The aggression factor (AF) compares how often you bet or raise with how often you call after the flop. It measures whether you take the initiative or let others push you around.
How it's calculated
An AF of 3 means you bet or raise three times for every call.
The healthy ranges on the side are guidelines for NL Hold'em cash games with 100 bb stacks, the same ones Flopmortem compares your game against. Use them to spot big deviations, not as an exact target: they depend on the table, your opponents and your style.
How to read your Aggression factor (AF)
A low AF is typical of the passive player who calls and waits to hit: they win little with their good hands and don't win pots without them. A very high one can point to too many bluffs, or to folding whenever you can't bet.
Since it's a ratio without a clear confidence interval, you need a fairly large sample to trust it.
How many hands you need to trust it
Flopmortem doesn't flag your Aggression factor (AF) as a leak until it has at least 30 opportunities. From there it calculates it with its 95% confidence interval: if the whole interval falls outside the healthy range, the leak is confirmed; if only the measured value is outside, it's a trend. The guide how many hands you need to trust your stats explains it.