It looks like I can solve my problem with two solutions. (I want to keep my specific font-size).

Solution 1
Remove the height as @C3roe comment says. If I want to preserve a similar height, I can use a padding to do it.

Solution 2
Modify the line-height to 1.25.
I know that browsers do stuff like line-height * 1.25. 14.4*1.25 = 18 (rounded value). It give me the right result, even if I keep the height:30px.

This is a solution, even if I don't know why. I recommend the article from "Vincent De Oliviera" about font metrics. It could explain why I have this problem.