Concrete Pad Calculator
A concrete pad calculator finds cubic yards as length × width × thickness (ft) ÷ 27. An 8 × 8 ft hot tub pad 4 in thick needs 0.79 yd³, about 36 bags of 80 lb.
How much concrete a pad takes
A concrete pad calculator works out the volume of a small rectangular slab in cubic yards. Multiply length × width × thickness, all in feet, then divide by 27. Cubic yards = L × W × (thickness in ÷ 12) ÷ 27. An 8 × 8 ft hot tub pad 4 inches thick needs 0.79 yd³, and the default 10 × 10 ft pad needs 1.23 yd³.
Most pads are small enough to mix from bags. To turn the cubic-yard result into 80-lb bags, multiply by 27 and divide by 0.6, the cubic feet one bag yields.
How pads differ from larger slabs
A pad carries a concentrated load in one spot: a condenser on four feet, a generator on rails, a hot tub full of water and people. The pad’s job is to stay level so the equipment does not rock, drain badly, or strain its connections. That makes the base under it as important as the concrete.
Size the pad a few inches larger than the unit on every side. The feet stay fully supported, and the edge is less likely to crack off.
Worked examples
Example 1: Hot tub pad, 8 × 8 ft, 4 in thick, $150 per yd³
- Volume: 8 × 8 × 0.333 = 21.3 ft³ ÷ 27 = 0.79 yd³
- With 10% extra: 0.87 yd³
- Cost: 0.869 × $150 = $130.37
- In 80-lb bags: 0.869 × 27 ÷ 0.6 = 39.1, so 40 bags
Example 2: AC condenser pad, 3 × 3 ft, 4 in thick
- Volume: 3 × 3 × 0.333 = 3 ft³ ÷ 27 = 0.11 yd³
- In 80-lb bags: 3 ÷ 0.6 = 5 bags, or 6 with 10% extra
Example 3: Default 10 × 10 ft pad, 4 in, $150 per yd³
- Volume: 1.23 yd³, or 1.36 yd³ with 10% extra
- Cost with allowance: $203.70
Typical pad sizes and volumes
| Pad | Size | Thickness | yd³ (net) | 80-lb bags (net) |
|---|---|---|---|---|
| AC condenser | 3 × 3 ft | 4 in | 0.11 | 5 |
| Heat pump, larger unit | 4 × 4 ft | 4 in | 0.20 | 9 |
| Small standby generator | 5 × 3 ft | 4 in | 0.19 | 9 |
| Hot tub | 8 × 8 ft | 4 in | 0.79 | 36 |
| Hot tub, heavy or poor soil | 8 × 8 ft | 6 in | 1.19 | 54 |
| Small shed or trash enclosure | 10 × 12 ft | 4 in | 1.48 | 67 |
Bag counts are rounded up and leave out the 10% allowance. Equipment sizes vary by model; use the manual’s dimensions.
How to build and measure the pad
- Mark the footprint. Take the unit’s base size and add 3 to 6 inches on each side.
- Dig out the soft soil. Remove sod and topsoil, then compact what is beneath.
- Add a gravel base. Four inches of compacted crushed stone is common. The gravel shed base calculator sizes it.
- Set the forms level, or with a slight fall. A pad outside should shed water away from the house.
- Measure inside the forms for length and width, and from the base to the form top for thickness.
Common mistakes
- Using the 10 × 10 ft default. Most equipment pads are far smaller. Change the size before reading the result.
- Pouring on topsoil. Organic soil settles and the pad tilts within a season or two.
- Building the pad too small. Feet that sit at the very edge can crack the corner off.
- Ignoring the filled weight of a hot tub. The water is the heavy part. Check the maker’s pad requirement against the filled weight, and size the tub’s volume with the hot tub calculator.
Related calculators
The concrete bag calculator converts yards to 40, 60, and 80-lb bags. For a full installed price with labor, use the concrete pad cost calculator. Pads that support structures or heavy equipment should be checked by a licensed contractor or engineer.
Frequently asked questions
How much concrete do I need for a pad?
Multiply length × width × thickness in feet and divide by 27. An 8 × 8 ft pad 4 inches thick is 64 × 0.333 = 21.3 ft³, or 0.79 yd³. With 10% extra it is 0.87 yd³, which is about 40 bags of 80-lb mix.
What size pad does an AC unit or generator need?
Usually only a little larger than the unit. Many condensers sit on a pad about 3 × 3 ft to 4 × 4 ft, and small standby generators on one around 3 × 5 ft. The installation manual gives the minimum size and clearances, and some makers supply a composite pad instead of concrete.
How thick should an equipment pad be?
Four inches is common for AC condensers, small generators, and most hot tubs on firm, well-compacted ground. Larger generators, package HVAC units, and big swim spas may call for 6 inches or reinforcement. The equipment manual's foundation section is the authority.
Should a small pad use bags or a ready-mix truck?
Under about 1 cubic yard, bagged mix is usually simpler. That is about 45 bags of 80 lb. Above that, ready-mix often makes sense, but check the supplier's minimum order and short-load fee first.
Does a pad need to be reinforced with rebar or mesh?
Light equipment pads are often poured with wire mesh or fibers for crack control, or none at all. Hot tubs and heavier equipment benefit from rebar or mesh, especially on clay or fill. Follow the manufacturer's requirements and local code.
Estimates only. Verify quantities with your supplier before purchasing.