How to use this calculator
- Pick a pad size. AC condensers usually sit on 3×3 or 4×4 pads. Sheds use the floor footprint. Hot tubs need at least 2 ft of clearance on all sides.
- Choose thickness. 4 inches is standard for AC and shed pads. Hot tubs and generators need 6 inches with rebar to handle the point loads.
- Add a waste %. 10% covers spills, the bag-mixing residue, and the over-dig at the edges of the form.
- Read your result. You get cubic yards (for ready-mix) and the equivalent number of 60 lb and 80 lb bags (for bagged installs).
Formula
Volume (yd³) = Length × Width × Thickness ÷ 27
Worked example
A 4 ft × 4 ft × 4 in AC pad equals 16 × 0.33 = 5.3 ft³ = 0.20 cubic yards. That is roughly 13 bags of 60 lb pre-mix or 9 bags of 80 lb. Bagged is the right call here — short-load fees would more than double the cost.
Tips for accurate results
- For AC condensers, allow at least 2 in of clearance around the unit on the pad.
- Hot tub pads should be 6 in thick with #4 rebar on 12 in centers — a filled hot tub can weigh 4,000+ lbs.
- Compact and level the gravel base before pouring; pads on un-prepped soil tilt as the soil settles.
- Use a 3,000 PSI mix for residential pads. Higher strength is rarely worth the cost at this size.