Rounding is a cost too

[Shadow Trader]

Fills are quoted to a fixed number of decimals, and brokers round in their own favor more often than not. A price of 1.108346 might be shown as 1.10835 on a five-digit platform, and the rounding direction can depend on whether the order is a buy or a sell.

For a trader making a few trades a day, that's nothing. For a scalper doing 80 trades a day, a tenth of a pip per trade is eight pips daily. That adds up to real money over a month.

The test is dull but revealing. Pull the tick data around a hundred fills and compare the executed price to the raw quote at that millisecond. A consistent bias in one direction shows up fast.

There's also the question of which side the rounding favors when converting between account currency and quote currency. A broker that rounds profit down and loss up to the next cent on every trade extracts a tiny fixed tax that doesn't appear anywhere in the fee schedule. On a standard lot it's invisible. On a micro lot, it can be a large fraction of the trade's profit.

To check, export a month of history, recompute each trade's profit from entry, exit, and size using high precision arithmetic, and compare to the figure on the statement. A column of differences that skews negative is the evidence.

Rounding in the broker's favor isn't illegal if it's disclosed. It just rarely is. How many decimals does the feed on the broker used here really carry, and has the rounding ever been checked?