Concrete Calculator — Cubic Feet
Slabs, footings and columns in real time. Outputs cubic feet, cubic yards and bag counts.
Ordering: ft³ vs yd³
Ready-mix trucks deliver concrete by the cubic yard (minimum order typically 1 yd³, with a short-load fee under 3 yd³). For small jobs under 1 yd³ (27 ft³), bagged mix is more cost-effective.
A 60 lb bag yields ~0.45 ft³. An 80 lb bag yields ~0.6 ft³. To pour 1 cubic yard from bags, you'd need 60 bags of 60 lb mix or 45 bags of 80 lb mix — heavy work that quickly becomes more expensive than ready-mix.
Slab Thickness Guide
| Application | Thickness |
|---|---|
| Sidewalk | 4 inches |
| Patio | 4 inches |
| Shed floor | 4 inches |
| Residential driveway | 5–6 inches |
| Garage floor | 5–6 inches |
| Heavy vehicle (RV pad) | 6–8 inches |
Worked Example — 10×12 Slab
A 10 ft × 12 ft shed slab at 4 inches thick. Thickness in feet: 4 ÷ 12 = 0.333 ft.
Volume = 10 × 12 × 0.333 = 40 ft³. Cubic yards = 40 ÷ 27 = 1.48 yd³.
Add 10% waste = 1.63 yd³. Order 1.75 yd³ (typical short-load delivery rounds up).
Worked Example — Sonotube Footing
A 12-inch diameter sonotube, 4 ft deep. Radius = 6 in = 0.5 ft. Volume = π × 0.5² × 4 = 3.14 ft³.
For 6 footings: 6 × 3.14 = 18.85 ft³ = 0.7 yd³. Use ~7 bags of 80 lb mix per footing.
After the Pour: Fill and Weight
Concrete footings often sit in an excavated hole larger than the footing itself — the gap gets backfilled with fill dirt once the concrete cures. Before ordering, it's also worth checking total weight: concrete runs about 150 lb per cubic foot, so a 1 cubic yard order (27 ft³) weighs roughly 4,050 lbs — use the weight calculator to confirm a delivery truck or pump can access your site safely.
Frequently Asked Questions
How many cubic feet of concrete in a cubic yard?
27 cubic feet equal 1 cubic yard. Concrete is ordered in yd³.
How thick should a concrete slab be?
4 inches for patios, sidewalks and shed floors; 5–6 inches for driveways; 6–8 inches for heavy vehicle areas.
How many 60 lb concrete bags per cubic foot?
About 2.2 bags. Each 60 lb bag yields roughly 0.45 ft³.
How many 80 lb concrete bags per cubic foot?
About 1.67 bags. Each 80 lb bag yields roughly 0.6 ft³.
What's the formula for a column footing?
Cylinder: V = π × r² × height. A 12-in diameter × 4 ft tall footing = 3.14 ft³.
Should I add waste factor?
Yes — 5–10% extra. Spillage, uneven excavation and over-pour are normal.
How much does 1 cubic foot of concrete weigh?
About 150 lbs. A cubic yard weighs ~4,050 lbs (2 tons).
How to calculate concrete for stairs?
Calculate each step as a rectangular volume (rise × run × width). Sum the volumes.