Concrete Pad Calculator
Pouring a small equipment pad — for an AC condenser, mini-split, generator, shed, or hot tub? Enter the pad length, width, and thickness to see how many bags it takes (small pads are usually a bag job, not a truck order) plus the cubic feet and yards, with a waste factor. For a full patio, garage floor, or driveway use the slab calculator; for installed price use the slab-cost calculator.
Total volume required
0.22Cubic Yards
Cubic feet
5.87
Cubic meters
0.17
Bags (80 lb)
10
Bags by size
40 lb
20
50 lb
16
60 lb
14
80 lb
10
Estimate includes your waste factor. Figures are estimates, not a quote.
How to use this calculator
- 1Measure the pad length and width in feet and the thickness in inches (4 inches suits most light equipment pads; 5–6 inches for a hot tub or heavy unit).
- 2Set Quantity if you are pouring more than one identical pad.
- 3Keep a 10% waste factor for a small form and uneven base.
- 4Read the bag count first — most utility pads are mixed from bags — and use cubic yards only if the pad is large enough to order ready-mix.
Formula & rounding
A pad uses the rectangular volume math: length × width × thickness in feet (thickness in inches ÷ 12), cubic feet ÷ 27 for cubic yards, with bag counts rounded up. What sets a utility pad apart is not the formula but the prep: a level, compacted gravel base, a footprint sized to the equipment plus working clearance, and enough thickness for the point load the unit puts on it. Size the pad to the equipment, not just to a number.
AC condenser pad, 3 ft × 3 ft × 4 in
- Volume = 3 × 3 × (4 ÷ 12) = 3 cubic feet
- With 10% waste: 3 × 1.10 = 3.3 cubic feet
- 80 lb bags = 3.3 ÷ 0.60, rounded up = 6 bags
- Cubic yards = 3.3 ÷ 27 = 0.12 cubic yards (well under a ready-mix minimum)
= ≈ 6 × 80 lb bags (0.12 cubic yards) for an AC pad
Hot tub pad, 8 ft × 8 ft × 6 in
- Volume = 8 × 8 × (6 ÷ 12) = 32 cubic feet
- With 10% waste: 32 × 1.10 = 35.2 cubic feet
- Cubic yards = 35.2 ÷ 27 = 1.30 cubic yards
- At ~1.3 yards, compare bags (≈59 × 80 lb) against a small ready-mix order.
= ≈ 1.30 cubic yards — the break-even point between bags and ready-mix
Common small concrete pad sizes
Typical footprints and thickness for light equipment pads. These are planning starting points — match the pad to the unit footprint and any manufacturer or code requirement.
| Pad use | Typical size | Common thickness |
|---|---|---|
| AC condenser / heat pump | 3 ft × 3 ft | 4 in |
| Standby generator | 4 ft × 5 ft | 4–6 in |
| Garden / storage shed | 8 ft × 10 ft | 4 in |
| Hot tub / spa | 7–8 ft square | 6 in (loaded) |
Estimate only. Size the pad a few inches beyond the equipment footprint for clearance, and confirm thickness and base with the manufacturer — a loaded hot tub is especially heavy. Large floors and driveways belong on the slab calculator.
Frequently asked questions
How much concrete do I need for an AC pad?
A typical 3 ft × 3 ft condenser pad at 4 inches thick is about 3 cubic feet, or roughly six 80 lb bags with waste. Enter your exact pad size for the bag count; pads this small are almost always a bag job rather than a ready-mix order.
How thick should a shed or equipment pad be?
Four inches is enough for most sheds, AC units, and generators on a compacted base. Step up to 5–6 inches for a hot tub or heavier equipment. Thickness drives the volume directly, so set it before reading the bag count.
What size concrete pad do I need for a hot tub?
Many spas sit on a pad roughly 7–8 feet square at 6 inches thick, but a filled hot tub is very heavy, so follow the manufacturer’s base specification. At about 1.3 cubic yards, that pad is near the point where ready-mix can beat bags.
Do equipment pads need a gravel base?
Yes. A point-loaded pad — an AC unit, generator, or hot tub — should sit on about 4 to 6 inches of compacted gravel so it does not settle or crack. Compact the base, make the pad a few inches larger than the equipment footprint, and slope it slightly for drainage.
Is a concrete pad the same as a slab?
The volume math is the same, but a pad usually means a small, standalone surface for one piece of equipment, sized to that unit and its base. For a full patio, garage floor, or driveway use the slab calculator, and for installed price use the slab-cost calculator.
Assumptions & sources
- Volume formula
- length × width × thickness (all in feet); cubic feet ÷ 27 = cubic yards.
- Bag yields
- ≈0.60 cu ft per 80 lb bag (0.45 / 0.375 / 0.30 for 60 / 50 / 40 lb); rounded up.
- Thickness
- 4 in for light equipment pads; 5–6 in for hot tubs and heavier units.
- Base prep
- Assumes a level, compacted gravel base (≈4–6 in) under the pad; point-loaded equipment needs a stable base, not just concrete.
- Waste factor
- Default 10% for small forms and an uneven base.
- Scope
- Small utility pads only — full slabs, floors, driveways, and installed price are handled on the slab and slab-cost pages.
See the methodology & sources for how these values, formulas, and rounding are chosen.
Helpful concrete guides
- Concrete bag sizes & yields — helps choose 40, 50, 60, or 80 lb bags for a small pad mixed on site.
This is an estimate, not a quote. Concrete quantities, bag yields, block coverage, and prices vary with product, brand, mix, region, supplier, tax, delivery, and on-site conditions. Always confirm with your supplier and round up for safety. For structural or code-related work, consult a qualified professional or your local building authority.