Does each 10-foot cube do 7d10?

Yes but each cube must be adjacent to another creating a chain of cubes. So in general each cube does 7d10 damage but only in the area that it is in. You could not combine the cubes to do an astounding 70d10 damage.

So, just for clarification, if I use it on a large or larger creature I would do 7d10 x the number of cubes on that creature. Thanks for the reply.

That would be a DM call. I wouldn't say you could combine the cubes for additional damage.

The area of the storm consists of up to ten 10-foot cubes, which you can arrange as you wish. Each cube must have at least one face adjacent to the face of another cube. Each creature in the area must make a Dexterity saving throw. It takes 7d10 fire damage on a failed save, or half as much damage on a successful one.

It seems to me that the wording here refers specifically to the spell's overall area of effect when specifying who makes a save and the damage based on the result of the save. Cubes seem to only be used as a tool to describe the shape of the area of effect. Based on that, I'd disagree with counting individual cubes to scale the damage dealt.

Your DM might make a different call, especially if you're using the spell specifically on a gargantuan creature or something, but otherwise there shouldn't really be any reason one target takes 2-4 times as much damage as someone right next to them because they happen to be on the border between the cubes defining the spell's shape.

Nope. It would still deal just 7d10... I am sure this answered over the past 1 year, just to be sure. The spell itself deals 7d10 points of damage, no matter how many cubes you are in contact with.

I would beg to differ as long as the creature takes up more than one cube every cube the creature is in should count for 7d10 damage.(bigger the creature the more health it tends too have)

It's not that each cube does 7d10 damage. The cubes describe the total area covered by the spell. Then, every creature within that area of the spell takes 7d10 damage. Ruling it your way would mean that a Large creature in a fireball would take more damage than a Medium creature in the same fireball because the Large creature takes up more of the area.

the spell description specifically states that each creature takes 7d10 fire damage on a failed save or half as much on a successful save.  It doesn't state that that damage is per cube.

The cubes are there only for a description on how the fire storm would look and to provide a way for the caster to change the shape.  Otherwise it's a 7th level fireball that does about the same amount of damage (11d6 for fireball vs 7d10 for firestorm)  Personally, I'd rather upcast fireball for the extra dice as the average damage is slightly better.

Of course, if you DM, you can call it how you want.

why would you ever upcast fireball to 7th level when you can just cast delayed blast fireball, a literal 7th level fireball that does better damage?

In my previous comment, I was exclusively comparing firestorm with fireball. I did not look at any other spell as my point was made with what I chose. It has nothing to do with whether or not I would upcast fireball at lvl 7.

Here's the thing....if i choose to upcast fireball at lvl 7 instead of casting delayed blast fireball,  thats my choice, not yours. Stay in your lane and play your own character. Let others play how they choose

I wasn't trying to dictate the way you should play the game, I was just stating an option that would work better by your own decision making metric: average damage.  

And by the way, 7th level fireball and firestorm actually do the same average damage, at 38.5 fire damage, you're just more likely to roll near that average damage with fireball because you're rolling more dice.

The reason to use this over fireball is simply the fact that you can configure this much better in the battlefield. Easily hitting almost every enemy while avoiding your allies. Never underestimate how much positioning is important.

7th level fireball would do 12d6 = 72+12=84 84/2= 42 avarage

firestorm 7thlevel 7d10 = 70+7=77 77/2=38,5 so fireball does more but in a way smaller area

As a wizard you may not have deleyed blast fireball prepared and need huge fire AOE. In that scenario upcasting fireball makes perfect sense. As a sorcerer you may have learn fireball as one of your 3rd lvl spells but haven't learned deleyed blast fireball as you preferred different spell of its lvl.

Wizards don't get firestorm

This might be a good spell to include if one were homebrewing a spell list for a red dragon.

This on a Tempest cleric with metamagic adept feat taking transmuted spell option. Change the fire to lightning or thunder, max the 7d10 with your channel divinity, in ten cubes thats a combined total of 700 lightning or thunder damage.

And im out.

What if you could cast it at a higher level and add more cubes?

Why isn't this a wizard spell?