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Lot size caps on gold and indices in prop accounts, and the contract size trap

Many prop firms set maximum lot sizes per instrument, or apply caps that are different for FX, gold and indices. The trap is that a lot doesn't mean the same thing across instruments, and it can differ between brokers the firm uses.

On most FX pairs, one standard lot is 100,000 units of the base currency. On gold, one lot is often 100 ounces, but some platforms use different contract sizes. On index CFDs, one lot might be one contract per point or ten or a hundred, depending on the setup. So a two lot position on gold can be very different in risk from two lots on EURUSD, and a lot cap that seems reasonable for FX can be much more restrictive, or much less, on another instrument.

Position size calculators sometimes assume FX style contract sizes. If you use the same calculation for gold or an index without checking the contract spec, the size you think you're trading can be far from what you're actually trading.

What I'd do with any prop account: open the contract specification for each instrument you plan to trade, check the value of one point or pip per lot, and compare it with the firm's maximum lot rules. Then set your position sizing around the actual risk per lot, not the number of lots.

It's easy to get wrong, and the firm won't forgive the breach.

Some firms also apply a total exposure cap across all positions, not just per trade. If you open a gold position and an index position at the same time, the combined lots or margin might hit the cap even if each trade is small on its own. That matters if you scale in, or hold correlated positions across instruments. I'd read the rules for how the cap is measured, whether in lots, notional value or margin, since each one behaves differently when you add positions across different instruments. The safest assumption, until you've read otherwise, is that the strictest version applies.

I'd also double check the minimum lot step on these instruments. On gold or indices, the step might be 0.01 or 0.1 depending on the setup, which affects how finely you can size positions. If your risk calculation produces a size like 0.37 lots and the platform only allows steps of 0.1, it will round, and you might end up either over your intended risk or well under it. Rounding down is the safer default when you're close to a cap or a risk limit. Some calculators round to the nearest step instead, which can go either way.