The Complete Guide to Position Sizing for Day Traders
Most traders lose money not because of bad entries, but because of poor position sizing. Here's how to fix that.
Why Position Sizing Matters More Than Your Entry
I've been trading for over a decade, and the single biggest difference between profitable traders and everyone else isn't their chart reading ability — it's how they size their positions.
Think about it: even with a 60% win rate, you can still blow up your account if your losers are 3x the size of your winners. Position sizing is the equalizer.
The 1% Rule
Never risk more than 1% of your total account on a single trade. This is the foundation everything else is built on.
For a $50,000 account, that means your maximum loss per trade is $500. If your stop loss is $2 away from your entry, you can trade 250 shares. Simple math, but most traders skip it.
Calculating Your Size
Position Size = Account Risk / (Entry Price - Stop Loss)
Let's say you want to buy AAPL at $225 with a stop at $220. That's $5 of risk per share. With a $50,000 account and 1% risk: $500 / $5 = 100 shares.
This keeps every trade the same "weight" in terms of risk, regardless of the stock price.
Common Mistakes
The two biggest mistakes I see:
- Sizing based on conviction — "I really like this setup so I'll go bigger." This is how accounts blow up.
- Ignoring volatility — A $5 stop on a stock that moves $10/day is very different from a $5 stop on a stock that moves $2/day.
Keep it mechanical. Keep it consistent. Your future self will thank you.