Position Sizing Across Multi-Asset Challenges: Why One Rule Beats Five

Ask a trader with five years in the markets what they risk per trade and they'll usually give you five answers. "Half a percent on crypto, maybe a quarter on forex because the stops are tighter, a third on indices, nothing on earnings weeks for equities, and I don't trade metals." That's five different rules for five different asset classes, which is five different places discipline can break and five different numbers to remember when the market is moving fast and you have thirty seconds to size a trade.

On a multi-asset cross-margin venue like JTX Markets, where all 293 instruments share one USDT wallet, there is a better way. One rule, one number, applied the same way to every trade regardless of what you're trading. The maths gets simpler, not more complex, and the discipline becomes a habit rather than a decision tree.

The one rule

Risk a fixed percentage of account equity on every trade, computed from the stop distance.

That's it. If the number is 0.5%, every trade — crypto, forex, equity, index, commodity — risks 0.5% of your account equity between your entry and your stop. The size changes to make the risk constant.

Formula:

• Size (in base units) = (Equity × Risk%) / (Stop distance × Mark price × Contract multiplier)

Risk is expressed as a decimal (0.5% = 0.005). Stop distance is expressed as a fraction of mark price (a $100 stop on a $10,000 mark = 0.01). Contract multiplier is 1 for most crypto, 100,000 for forex, varies for indices and commodities — but the formula handles it because you're sizing from USDT risk down to native units at the end.

The reason this works across every asset class is that it factors out all the per-instrument details into a single USDT risk number. Whether the position is 2.5 BTC-PERP or 0.3 lots of EURUSD, if your stop is 1% away from entry and you've sized correctly, the dollar loss on hitting the stop is identical.

Worked examples on a $25,000 account

Say you're trading a $25,000 funded account from a challenge tier and your rule is 0.5% per trade. That's $125 of risk per position.

BTC-PERP long at $80,000, stop at $78,400 (−2.0%):

• USDT risk per BTC = $80,000 × 0.02 = $1,600.
• Size = $125 ÷ $1,600 = 0.078 BTC.
• Position notional = $6,240. Leverage used = 25:1.

EURUSD long at 1.0850, stop at 1.0820 (−28 pips, −0.28%):

• USDT risk per lot = $100,000 × 0.0028 = $280.
• Size = $125 ÷ $280 = 0.446 lots ≈ 45,000 EUR.
• Position notional = $48,825. Leverage used = ~1950:1 of margin on the position, which sounds extreme but is normal on forex where you're risking 28 pips.

SPX500 long at 5,800, stop at 5,760 (−40 points, −0.69%):

• With a contract multiplier of 100 (each contract = $100 × index level), USDT risk per contract = 40 × $100 = $4,000.
• Size = $125 ÷ $4,000 = 0.031 contracts.
• Position notional = ~$18,000. Leverage used = ~720:1 on the notional.

AAPL-PERP long at $230, stop at $225 (−2.17%):

• USDT risk per unit = $230 × 0.0217 = $5.00.
• Size = $125 ÷ $5 = 25 shares.
• Position notional = $5,750. Leverage used = 230:1.

Same account, same rule, four completely different position sizes — because the stop distance and the contract maths are different for each. But every one of these trades, if it hits its stop, costs the account the same $125.

Why one rule beats five

Three concrete advantages:

• The maths is identical every time. You don't have to remember whether forex is 0.3% or 0.4%. One number, memorised once, applied forever. Under stress — which is the only time position sizing really matters — the formula is reflexive.
• Portfolio risk aggregates cleanly. If you have four open positions each risking 0.5%, your maximum loss if everything hits stops is 2% of equity. On a per-asset-class rule system, aggregating this requires actual arithmetic. On one rule, it's just (positions × risk per position).
• The cross-margin wallet rewards it. On JTX, every instrument is collateralised by the same USDT pool. Margin, mark-to-market, liquidation thresholds — all computed off the one equity number. Sizing off that one equity number aligns your risk calculation with the engine's actual accounting, so there's no gap between "what I think I'm risking" and "what the engine thinks I'm risking".

The per-asset-class rule exists for historical reasons — most retail traders came from a world where their forex broker, their crypto exchange, and their equity broker were three separate accounts with three separate risk pools. On a venue where all three live in one wallet, carrying three different rules in your head is just friction.

How to pick the number

Three ranges that work for different trader profiles:

• 0.25% per trade — conservative, suited to a trader running 8-15 open positions at once or trading a strict prop-firm challenge where the daily-loss rule is 4-5% and you want cushion.
• 0.5% per trade — the standard, suited to a trader running 3-6 open positions and comfortable with meaningful daily P&L swings.
• 1% per trade — aggressive, suited to a trader running 1-2 concentrated positions with a strong directional view, and only on a personal account where there's no challenge rule to breach.

Above 1% per trade, the maths of ruin curves start to bite. A string of five losers at 2% each is a 10% drawdown that's going to take a 11% recovery to erase, and that compounds fast on a bad run.

Below 0.25%, you're probably under-sizing — the position isn't big enough to make the research time worth it, and small positions tend to breed small discipline.

For more on the specific maths of surviving a drawdown without blowing a challenge, the earlier position-sizing-prop-firm-challenges post has the loss-sequence tables and recovery curves.

Where this breaks

Two edge cases where the one-rule formula needs amendment:

• Correlated positions. Three EURUSD-related trades (EURUSD long, EURJPY long, EURCHF long) are effectively one EUR-long trade. Sizing each at 0.5% means you're risking 1.5% on EUR, not 0.5%. For correlated clusters, treat the cluster as one position and divide the per-trade risk across its legs.
• Wide-stop breakouts. If you're trading a 10% stop (common on crypto breakout entries), 0.5% risk divided by 10% stop distance gives you a 5% notional position. On a $25K account that's $1,250 of exposure — probably too small to matter. In these cases, consider either accepting the small size or treating the breakout as a different trading regime with a different (larger) risk budget.

Neither of these breaks the rule — they just mean the rule applies at a level above the individual trade (cluster-level, regime-level) in those cases.

Summary

One number. Applied the same way to every trade across every asset class. The size changes; the risk doesn't. On a cross-margin venue, this is both easier to execute and more aligned with how the engine actually accounts for your risk. If you're carrying five rules in your head for five asset classes, consolidating them is probably the highest-leverage change you can make to your trading process this quarter.

If you want to see how this plays out across a two-week trading run, the funded challenge tiers are the lowest-friction way to drill the discipline on a real engine without the full capital commitment. Numbers compound either way.