Things that impact health: Level (each class gets different amount), Endurance Trait, Strength Trait, Skill (Body Development etc), HP bonus from equipment.
Notes:
1) Equipment bonus that impact Endurance and Strength will impact total health
2) The every 2 points of Strength value is only triggered at 2, 4, 6, 8 etc so odd numbers of Strength won't increase it (Unless you have Body Development II).
3) Remember to consider trait bonus from equipment as its easy to swap out a +1 or +2 STR item (belt, gloves etc) and forget how that swap out messes with calculations.