Position Size Calculator — for prop firm futures traders

Max contracts = floor( $ risk per trade ÷ (stop ticks × tick value) ). If you risk $200 with an 8-tick ES stop ($100/contract risk), max = 2 contracts.

Trail floor depends on the firm:

• Topstep: trail tracks intraday equity HWM (Combine) or daily EOD balance (XFA), and locks at the starting balance once HWM = SB + drawdown amount. Funded account = $0 net P&L is your floor permanently.
• Apex 4.0: trail locks at starting balance + $100 (the "safety net"). E.g. 100K → lock at $103,100. Both EOD and intraday options exist; the $ floor is the same.
• MFFU Core / Rapid: same SB+$100 lock as Apex. Core is EOD; Rapid is intraday.
• TPT Test: SB lock (same as Topstep). EOD trail during the Test phase. PRO shifts to intraday and PRO+ shifts back to EOD — same $ drawdown numbers, only the trigger timing differs (use Test preset for funded-phase math too). No DLL firmwide since Jan 2025 — trail is the only hard guardrail.
• Tradeify Growth / Select: SB+$100 lock (same archetype as Apex/MFFU). EOD trail across all programs. Growth has an evaluation DLL; Select does not. Lightning (instant-funded, not listed) follows the same lock pattern with its own DD numbers.
• Bulenox Option 1 / Option 2: SB+$100 lock. Option 1 = intraday HWM trail, no DLL; Option 2 = EOD trail with DLL. The choice between them is permanent at signup — you cannot switch.
• Earn2Trade GAU: SB lock (same archetype as Topstep family). EOD-only trail. Has DLL during evaluation. Drawdown numbers diverge slightly from Topstep at 100K+ — use the preset, don't reuse Topstep numbers.
• Custom: no firm rules — the calc only shows $ risk per trade and % of balance.

Buffer to trail floor = current balance − trail floor (the floor depends on your HWM if not yet locked).

N losses in a row to hit the floor = floor(buffer ÷ $ risk per trade). This is the most useful number on the page — it answers "if I keep taking this size, how many in a row before I'm out?"

What this calc does NOT model: commissions/fees (be conservative; subtract a few dollars per RT from your real buffer), unrealized P&L drawdowns on the trail (intraday trail tracks unrealized HWM), profit-target progression, consistency-rule blocks on payout (use the consistency calc for that).

Most position-size calculators assume a static account: "you have $50K, you risk 1%, that's $500 per trade, here are your contracts." That's fine for retail brokerage. It's dangerous on a prop-firm account.

On a prop firm, you don't have $50K to draw from. You have a trail-DD buffer that's typically $1,500–$3,000 wide at the start, and that buffer moves up with profits but stops moving once it locks (Topstep at SB, Apex/MFFU at SB+$100).

If you're sizing for "1% of the $50K", you're risking $500/trade on a $2,000 buffer. Three losses in a row = $1,500 down = 75% of the buffer gone. Four losses = blown.

The honest sizing question on a prop firm isn't "what % of my balance am I risking" — it's "how many of these stop-outs can I survive in a row before the trail floor catches me?" That's the number this calc puts on the page.

"My broker shows my account at $52,000. Why does the calc say my buffer is $2,100 and not $4,100?"
Because once you hit a profit, the trailing drawdown moves up with you (until it locks). If you started at $50K, peaked at $52,100, and your DD is $2,000, your floor is now $50,100 — not $48,000. The "balance" you see and the "buffer" the trail enforces are different numbers. Set the "Trail HWM" field above to your actual peak to model this.

"What's the right risk %?"
On a prop account, think in $ not %. A common conservative target is to size so that 5–7 consecutive losses don't blow the trail. With a $2,000 buffer, that's ~$285–$400/trade max. Anything more and you're a coin-flip away from a bad week ending your account.

"Does this work for Apex EOD vs Intraday?"
Both lock at SB+$100, so the $ floor is the same. The difference: intraday trail tracks unrealized HWM (any wick to a profit ratchets it up); EOD only tracks the closing balance. EOD is more forgiving for traders who hold through wicks. The buffer math here uses your stated HWM — use whichever your account type tracks.

"What about commissions?"
Not included in the DD math (be conservative — subtract $4–$8 per round-trip per contract from your buffer in real life). Use the futures P&L calculator if you want net-of-fees math on a specific trade.