How much chocolate ganache for a 12" drip cake?

[LaundryFairy]

Could you tell me how much chocolate ganache you would need for a 12" drip cake? I don't need to fill the cake, as it will be buttercream filling.

Or if you have a quantity for an 8", that's good too as I can work it out from there. All help gratefully received!

i think 400gm of chocolate and 400ml of double cream and 2tbsp of butter.this recipe would be enough for a 12" cake.although for an 8"cake half of the gsnche is required.