Mechanism
- Limits fill only if the market trades through your price with size.
- In thin books, you often get “some” but not “all.”
- Chasing the rest usually means paying worse prices, increasing risk per unit of edge.
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.
How it kills accounts
Partial fill → chase fill with market order → worse average entry → widen stop “to be safe” → risk up → drawdown → forced de-risk at the worst time.
How it kills accounts:
- Edge looks fine in backtest.
- Live spreads widen at the exact wrong moments.
- Stops trigger inside noise, so you widen stops.
- Same size + wider stop = silent leverage increase.
- A normal spike becomes structural damage.
Rule that survives
- Predefine a minimum fill to treat the trade as “on.”
- If under-filled: either cancel or accept the smaller size. No chasing.
- Stops and targets must be based on average fill, not intended fill.
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.
Example archetype
You want 5 units, get 2. You feel “unfinished” and market in the rest. Now your cost basis is worse, your stop feels tight, and you start managing from discomfort.
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.
Deep dive
Deep dive
Partial fills are where good rules go to die: you feel “committed” but not “satisfied.” That emotional gap produces chasing behavior.
Decide in advance how you handle:
- Under-filled entries
- Staggered exits
- Stops for partially filled positions
Variants merged
This page consolidates closely-related entries into one stronger canonical reference. Retired versions now redirect here.
Variant merged: Partial Fills Hide Risk
Why it’s included: Variant emphasis: partial fills create “ghost exposure” and broken sizing. Your stop distance and risk budget are defined by what actually filled, not what you intended to fill.
Truth line: A partial fill can create exposure you don’t notice until the market moves.
Mechanism add-on: When an order is partially filled, you may think you’re “not in” or “not fully in,” but you still carry risk. If you then submit another order, you can accidentally double up.
Failure add-on: Partial fill → you resend order → position is larger than intended → stop triggers → loss exceeds plan → you ‘fix’ by widening stop → now you’re managing surprise exposure instead of trading.
Rule add-on: Treat partial fills as live positions. Always check actual position size before sending another order.
Variant merged: Partial Fills Create Ghost Risk
Why it’s included: Variant emphasis: partial fills create “ghost exposure” and broken sizing. Your stop distance and risk budget are defined by what actually filled, not what you intended to fill.
Truth line: A partial fill is a position. If you ignore it, you carry exposure you did not model.
Mechanism add-on: In thin conditions, limits can fill partially. Your platform may show an average price and an unfilled remainder.
Failure add-on: Limit partially fills → trader assumes full fill → hedges/stops sized for full position → mismatch → price moves → confusion → market order to 'fix it' → costs spike.
Rule add-on: Treat partial fills as first-class states in the plan. Define what to do at 25%, 50%, 75% filled (cancel, reprice, wait, or switch).
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.