How should I calculate profit factor after trading costs?

Classify outcomes after costs

Profit factor = sum of positive net P&L / abs(sum of negative net P&L)
Fictional outcomes: +60, +40, -50 → 100 / 50 = 2

The sign of the net result determines which side receives the trade. A gross gain that becomes a net loss after commission and swap belongs in the loss sum. Applying costs after first calculating the ratio produces a different and potentially misleading measure.

Profit factor is a money ratio, not the percentage of winning trades and not average R. Two samples with different win rates can have the same profit factor because the sizes of their gains and losses differ.

Handle the denominator honestly

Gains without any losses produce an unbounded ratio. That is not a finite score that should be capped at an attractive-looking number. A sample with no nonzero outcomes does not provide a meaningful ratio at all.

For example, three small gains in a new strategy can create a no-loss sample. The missing loss denominator says that the sample has not observed a loss, not that the process is incapable of losing. Check the trade count, date coverage and the size of the largest gain before comparing it with a longer-established sample.

Compare like with like

Use the same net formula, currency and eligibility rules across strategies or periods. Combining incompatible account currencies can change the money sums without a legitimate conversion. Changing filters between the gain and loss calculation can invalidate the ratio entirely.

Read profit factor alongside expectancy, concentration and realised drawdown. A single large winner can dominate it, and excluding open positions or trades with incomplete records changes the scope. Keep the underlying sample available for inspection so a reviewer can reproduce the numerator and denominator. The statistic describes recorded outcomes under those choices; it does not predict the next trade.