Have to cover 4" 6" and 8" cakes with grass tip and was wondering how much buttercream I would need? Anyone has an idea?

Will you be icing them before you use the grass tip or just a crumb coat.  

Also...how tall will you be making each tier and will you be putting buttercream between the layers? We need a little more info to be able to help you out, ok?

Sorry ...4" tall after I crumb coat , which I will probably do in the red as the white may show through if I miss a spot.

The rule of thumb I learn yrs ago was one batch of b'cream using 2# of sugar for every cake mix (or batch of batter)  used....... but I always found that to be more than I needed.  I would guess 2 batches of icing -( maybe 3 - but I doubt it.)

Thank you for your help