In Project Zomboid the exp to level your skills exponentially increases, here’s how much exp you need to level your skills.

Skill EXP Chart

LevelRegular SkillsPassive Skills*
1751500
21503000
33006000
47509000
5150018000
6300030000
7450060000
8600090000
97500120000
109000150000
*Passive skills are Strength and Fitness