Position sizing: stocks vs forex math

Why a 2% stop in forex is not a 2% stop in equities

If you've been trading forex and you start trading equities, the first thing that catches you is volatility. A liquid US large-cap regularly has an average true range (ATR) of 1.5-3% per day. A major forex pair like EUR/USD typically has an ATR of 0.4-0.8% per day. That's a 3-4x ratio. A 'tight' stop on a stock is roughly 1.5x ATR — about 2-3% of price. A 'tight' stop on EUR/USD is closer to 30-50 pips, or about 0.3-0.5% of price. If you apply forex stop distances to stocks, the stop sits inside the normal day's noise and you get chopped out every time.

The fix is not 'use looser percentage stops on stocks.' The fix is to size the position so the dollar amount at risk is the same regardless of instrument. Risk per trade is set in dollars first. Stop distance is calibrated to the instrument's native volatility second. Position size — share count or lot size — is whatever falls out of the math.

Worked example with a $500 account and a 2% risk cap per trade ($10 of risk). Setup: long a stock at $100, stop at $98. Stop distance is $2 per share. Share count = $10 / $2 = 5 shares. Position notional is $500 — the entire account in this one trade. That sounds aggressive, but the risk is still only $10 because the stop is just 2% away. Same risk budget on EUR/USD: long at 1.0800, stop at 1.0760 (40 pips). 40 pips on a 0.02 standard lot is $8 of risk, so you'd round up to 0.025 lots for $10 of risk. Same dollar risk, different instrument units. The risk equation is identical.

Two practical notes. First, fractional shares change the game on a $500 account. You can buy 0.5 shares of a $300 stock if your sizing math calls for it. Before fractional shares, retail traders with small accounts were structurally locked out of high-priced stocks; now they aren't. Second, for overnight equity holds, remember the gap-risk lesson — your stop may not execute at the price you set. Sizing for a 1.5-2x worst-case stop slippage on positions through scheduled catalysts is the realistic adjustment. If your intended stop says $10 of risk, plan that an unlucky earnings gap could turn it into $15-20.

Recap: equities have wider ATR percentages than forex. Use the same dollar risk per trade across instruments by sizing position = risk budget / stop distance. Fractional shares make this workable on $500 accounts. Adjust for gap risk on overnight equity holds.