Lot step and rounding: small accounts hit this wall first

[Shadow Trader]

Brokers set a minimum lot size and a step, such as a minimum of 0.01 with steps of 0.01. A calculated position of 0.037 lots has to be rounded to 0.03 or 0.04, and which way depends on the platform or the script.

On an account of a few hundred dollars, that rounding is a big piece of the risk. A planned risk of one percent can quietly become 1.3 percent or 0.7 percent.

It matters more with tight stops, because the position size is calculated from the stop distance. A five pip stop on 0.03 lots costs a very different amount than on 0.04, and the gap is large in proportion.

Position size calculators rarely say which direction they round. Some round down to stay safe, some to the nearest step, and some just truncate.

The effect carries into scaling. A trader who halves a position at the first target needs a size that can be halved. At 0.03 lots, the only choices are 0.01 and 0.02, which isn't half of anything.

Swap and commission also scale in steps, and a minimum commission per ticket can make the smallest sizes proportionally the most expensive.

A quick test is running the size calculation for a dozen stop distances and checking the actual risk in dollars after rounding. The spread of results is often wider than expected.

Does the platform round up or down, and has it ever been tested on real numbers?