Position sizing for prop challenges: how much to risk per trade
Educational only, not financial advice. Simulations and backtests are models, not promises. Check every rule with your own firm.
"Risk 0.5% per trade" is the default advice. Whether it’s good advice depends on one number most traders never measure: their expectancy per trade after costs.
Step 1: get the lot size right
lots = (account × risk%) ÷ (stop distance × value per point per lot + commission per lot)
Example: a $100,000 account risking 0.5% ($500) on EUR/USD with a 20-pip stop. One standard lot is worth $10 a pip, so a 20-pip stop costs $200 per lot, and $500 ÷ $200 = 2.5 lots. Add $7 round-trip commission per lot and it becomes $500 ÷ $207 ≈ 2.41 lots.
The common mistake is the contract size. Index CFDs can be 1, 10 or some other number of units per lot depending on the firm and platform, and gold "pips" mean 0.1 on some platforms and 0.01 on others. Check your firm’s symbol specification. The lot size calculator lets you edit the contract size for exactly this reason.
Step 2: check it against the daily limit
A challenge usually ends on a daily loss before it ends on the max drawdown. Count how many full losses you can take today: with a 5% daily limit and 1% risk you can lose four trades and still be inside, and the fifth would breach. With 2% risk, only two. Spread widening and slippage make real losses bigger than planned, so leave a buffer. Firms define it differently: FTMO and FundedNext set the amount as a % of the initial balance, measured from your start-of-day balance, while Lark Funding uses 5% of your 5:00 pm EST balance. Read your firm’s wording.
Step 3: choose the risk, based on your edge
Here’s the part most sizing guides skip. The same rules give opposite answers depending on your expectancy. Simulated with the pass-odds calculator on Lark Funding 1-Step rules (10% target, 7% static max loss, 5% daily), up to 2 trades a day, no time limit:
| Your trading | 0.25% | 0.5% | 1% | 2% |
|---|---|---|---|---|
| Edge +0.05R (50% wins at 1.2:1, 0.05R costs) | 91.0% | 74.0% | 59.6% | 51.5% |
| No edge, 0.05R costs (−0.05R) | 1.6% | 9.9% | 22.1% | 30.9% |
With a real edge, small risk wins: the edge gets hundreds of trades to add up, and the drawdown is rarely threatened. The catch is time. At 0.25% risk the average challenge took about 680 trades. With no edge, small risk is close to a guaranteed fail, because costs grind the account down. That’s the same logic as bold play beating martingale.
What Kelly says
The Kelly criterion picks the bet size that maximizes long-run growth. For a trade that wins b times the risk with probability p, the Kelly fraction is p − (1 − p) ÷ b. Two things follow:
- With zero or negative edge, Kelly says bet zero. No sizing rule turns a losing system into a winning one.
- With an edge, full Kelly is usually far too aggressive for a 5–10% drawdown limit. Fractional Kelly (for example half) gives up a little growth for much smaller drawdowns (MacLean, Thorp and Ziemba 2010). And your real edge is an estimate with error, which argues for smaller still.
A challenge is also not a long-run growth problem. It’s a race to a target before a floor, often against a clock. Browne (1999) shows the policy that maximizes the chance of reaching a goal by a deadline takes more risk as you fall behind, which is what you see in the table above for a trader with no edge.
A practical sizing checklist
- Measure your expectancy after costs on a large sample: at least a couple of hundred trades, from live or forward-tested results rather than an optimized backtest.
- Compute costs in R: average spread + commission + slippage, divided by your average stop. If that’s 0.1R and your gross edge is 0.05R, you don’t have an edge.
- Read the firm’s risk rules. Lark Funding’s free restart requires that no single trade lost more than 1%, and it lists an "All-or-Nothing" prohibited strategy. FundedNext caps open risk at 3% on its funded accounts. Topstep and Apex cap contracts.
- Fit the daily limit: make sure you can take at least two or three full losses in a day without breaching.
- Run your numbers through the pass-odds calculator, and check the time it takes, not just the pass rate.
Sources
- Kelly, J. L. (1956). A new interpretation of information rate. Bell System Technical Journal 35(4), 917–926. doi:10.1002/j.1538-7305.1956.tb03809.x [Peer-reviewed]
- Thorp, E. O. (2006). The Kelly criterion in blackjack, sports betting, and the stock market. Handbook of Asset and Liability Management Vol. 1, 385–428. doi:10.1016/S1872-0978(06)01009-X [Book]
- MacLean, L. C., Thorp, E. O. & Ziemba, W. T. (2010). Long-term capital growth: The good and bad properties of the Kelly and fractional Kelly capital growth criteria. Quantitative Finance 10(7), 681–687. doi:10.1080/14697688.2010.506108 [Peer-reviewed]
- Browne, S. (1999). Reaching goals by a deadline: Digital options and continuous-time active portfolio management. Advances in Applied Probability 31(2), 551–577. doi:10.1239/aap/1029955147 [Peer-reviewed]
- Dubins, L. E. & Savage, L. J. (1965). How to Gamble If You Must. McGraw-Hill. [Book]
"Peer-reviewed" means published in a refereed journal. Working papers and books aren’t peer-reviewed. Figures from "our own tests" are our backtests and simulations, not peer-reviewed research.