How to calculate how much concrete you need
All concrete is measured by volume. The formula for a slab is: length × width × thickness. Divide by 27 to get cubic yards (the unit ready-mix trucks use). This calculator handles slabs, footings (rectangular), and round columns, then adds everything up so you can order or buy bags in one go.
Bags vs. ready-mix concrete
For small jobs under about 1 cubic yard, bagged concrete is usually easiest — available at any hardware store. For larger pours, ordering ready-mix by the truck is cheaper and much faster. Most ready-mix trucks carry 8–10 cubic yards. Add 5–10% extra to account for spillage, uneven sub-base, and waste.
Bag coverage reference
| Bag size | Yields | Bags per yd³ |
|---|---|---|
| 40 lb | 0.011 yd³ (0.30 ft³) | ~90 |
| 60 lb | 0.017 yd³ (0.45 ft³) | ~60 |
| 80 lb | 0.022 yd³ (0.60 ft³) | ~45 |
Common slab thicknesses
Sidewalks and patios: 4 inches. Driveways: 4–6 inches (6 for heavy vehicles). Garage floors: 4–6 inches. Structural slabs: consult an engineer. Always pour on compacted gravel sub-base for best results.
Tips for a good pour
Dampen the sub-base before pouring so it doesn't pull water from the concrete too fast. Work in sections for large slabs, and never add extra water to the mix — it weakens the finished concrete significantly. Control joints every 8–12 feet prevent cracking.
Methodology & sources
How this calculator works: concrete is estimated by volume. Slabs and footings use length × width × thickness; columns use π × radius² × height. All dimensions are converted to feet, and the total cubic feet is divided by 27 to get cubic yards (1 yd³ = 27 ft³, by definition). Bag counts use published dry-mix yields — a 40 lb bag ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, and 80 lb ≈ 0.60 ft³ — so bags = total cubic feet ÷ bag yield, rounded up. A waste allowance of 5–10% is recommended for spillage and uneven sub-base.
Sources: bag yields follow the manufacturer figures published by QUIKRETE® and Sakrete® (standard 40/60/80 lb concrete mixes); the cubic-yard conversion and slab-thickness guidance follow common U.S. residential concrete practice. Estimates are for planning only — order confirmation should be made with your ready-mix supplier. Last reviewed: by the Toolchive Team.