I have a fighter that's level 8 with 16 con. For HP on level up it's set to calculate automatically using the average but the amount of hp he has is wrong.

He should have 76 HP but he has 70 for some reason. I can override it but it resets after a long rest. How do I fix this?

76 HP is correct if you set it to Fixed HP.

Each time you gain a level, you gain 1 additional Hit Die. Roll that Hit Die, add your Constitution modifier to the roll, and add the total (minimum of 1) to your hit point maximum. Alternatively, you can use the fixed value shown in your class entry, which is the average result of the die roll (rounded up).

https://www.dndbeyond.com/sources/basic-rules/step-by-step-characters#ClassFeaturesandHitDice

Rounding the average result of the hit dice means that a d10 becomes 6.

Fixed HP Formula:

Assuming no multiclass.
H = Class Hit Dice
C = Con Mod
L = Level
Max HP = H + C + [ (L-1) * ([H/2]+1+C) ]

Max HP = 10 + 3 + [ (8-1) * ([10/2]+1+3) ]
Max HP = 13 + [ (7) * (5+4) ]
Max HP = 13 + 63
Max HP = 76

HP Gain in Practice:

1st Level
Gain 13 (10+3)
Total HP 13

2nd Level
Gain 9 ([10/2]+1+3)
Total HP 22 (13+9)

3rd Level
Gain 9
Total HP 31

4th Level
Gain 9
Total HP 40

5th Level
Gain 9
Total HP 49

6th Level
Gain 9
Total HP 58

7th Level
Gain 9
Total HP 67

8th Level
Gain 9
Total HP 76

Yeah, that's what they're saying. They should have 76 but don't. OP, just to confirm that you have it set correctly, if you go into the preferences for the character sheet in question (either from the dropdown in their name or by clicking on the editor icon - the anvil - and clicking on 'home'), please confirm 'Hit Point Type' is set to 'Fixed' and not 'Manual.' If it is set to fixed, I think you'll have to post a link to the character so a mod can take a look (this means posting the URL that you use when you use the character, not the URL from the share link). To get the manually edited max HP to stick in the meantime, when you click the long rest button, uncheck the box regarding resetting Max HP on long rest. You'll have to uncheck that box each time, though.

I also want to check, are you using experience or milestone leveling? If experience, do you have 8 fighter levels or do you have unused levels?

Maybe post a link to the character so we can get information quickly and solve your problem faster.

Quote from Gamemaster76 >>

I have a fighter that's level 8 with 16 con. For HP on level up it's set to calculate automatically using the average but the amount of hp he has is wrong.

He should have 76 HP but he has 70 for some reason. I can override it but it resets after a long rest. How do I fix this?

 On the actual Character Sheet, tap/click the HP and see if the “Max HP Modifier” has a -6 listed.

Quote from DxJxC >>

I also want to check, are you using experience or milestone leveling? If experience, do you have 8 fighter levels or do you have unused levels?

Maybe post a link to the character so we can get information quickly and solve your problem faster.

 I thought of that, but if that were the problem wouldn’t the HP be off by 9 instead of 6 because of the +3 Con mod, or does that get automatically applied even if the level is still unused?

Quote from pocketmouse >>

Yeah, that's what they're saying. They should have 76 but don't. OP, just to confirm that you have it set correctly, if you go into the preferences for the character sheet in question (either from the dropdown in their name or by clicking on the editor icon - the anvil - and clicking on 'home'), please confirm 'Hit Point Type' is set to 'Fixed' and not 'Manual.' If it is set to fixed, I think you'll have to post a link to the character so a mod can take a look (this means posting the URL that you use when you use the character, not the URL from the share link). To get the manually edited max HP to stick in the meantime, when you click the long rest button, uncheck the box regarding resetting Max HP on long rest. You'll have to uncheck that box each time, though.

 yeah that was the problem, I thought I had it set for Fixed but I was wrong, Thanks!