How to Calculate the Thickness of FRP Tank in a Given Volume?

No need. Farm the job out to a plastics tank company and use their quotation and proposal!

If push comes to shove, though, carry out a force balance along a theoretical vertical cut at various depths in the tank to establish the tensile stress in the walls (clearly the stress will be greatest at the bottom of the tank). Make sure that the material is thick enough to withstand the tensile stress with a suitable safety margin. This is the same technique that is used for gas-containing pressure vessels.

I note two answers above. Both are correct, I believe.

The second will be quite an exercise. You'll have to learn about the cured properties of reinforced plastic composites. If you really mean FRP/FRG or any non-thermoplastic tank material.

Compatability will be a concern. You don't say what the fluid is. There's so much you don't say.

I predict that you will go with answer #1 above.

Keep us informed.

You should be able to find the allowable stress of GRP from the web etc. From your username it sounds like you might be in the industry.

But as a rough guide - Du Pont used to make RO modules in GRP. They were 200mm dia and something like 20mm thick, working pressure 70 bar. So on that basis the working stress was around 35 MPa. Sounds about right, if you check ordinary plastic pipe, uPVC, ABS, PE etc the stress is about 10 MPa. Worth checking with the GRP supplier though, as the strength will depend on how much glass fibre they put in.