week and your “disciplined 1% risk” system is quietly running 1.6% risk at the exact worst moments.

Tell: if the trade only works when the spread is tight and price is smooth, it’s not an edge, it’s a regime bet.

Simple log: record stop price and actual fill price for every stop. Build a histogram by session. If the tail exists, respect it.

Glossary: slippage, markout, liquidity.

Field checklist

• Measure spread before entering. If it’s abnormal, you’re trading the wrong product at the wrong time.
• If volatility expands, reduce size first. Don’t “solve” it by widening stops with the same size.
• Avoid the predictable liquidity holes: rollover, session open/close, first minutes after data.
• Assume your stop may fill worse than your entry. Price the worst-case, not the brochure.
• If you cannot explain where liquidity comes from, trade smaller.