Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 10 ft; Width 12 ft; Thickness 4 in = 0.3333 ft
- Volume = 10 × 12 × 0.3333 × 1 = 40 ft³
- Net volume = 40 ft³ ÷ 27 = 1.4815 yd³ (× 0.0283168 = 1.1327 m³)
- Order quantity = 1.4815 yd³ × (1 + 10%) = 1.6296 yd³ = 44 ft³
- 80 lb bags = 44 ft³ ÷ 0.6 ft³ per bag = 73.33 → 74 bags (always round up)
- Weight = 44 ft³ × 150 lb/ft³ = 6,600 lb
How to measure
A pad should extend beyond the object it carries. For an AC condenser, the pad is normally a few inches larger than the unit on every side. For a shed, make the pad slightly larger than the floor frame so that rain does not undermine the edge. Check the manufacturer’s installation sheet for minimum pad size.
Pad thickness depends on the weight. Light equipment sits on 3 to 4 in; heavy sheds, hot tubs and generators with large fuel tanks call for 4 to 6 in with reinforcement. Ask the manufacturer.
Quantities for common pads
These are the sizes people ask about most. All use a 4 in thickness and a 10% waste allowance, except where noted.
| Use | Size | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| AC condenser | 3 × 3 ft | 0.12 | 6 | 8 |
| Generator | 4 × 6 ft | 0.33 | 15 | 20 |
| Small shed | 8 × 10 ft | 1.09 | 49 | 66 |
| Shed | 10 × 12 ft | 1.63 | 74 | 98 |
| Hot tub (5 in) | 8 × 8 ft | 1.09 | 49 | 66 |
Worked example: a 10 × 12 ft shed pad
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 10 ft; Width 12 ft; Thickness 4 in = 0.3333 ft
- Volume = 10 × 12 × 0.3333 × 1 = 40 ft³
- Net volume = 40 ft³ ÷ 27 = 1.4815 yd³ (× 0.0283168 = 1.1327 m³)
- Order quantity = 1.4815 yd³ × (1 + 10%) = 1.6296 yd³ = 44 ft³
- 80 lb bags = 44 ft³ ÷ 0.6 ft³ per bag = 73.33 → 74 bags (always round up)
- Weight = 44 ft³ × 150 lb/ft³ = 6,600 lb
The pad needs 1.63 yd³ or 74 bags of 80 lb mix. That is on the border: 74 bags is about 3 tons to carry, so ordering a small truck load becomes attractive. The cost calculator helps decide.
Pad or foundation?
A flat concrete pad is not a frost-protected foundation. In cold climates a slab can heave with the ground, which may be acceptable for a light shed but not for equipment with rigid connections. Where heave matters, a thickened edge or footings below the frost line may be required. Check local requirements for sheds and outbuildings, including permits and setbacks.
Slope the pad slightly away from the equipment so water drains away, and raise it a couple of inches above grade to keep the base of the unit dry.
Tips for small pads
- Compact the base and set the forms level. Small pads show every error.
- Add reinforcement for heavier pads. A few rebar pieces tied in a grid are inexpensive.
- Use a bagged mix with the water the bag specifies: see the bagged concrete guide.
- Let the pad cure before placing heavy equipment. A week is typical for light equipment, and follow the manufacturer for anything critical.
Mistakes to avoid
- A pad exactly the size of the equipment, leaving no margin for settling.
- Pouring directly on topsoil.
- Forgetting conduit sleeves or anchor bolts that must be cast in.
- Not checking local setback and permit rules before you pour.
Pad size rules of thumb
Make the pad larger than the thing standing on it: extend it 3 to 6 in beyond an air conditioner or generator on each side, and a foot or more around a shed. A 3×3 ft pad at 4 in is 0.12 yd³ with waste, or 6 bags of 80 lb mix. A 4×8 ft pad, the footprint of a small utility shed corner or a pair of trash cans, is 0.43 yd³, or 20 bags. For equipment with vibration, ask the manufacturer for a pad specification, and keep the pad separate from the house foundation so that vibration is not transmitted.
Frequently asked questions
How much concrete for an AC pad?
A 3 × 3 ft pad at 4 in is 3 ft³, which is 5 bags of 80 lb mix with no waste. Check the manufacturer’s minimum pad size.
How thick should a generator pad be?
Four inches is common for a residential standby generator, but follow the manufacturer’s installation manual and local code.
How many bags for a 10 × 12 shed pad?
About 67 bags of 80 lb mix at 4 in thick with no waste, and 74 with 10% waste.
Does a shed pad need rebar?
Often a light mesh or a rebar grid is used. It depends on the shed weight, soil and local practice.
Can I pour a pad on dirt?
You can but a compacted gravel base is better for drainage and uniform support, particularly in freeze-thaw climates.
What about a pad for a hot tub?
Hot tubs are heavy when filled. Ask the manufacturer for the pad requirement and use reinforcement.
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.