You will need around one 25-kg bag of pre-mixed concrete to fill a foot height of an 8-inch Sonotube if you use 150 lb/ft³ concrete. To arrive at that bag number:
Find the volume of a 1′ high 8" diameter column:
Volume = π × (8 in)² × 12" / 4 = 603.2 in³ ≈ 0.3491 ft³
Calculate the equivalent weight of the column:
Weight = 0.3491 ft³ × 150 lb/ft³ = 52.3599 lb ≈ 23.75 kg
Determine how many bags you need:
Bags = 23.75 kg / 25 kg = 0.95 bag ≈ 1 bag
Read more