Concrete Pad Calculator
Yards and 80-lb bags for any concrete pad
What is concrete pad calculator?
A concrete pad is just a small slab — what changes is the purpose and the load it carries. The math is identical to a patio: length × width × thickness gives volume, and bag yield (0.60 ft³ per 80-lb bag) gives the bag count. The reason a dedicated pad calculator exists is the sizing rules, not the math.
Equipment placement drives the dimensions. AC condensers, heat pumps, and mini-split outdoor units want the equipment footprint plus a 2 in border on each side. Standby generators (Generac, Kohler, Briggs) publish minimum pad dimensions in their install manuals — 4 × 5 ft is typical for a 14-22 kW residential unit. Shed pads should match the shed footprint exactly so the wall sits flush on the edge.
Thickness depends on point load, not total weight. A 200 lb AC condenser on four feet creates a higher PSI on the pad than a 5,000 lb shed spreading the load across an 8 × 10 footprint. 4 in is the universal residential default — only switch to 6 in for hot tubs, large generators, or vehicle-bearing pads.
The substrate matters more than people think. Always pour over at least 4 in of compacted gravel (3/4 in minus or paver base). The gravel handles drainage and frost movement; the concrete handles the load. Skipping the gravel is the #1 cause of pad cracking.
For pads under 1 cubic yard (~50 of 80-lb bags), bagged concrete from a hardware store is cheaper and more practical than ready-mix. Above that, call the truck.
When to use a concrete pad calculator
- AC condenser or heat pump pad — A 36 × 36 × 4 in pad sized to the equipment footprint plus a 2 in border on each side. The Slab/Pad tab handles it directly — about 2 of 80-lb bags.
- Whole-home generator pad — Generac and Kohler standby units want a 4 ft × 5 ft × 4 in minimum pad. That's roughly 0.25 cu yd or 12 of 80-lb bags.
- Shed or playhouse base — An 8 × 10 × 4 in shed pad runs about 1 cubic yard — the bag-vs-ready-mix breakeven. The result tells you which way to go.
How to use the Concrete Pad Calculator
- Measure the equipment footprint — Add at least 2 in of pad on each side of the equipment for a clean border. Spec sheets often state the minimum pad size — use that as the floor.
- Pick a thickness — 4 in is standard for AC, generator, and shed pads. Heavy commercial equipment may want 6 in. The pad needs to support the static weight without cracking.
- Enter dimensions in the Slab tab — Pads default to the Slab/Pad shape. Length × width in feet, thickness in inches. Output is cubic yards plus a bag count.
- Decide bags vs ready-mix — Most equipment pads are well under a cubic yard, so bagged mix is fine. Above ~50 bags, call ready-mix. The calculator highlights the breakeven.
Worked examples
AC pad: 36 × 36 × 4 in
Input: Slab tab: 3 ft × 3 ft × 4 in
Output: 0.11 cu yd / 5 of 80-lb bags (with 10% waste)Generator pad: 4 × 5 × 4 in
Input: Slab tab: 4 ft × 5 ft × 4 in
Output: 0.25 cu yd / 11 of 80-lb bagsShed pad: 8 × 10 × 4 in
Input: Slab tab: 8 ft × 10 ft × 4 in
Output: 0.99 cu yd / 45 of 80-lb bags — ready-mix territory