Concrete Calculator
Pick a shape, enter the size, and get the volume of concrete in cubic yards (or m³), the number of bags if you mix it yourself, and the cost of bags versus a ready-mix truck.
How to use the concrete calculator
- Choose the shape: a flat slab (patio, shed base, driveway), a footing or wall, a round column, or post holes.
- Enter the size. Slabs use length × width × thickness. Columns and post holes use diameter and height/depth.
- Add 5–10% extra for spillage, uneven ground and over-excavation.
- Choose your bag size or enter your local ready-mix price to compare costs.
Concrete volume formulas
Slab or footing (cu ft) = length (ft) × width (ft) × thickness (in) ÷ 12
Column or hole (cu ft) = π × (diameter (in) ÷ 24)² × height (ft)
Cubic yards = cubic feet ÷ 27
Bags = cubic feet ÷ yield per bag (80 lb ≈ 0.60 cu ft)
Example: a 10 × 10 ft patio
A 10 × 10 ft slab, 4 inches thick, is 10 × 10 × 4 ÷ 12 = 33.3 cu ft, or 1.23 cubic yards. With 10% extra you need 36.7 cu ft (1.36 cu yd). That is about 62 bags of 80 lb concrete — a lot of mixing — so a 1.5-yard ready-mix order is usually the better choice.
How thick should a slab be?
| Project | Typical thickness |
|---|---|
| Walkway, patio, shed base | 4 in (10 cm) |
| Driveway for cars | 4–5 in (10–13 cm) |
| Driveway for trucks / RV | 5–6 in (13–15 cm) |
| Garage floor | 4–6 in (10–15 cm) |
Always follow local building codes and your engineer's plans for structural work.
Bag yields
Pre-mixed bags are sold by weight. Typical yields are about 0.60 cu ft for an 80 lb bag, 0.45 cu ft for 60 lb and 0.30 cu ft for 40 lb. Check the yield printed on your brand's bag and adjust if needed.
Frequently asked questions
How many bags of concrete are in a yard?
One cubic yard is 27 cubic feet. That is about 45 bags of 80 lb concrete (0.60 cu ft each), 60 bags of 60 lb, or 90 bags of 40 lb.
How much concrete do I need for a 10x10 slab?
A 10 × 10 ft slab 4 inches thick needs about 1.23 cubic yards. With 10% extra, plan for about 1.36 cubic yards or roughly 62 bags of 80 lb mix.
How much extra concrete should I order?
Most contractors add 5–10%. Use the higher number for uneven ground, hand-dug holes or when you can't easily get more concrete during the pour.
Is it cheaper to buy bags or ready-mix?
For small jobs under about one cubic yard, bags are usually practical. For larger pours a ready-mix truck saves a lot of labor, and the cost per yard is often similar or lower, though small loads can carry a delivery fee.
How much concrete for a fence post?
A 10-inch hole that is 30 inches deep holds about 1.36 cu ft. Subtract a little for the post itself. That is about 2–3 bags of 80 lb mix per post, depending on post size.