Prop-firm-aware position sizing. Shows which rule is binding so you know what's limiting your size — risk %, daily loss, or drawdown buffer.
Apex uses intraday trailing drawdown. No separate daily loss limit — the trailing floor functions as both. Max contract size is enforced per account.
Generic position size calculators answer the wrong question. The classic "1% rule" asks: given my stop distance and account size, how many contracts keep my per-trade risk at 1%? That math is correct, but it ignores two prop-firm-specific rules that can invalidate it instantly. Your 1% might be 6 contracts on a $100K account, but if your remaining daily loss buffer only absorbs 4 contracts at your stop distance, taking 6 contracts means a single fill through your stop blows the daily limit and locks your account for the session.
The three essentials. A prop-firm-aware calculation takes the minimum of three separate constraints. First, your personal risk percentage — typically 0.5% for conservative traders, 1% for standard, 2% for aggressive. Second, your remaining daily loss buffer — resets each session, but eats away fast if the first trade loses. Third, your trailing drawdown buffer — doesn't reset, and for firms like Apex and MFFU Rapid it can shrink intraday on unrealized profits even when you close flat.
Why the 20% safety margin. Each buffer constraint applies an 0.8 multiplier before dividing by risk per unit. This accounts for slippage, adverse fills, and commissions. On a $2,000 daily buffer, sizing to consume all $2,000 means any fill more than one tick beyond your stop price triggers the daily limit. The 20% margin leaves roughly $400 of headroom — enough to absorb a few ticks of slippage on ES or CL without breaching. Traders with pristine fills on highly liquid instruments often lower this to 10%; traders in thinner markets like RTY or CL during news go to 30%.
Reading the binding constraint. The rule showing BINDING is what's limiting your size right this second. If risk % is binding, you're well inside firm limits — this is the healthy state. If daily is binding, you've already taken earlier losses and the session's buffer is shrinking; reduce risk or wait for tighter-stop setups. If DD is binding, you're close to your account's hard floor — this is the danger zone, and anything but your highest-probability setup should be skipped. Watching which constraint binds throughout the day is one of the single best signals of account health a prop trader can track.
What this calculator does NOT account for. Firm hard contract caps (e.g. Topstep's scaling plan tiers, Apex's per-account maxes) aren't in the essentials panel — they're displayed as context in the firm notes. Consistency rule ceilings (e.g. "no single day can exceed 30% of your profits since last payout") aren't included either, because those bind on profit-taking not risk-taking. For a full multi-constraint view including consistency ceilings and exact scaling tiers, use the firm's official dashboard as the source of truth and use this tool for the essentials.
Position sizing is the per-trade tool. For the bigger picture — understanding how your drawdown buffer evolves over a sequence of trades, modeling account growth under a scaling plan, or computing the true cost of a firm's challenge before committing — use the rest of the calculator suite. All tools share the same firm-rules database, so what you set here carries semantics across the rest.