Stop-Loss Explained (And How It Pairs With Position Size)
A stop-loss is how you pre-decide “I’m wrong here.” Without it, position sizing is guesswork and small mistakes become account problems. Here’s the plain-English version — and how it pairs with the 1% rule.
Learn how stop-losses define risk per share, why moving them wrecks the plan, and how to pair stops with a position size calculator so one loser can’t wreck the account.
What a stop-loss is
A stop-loss is an order (or a hard mental rule) to exit if price hits a level that invalidates your idea.
It’s not a prediction of the bottom. It’s a risk boundary. Good traders lose often; they lose small.
Broker stop types vary (stop-market vs stop-limit). Know the difference: gaps and slippage can fill worse than your line.
Stops and position size are one system
Position size = (Account × Risk %) ÷ (Entry − Stop). Wider stop → fewer shares for the same dollar risk.
If you pick a stop after you pick a share count, you’re doing it backwards. Level first, size second.
Use a stop-loss planner together with a position size calculator until the habit is automatic.
Where to place the stop (beginner heuristics)
Technical invalidation: below support for longs, above resistance for shorts — zones, not ego pennies.
Volatility-aware: noisy names need more room or you’ll get shaken out constantly (which means fewer shares).
Never place a stop where a normal wiggle lives just to “feel tight.” Fake precision is still a guess.
Mistakes that erase the benefit
Moving the stop farther after entry without cutting size — silent risk increase.
No stop at all because “it’ll come back.” Sometimes it doesn’t.
Stops so tight the market’s noise stops you out before your thesis had a chance — then revenge-sizing the next trade.
Practice the full workflow in a paper simulator: thesis → stop → size → execute → journal.
Next steps
Concepts stick when you apply them. Open a related calculator, run your own numbers, and — if you’re still learning execution — practice with paper trades before increasing real size. Browse more guides or the full set of free trading tools.
Free tool
Stop-Loss Calculator
Enter entry price and either a stop % or stop price to see dollar risk per share, total risk for your share count, and the stop level that matches a target risk %. Pair with position sizing before you place the order.
Other free tools
- Stock Market Simulator — Practice trading with fake money.
- Trading 212 Portfolio Charts — Connect T212 and flex your P&L.
- Position Size Calculator — Risk the right amount on every trade.
Ready to put this into practice?
Trading 212 — Commission-free stocks & ETFs with fractional shares. You must sign up and deposit funds — signup alone does not unlock the free share. Bonus shares are chosen at random and vary by region and campaign (UK/EU promos often land around £8–£25, with a max up to £100).
- Zero-commission stock & ETF investing
- Fractional shares from $1
- Free share up to £100 after signup + deposit (random value; T&Cs apply)
Free share only after signup + deposit · random bonus up to £100 · capital at risk · T&Cs apply
This is educational content, not financial advice. Investing carries risk — you can lose money. Do your own research and consider a qualified advisor for personal decisions.