How to calculate it yourself
Cylinder volume = π × radius² × depth (all in feet). For a Bigfoot bell base, add the bell volume from the manufacturer spec sheet. Multiply by pier count, then add 10% waste. Most decks use 6-12 piers depending on size and span.
Common scenarios
Small 8×10 deck (4 piers)
4 piers × 12" × 36" = 4 × 2.36 ft³ = 9.4 ft³ + 10% = 10.5 ft³ ≈ 18 bags of 80 lb. About $90-110 in pre-mix. Under 0.5 yd³ — bagged is the right call.
Standard 12×16 deck (6 piers)
6 piers × 12" × 48" (frost zone) = 6 × 3.14 = 18.8 ft³ ≈ 32 bags of 80 lb. About $160-190. Bagged is still cheaper than a short-load delivery here.
Large 16×24 deck with stair landing (10+ piers)
10 piers × 12" × 48" = 31.4 ft³ ≈ 1.16 yd³. This is the breakeven where ready-mix becomes worth it — at 1+ yd³ the short-load fee gets divided across enough volume to compete with bagged.