Lot step rounding: small numbers, steady drift

[Fairman]

Lot step rounding: small numbers, steady drift

A position size calculator says 0.037 lots. The platform takes 0.03, or 0.04, or rejects the order. Which depends on the broker's step size and rounding rule.

On small accounts the drift is real. Rounding down every trade slightly undersizes the position and trims profit. Rounding up slightly oversizes it and adds risk. Over hundreds of trades it shows up as a gap between planned and actual risk per trade.

Some brokers allow 0.001 lot steps. Others allow only 0.01. Calculators that do not know this produce numbers that cannot actually be traded.

A quick test on a demo or tiny live account shows how the platform handles odd values and explains a few mysteries about position size.

Another thing worth testing is what happens when a partial close would leave a remainder below the minimum lot. Some platforms refuse the close, some round the remainder up, and some close the whole position. The behaviour is rarely documented and can surprise during a fast exit.

Position sizing is where the drift becomes expensive. If the calculator says 0.037 and the platform rounds down to 0.03, the trade carries about 19 percent less risk than planned. Do that on every trade, and the account is consistently under-using its edge. Round up instead and the opposite happens, with a hidden overshoot that shows up only on the worst days.

A cleaner approach is letting the calculator round to the broker's step and then showing the true risk after rounding, so the number on the screen is the number that will be taken. Some traders deliberately round down, accepting the small shortfall for safety. Whatever the choice, it works best when it is made on purpose.

Brokers occasionally change step sizes without announcement, so the check is worth repeating once or twice a year, especially after a platform update.

What lot step does the broker allow, and does the sizing method fit it?