Concrete Calculator
Work out how much concrete a slab, footing or round column needs — volume in cubic yards and cubic metres, plus the number of 60 lb and 80 lb bags to buy.
Bag yields used: 80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft. Volume includes the waste percentage.
How the math works
footing: cu ft = length × (width ÷ 12) × (depth ÷ 12)
column: cu ft = π × (diameter ÷ 24)² × height
cu yd = cu ft ÷ 27 · m³ = cu ft × 0.0283168
bags = volume ÷ yield (80 lb = 0.60 cu ft · 60 lb = 0.45 cu ft)
Slabs and footings are boxes: multiply length, width and depth once all three are in the same unit. A round column is a cylinder — π × radius² × height, where the radius in feet is the diameter in inches ÷ 24. One cubic yard is 27 cubic feet; one cubic metre is 35.31 cubic feet.
Divide that volume by the bag yield. An 80 lb bag makes about 0.60 cubic feet and a 60 lb bag about 0.45, which is why a cubic yard takes 45 or 60 bags. Counts round up, since you cannot buy part of a bag. The waste factor covers an out-of-square form, an over-dug trench, spillage and the mix left in the barrow: 5% for a formed slab, 10% for footings and cardboard tubes.
Typical dimensions
| Project | Typical thickness or size | Notes |
|---|---|---|
| Walkway, patio | 4 in (10 cm) | Over 4 in of compacted gravel; mesh for patios |
| Driveway | 5–6 in (13–15 cm) | Rebar or mesh; thicker where vehicles turn |
| Shed base | 4 in (10 cm) | Mesh if the shed holds anything heavy |
| Strip footing | 8–12 in deep, 16–20 in wide | Set by your local code and soil |
| Round column | 8–12 in (20–30 cm) diameter | Deck piers must reach below the frost line |
Ordering numbers, not a structural design: local code decides footing size, reinforcement and frost depth.
Bagged mix or a ready-mix truck?
Up to about one cubic yard, bagged mix is practical: 45 × 80 lb bags, some 3,600 lb of material to load, mix and place in a day. Beyond that, ready-mix is cheaper per yard, far more consistent and much less work — ask the plant about minimum loads and short-load charges before you book. Mix with the water stated on the bag, usually about 3 quarts per 80 lb, and never add extra to loosen it: it weakens the concrete and causes shrinkage cracks.
Pouring and curing
Screed off the forms, float, then finish. Tool or cut control joints every 8–10 ft to about a quarter of the slab depth, so cracks happen where you want them. Stay off it for 24–48 hours, keep vehicles off for a week, and expect full design strength at 28 days.
FAQ
- How many 80 lb bags of concrete are in a cubic yard?
- 45 bags. An 80 lb bag yields about 0.60 cubic feet and a cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45. With 60 lb bags, which yield about 0.45 cubic feet, it takes 60 bags.
- How much concrete do I need for a 10 × 10 slab at 4 inches?
- About 1.23 cubic yards before waste: 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, and 33.3 ÷ 27 = 1.23. That is roughly 56 bags of 80 lb mix, so buy 59 if you allow 5% for waste.
- How much does a cubic yard of concrete weigh?
- About 4,050 lb (1,840 kg). Cured concrete weighs roughly 150 lb per cubic foot, and a cubic yard is 27 cubic feet. That is why pouring a full yard from bags means moving more than 3,600 lb of material.
- How long before I can walk on new concrete?
- Usually 24 to 48 hours for foot traffic, and about 7 days before you drive on it. Concrete reaches full design strength at around 28 days, so keep heavy loads off it for the first week and keep the surface damp or covered while it cures in hot weather.