Failing a prop firm evaluation is almost never one bad trade, it is a wrong lot size. Position sizing converts your risk percentage and stop loss distance into the correct lot size, so every trade risks the same controlled amount.
Lot size calculator
Pip value: $10 per pip per standard lot applies to most USD-quoted major pairs. For gold (XAUUSD) one pip is commonly $10 per lot too, but check your broker contract specs.
The formula behind it
Lot size = (balance × risk %) ÷ (stop loss in pips × pip value per lot). The result is in standard lots. 0.01 lots is one micro lot, 0.10 is a mini lot, 1.00 is a standard lot. The same logic works for index and commodity CFDs if you use the dollar-per-point value per lot instead of pip value.
Worked example
$10,000 account, 1% risk ($100), 30 pip stop on EURUSD: $100 ÷ (30 × $10) = 0.33 lots, rounded down to 0.33. A 60 pip stop on the same setup would halve you to 0.16 lots. Wider stop, smaller size, same dollars at risk. See the full position sizing guide for the common mistakes that blow up evaluations.
FAQ
What risk percentage should I use in a prop evaluation?
Most evaluation passers run 0.25% to 1% per trade. At 0.5% risk you can absorb roughly 20 consecutive losses before hitting a 10% drawdown limit, at 1% only 10. Your win rate decides how much cushion you actually need.
Why does my platform reject my calculated lot size?
Brokers enforce minimum lot steps (usually 0.01) and maximum exposure limits. The rounded-down value in the result is the size to enter. If it rejects 0.33, your broker may use 0.10 or 0.01 steps only at different tiers.