How to Calculate Concrete Volume: Slabs, Footings & Post Holes
To calculate concrete in Cubic Yards for a slab: multiply Length (ft) × Width (ft) × (Thickness (in) ÷ 12), divide by 27, and add 10% for spillage. For retail pre-mix, 1 cubic yard = 45 bags of 80-pound Quikrete (0.60 cu ft/bag) or 60 bags of 60-pound Quikrete (0.45 cu ft/bag).
Concrete is ordered and batched in Cubic Yards (27 cu ft) for truck deliveries, but purchased in individual 50 lb, 60 lb, or 80 lb bags at retail home improvement centers. Calculating exact volumetric requirements prevents costly dispatch short-load penalties or running out of material mid-pour.
1. Rectangular Slab / Pad Formula
Applies to patios, sidewalks, driveway extensions, shed pads, and pool surrounds.
Example: A 10 ft × 10 ft slab at 4 inches thick:
(10 × 10 × (4 ÷ 12)) ÷ 27 = 33.33 ÷ 27 = 1.23 cu yd (+10% waste = 1.36 cu yd / 61 bags of 80 lb).
2. Round Column / Post Hole / Pier Formula
Applies to cylindrical Sonotubes, deck footing piers, and fence post holes.
Example: Four 12-inch diameter holes dug 4 ft deep:
π × (0.5)² × 4 × 4 = 12.57 cu ft = 0.47 cu yd (approx 21 bags of 80 lb).
Standard Pre-Mix Concrete Bag Yields & Coverage
| Bag Weight | Volume Yield (Cu Ft) | Bags per Cubic Yard | Coverage @ 4" Thickness | Typical Application |
|---|---|---|---|---|
| 80 lb (36.3 kg) | 0.60 cu ft | 45 bags | 1.8 sq ft | Structural slabs, footings, steps (best value) |
| 60 lb (27.2 kg) | 0.45 cu ft | 60 bags | 1.35 sq ft | Walkways, small repairs, curbs (easier lifting) |
| 50 lb (22.7 kg) Fast-Set | 0.375 cu ft | 72 bags | 1.1 sq ft | Mailboxes, fence posts (sets in 20-40 mins, no mixing) |
- Always order +10% over net geometry: Formwork flexes outward under the massive hydrostatic pressure of wet concrete (approx 145 lbs/cu ft), and uneven dirt subgrades consume 5% to 8% more material than mathematical calculations assume.
- Compact your subbase: Never pour concrete directly over loose topsoil or mud. Install a minimum 4-inch layer of crushed stone (#57 gravel) and compact thoroughly with a plate compactor to prevent future settling cracks.
- Control joint spacing rule: Cut control joints into your slab at intervals (in feet) no greater than 2 to 2.5 times the slab thickness in inches. For a 4-inch slab, joints should be spaced every 8 to 10 feet.
Frequently Asked Questions
How many 80 lb bags of concrete are in a cubic yard?
There are exactly 45 bags of 80 lb pre-mix concrete in one cubic yard. One 80-pound bag yields 0.60 cubic feet of cured concrete. Since one cubic yard contains 27 cubic feet: 27 / 0.60 = 45 bags.
How many 60 lb bags of concrete make a cubic yard?
It takes 60 bags of 60 lb concrete to make one cubic yard. A 60-pound bag yields 0.45 cubic feet of concrete: 27 / 0.45 = 60 bags.
When should I order a ready-mix truck instead of mixing bags?
As a rule of thumb, projects requiring more than 1.0 to 1.5 cubic yards (45 to 65 bags of 80 lb concrete) are significantly easier, faster, and more structurally uniform when ordered from a ready-mix concrete plant. Hand-mixing or using a small portable mixer for more than 50 bags causes cold joints and extreme physical fatigue.
How thick should a concrete slab be for a patio vs. a driveway?
A standard pedestrian walkway or backyard patio should be at least 4 inches thick poured over a 4-inch compacted gravel base. A residential vehicle driveway or garage slab should be at least 5 to 6 inches thick with steel rebar or wire mesh reinforcement (minimum 3,500 to 4,000 PSI concrete).