Small-lot commissions and the rounding that adds cost

[Shadow Trader]

Commission is usually quoted per standard lot per side. A 0.01 lot trade should pay 1 percent of that. In practice, brokers often round up to the nearest cent, or apply a minimum per trade.

That matters on tiny accounts. If the commission is 3.50 dollars per lot per side, a 0.01 lot costs 3.5 cents per side. Rounded up to 4 cents, the effective rate is 14 percent higher. A minimum charge of 10 cents makes it nearly triple.

The effect is hidden because the commission column in the history shows a small number, and no one multiplies it back to a per-lot rate.

A quick check: take ten trades of different sizes, compute the commission divided by the volume, and see whether the rate stays constant. It often doesn't.

For a scalper using small size while testing, the true cost per trade might be quite different from what was assumed in the spreadsheet, and the strategy's edge might disappear in the rounding.

Does the commission per lot stay constant across trade sizes on the account used here?