What's the health formula in Fallout 4?

Your character starts with a base HP of 80, with 5 extra points for every point of endurance. When you level up you gain a base of 2.5 HP, along with half of your endurance.

This effect is retroactive, your HP will raise accordingly if your endurance increases. Note that this does not account for any enhancements from gear, stims, or food.

In order to account for buffs, you can use the complete formula:

80 + (CurrentEndurance * 5) + (Level - 1) * (PermanentEndurance * .5 + 2.5)

PermanentEndurance refers to all endurance accrued via bobbleheads, the SPECIAL book, perk assignment, or initial SPECIAL assignment at the beginning of the game. CurrentEndurance accounts for endurance bonuses from gear and buffs.

You can find more information in the Fallout Wiki article.

For notation that is easier on the eyes: Health = 80 + (END * 5) + [(2.5 + (END/2)) * (Level-1)] + Perks

I have been doing some min/max testing on health and it seems you gain the initial endurance boost from character creation AND the theoretically retroactive boost that you missed from all previous levels upon putting points into endurance. I tested the formula for a level 30 character that has a current value of 11 endurance who started out with 2 endurance, AND my other level 50 character who has 10 endurance and started out with 9 and that formula is correct for both. This means that your hp value is based upon whatever your current endurance is(Not Retroactive). Hope this detail could clear things up for other theorycrafters and min/maxer's out there.