Circular Wall Diameters, ID or OD?

For any “wall” spell that allows you to create the wall in a “circle that has a 20-foot diameter…,” is that the inner diameter, or the outer diameter? For spells like wall of fire, wall of water, or wind wall it’s a little less important since they are only 1 foot thick, but a wall of thorns is 5 feet thick, so that makes a big difference. How do you rule it?

The outer diameter.

I’d probably measure center to center, so neither.

Playing on a grid, I’d likely say inner. I’d count 8 squares across and put the wall on either side.

But then playing on a grid, most all my circles and spheres become squares and cubes, so I may not be the best source. 

Personally, I would rule whatever way benefits the player most. But I am the crazy DM that gave everyone at my table an artifact item at level 1 so...

Quote from Xalthu >>

Playing on a grid, I’d likely say inner. I’d count 8 squares across and put the wall on either side.

But then playing on a grid, most all my circles and spheres become squares and cubes, so I may not be the best source. 

 My circles get squared too during grid play so an effect creating a ringed wall or circle up to 20 feet in diameter would fill a 4 x 4 squares at the maximum, as limited by the spell.

Quote from Plaguescarred >>

Quote from Xalthu >>

Playing on a grid, I’d likely say inner. I’d count 8 squares across and put the wall on either side.

But then playing on a grid, most all my circles and spheres become squares and cubes, so I may not be the best source. 

 My circles get squared too during grid play so an effect creating a ringed wall or circle up to 20 feet in diameter would fill a 4 x 4 squares at the maximum, as limited by the spell.

 Ahh, diameter, not radius, I read it wrong. So yes, a 4x4 square.

Well, thank you all for your replies!! I genuinely appreciate the different takes on this situation.

Quote from IamSposta >>

Well, thank you all for your replies!! I genuinely appreciate the different takes on this situation.

 You're welcome! May i ask what is your take on this?

I’m not sure, which is why I asked. RAW I think it’s OD, but RAI I think it might be ID. I like SagaTympana’s idea of splitting the difference though.