A stop loss is an order placed with the broker that closes a position automatically when price reaches a predefined level, capping the loss on the trade. It converts an open-ended emotional decision ("when do I admit I'm wrong?") into a pre-committed mechanical one. On a long position the stop sits below entry; on a short, above. Once triggered, most retail stops convert into market orders, which means the actual fill is subject to slippage.
The stop loss is the single most consequential discipline tool a retail trader has. It defines the trade's maximum risk in advance, which is what makes consistent position sizing possible. Without a hard stop, every losing trade has the optionality to become catastrophic, and the long-tail loss is what kills accounts.
Formula
Risk per trade ($) = |Entry price - Stop price| x pip value x lot size
Stop distance (pips) = |Entry price - Stop price| in pips
Worked example
Account: $10,000. Risk per trade: 1% = $100. EUR/USD entry: 1.0850. Stop placement (below recent swing low): 1.0820 — a 30-pip stop.
- Lot size that risks exactly $100 = $100 / (30 pips x $10/pip per lot) = 0.33 lots
- If the stop hits at 1.0820: loss = 30 x $3.30 = $99 (within the 1% budget)
- If the stop hits with 5 pips of adverse slippage at 1.0815: loss = 35 x $3.30 = $115 (1.15% — slightly over)
Why it matters
Stops are the upstream input to every other risk metric a trader tracks. R-multiples, expectancy, win rate at a given R:R — none of them mean anything if the stop on a losing trade was actually hit at a different price than planned. Logging the planned vs. actual stop is one of the most useful things a trade journal does.
Common pitfalls
- Moving the stop further away after entry to avoid being stopped out — erases the discipline entirely. This is one of the most common mistake tags in trader journals.
- Placing stops at obvious round numbers or 1-pip below a swing low, where stop-hunting algorithms cluster.
- Not accounting for spread and slippage — a 20-pip stop on a 2-pip spread pair is effectively a 22-pip stop on entry.