Concrete Pad Calculator: AC, Shed & Equipment Pads
This concrete pad calculator sizes small slabs like AC condenser, generator, shed and hot-tub pads from your length, width and thickness. Because these pads are small, it also tells you exactly how many bags to buy so you don't over-order for a quick weekend pour.
Estimated concrete
Estimates only. Add ~5–10% for waste. Bag yields: 40 lb ≈ 0.30, 60 lb ≈ 0.45, 80 lb ≈ 0.60 cu ft. Concrete ≈ 150 lb per cu ft.
How the Concrete Pad Calculator works
- 1
Enter your dimensions
Type the length, width and thickness of your slab, footing or pad and choose your units, feet, inches or meters.
- 2
Get instant volume
See the result in cubic yards, cubic feet and cubic meters, plus weight, the moment you type, no page reloads.
- 3
Buy the right amount
We convert volume into the exact number of 40, 60 and 80 lb bags, truck loads and cost so you order without waste or shortfalls.
The formula
ft3 = Length_ft x Width_ft x (Thickness_in / 12) x quantity; yd3 = ft3 / 27
- ·Length_ft = pad length (feet)
- ·Width_ft = pad width (feet)
- ·Thickness_in = slab thickness (inches)
- ·quantity = number of identical pads
- ·27 = cubic feet per cubic yard
A default 10% waste factor is added to cover uneven subgrade and spillage. Small AC and shed pads are usually 4 inches thick; heavier equipment pads run 5-6 inches.
Worked examples
Example 1: 3x3 ft AC condenser pad, 4 in thick
- Volume = 3 x 3 x (4 / 12) = 3 ft3
- Add 10% waste: 3 x 1.1 = 3.3 ft3
- 80 lb bags: 3.3 / 0.60 = 5.5, round up
Result: 3.3 ft3 (0.12 yd3); about 6 eighty-lb bags
Example 2: 8x10 ft shed pad, 4 in thick
- Volume = 8 x 10 x (4 / 12) = 26.67 ft3
- Cubic yards = 26.67 / 27 = 0.99 yd3
- Add 10% waste: 0.99 x 1.1 = 1.09 yd3
Result: about 1.1 cubic yards
Concrete calculators by project type
Concrete bag yield & coverage
How much mixed concrete a standard bag yields, and how far a cubic yard goes at common slab depths.
| Bag size | Yield (cu ft) | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
- 40 lb
- 60 lb
- 80 lb
Related guides
Concrete pad calculator FAQs
How many bags of concrete for a 3x3 AC pad?
A 3 ft × 3 ft pad at 4 inches thick is 9 × 0.333 = 3 ft³, which is about five 80 lb bags or seven 60 lb bags. Round up one bag to cover spillage.
How thick should an equipment or AC pad be?
Four inches is standard for AC condensers, generators and shed pads on a compacted base. Heavier equipment or mini-split multi-zone units may call for a 5–6 inch pad with mesh reinforcement.
Can I use bagged concrete for a shed or generator pad?
Yes, most small pads are well under a cubic yard, so bagged concrete is the practical choice. A 4×4 pad at 4 inches only needs about nine 80 lb bags.
Do I need gravel under a concrete pad?
A 3–4 inch compacted gravel base is recommended to promote drainage and prevent the pad from settling or heaving. Skipping it can cause the pad to crack or tilt under load.
How many 80 lb bags make a cubic yard for a pad?
It takes about 45 eighty-pound bags to make one cubic yard, since each bag yields roughly 0.60 ft³. Most pads use only a small fraction of a yard, so bag counts stay manageable.
Should I add rebar or mesh to a small pad?
For light AC or shed pads, welded wire mesh helps control cracking and is usually enough. Add rebar for heavier equipment or pads that will bear concentrated point loads.
What size pad do I need for a mini-split or heat pump?
A 3x3 ft or 3x4 ft pad that extends about 6 inches beyond the unit's footprint on all sides is standard. Keep it 4 inches thick for a residential condenser and set it level so the compressor drains and runs quietly.
How long before a pad can support equipment?
You can walk on it after 24-48 hours, but concrete only reaches roughly 70% strength at 7 days and full strength at 28 days. For a heavy AC unit or generator, wait about 7 days before mounting the equipment.
What PSI concrete should I use for an equipment pad?
A 3,000-3,500 PSI mix is plenty for most residential AC, shed, and generator pads. Bagged concrete mixes typically cure to about 4,000 PSI, so a standard bag exceeds what a light pad needs.
Should a concrete pad be sloped for drainage?
Yes, give it a slight slope of about 1/8 to 1/4 inch per foot away from any structure so water sheds off. A pad that ponds water can heave in freeze-thaw climates and corrode equipment feet.
How deep should the gravel base under a pad be?
Plan on about 4 inches of compacted crushed gravel under a residential pad. The base spreads load, improves drainage, and reduces cracking from soil movement, which matters more than adding extra concrete thickness.
Should I isolate the pad from an adjacent slab or foundation?
Yes, place a 1/2 inch isolation joint (foam or fiber board) where the pad meets a house foundation, driveway, or existing slab. This lets the pad move independently and prevents cracks from telegraphing between the two.
Do I need a permit to pour a small concrete pad?
Small AC or shed pads usually do not require a permit, but rules vary by municipality and by whether the pad supports a structure. Check locally, since larger or structural pads and anything near property lines often do need approval.