Concrete Calculator
Calculate how many cubic yards and 80-lb bags of concrete you need for a slab, footing, patio, or column.
What is Concrete Calculator?
The concrete calculator tells you how much concrete to order or buy for a rectangular slab, footing, patio, driveway, or column. It converts your dimensions into cubic yards (the standard unit for ready-mix trucks) and the equivalent number of 40-, 60-, or 80-pound premix bags for smaller DIY jobs.
Formula
Volume = length × width × thickness (all in feet). 1 cubic yard = 27 cubic feet.
Premix bag yields (approximate):
- 80-lb bag ≈ 0.60 cu ft
- 60-lb bag ≈ 0.45 cu ft
- 40-lb bag ≈ 0.30 cu ft
A 10% waste allowance is standard for small jobs. Large pours can use less (5–8%).
Worked example
A 10' × 10' patio slab, 4" thick, 10% waste:
- Volume = 10 × 10 × (4/12) × 1.10 ≈ 36.7 cu ft
- = 1.36 cubic yards
- ≈ 62 × 80-lb bags, or 82 × 60-lb bags
- Ready-mix cost at $160/yd³ ≈ $217
How to use this calculator
- Measure length and width of the pour in feet.
- Enter the thickness in inches (slabs are typically 4", driveways 5–6", footings 8–12").
- Keep the 10% default waste allowance for most DIY jobs. Increase to 15–20% for irregular shapes or when you are pouring for the first time.
- For jobs over 1 cubic yard, ordering ready-mix is almost always cheaper and easier than carrying dozens of bags.
Frequently asked questions
At what size should I order ready-mix vs buy bags?
Ready-mix minimums are typically 1 cubic yard (and short-load fees apply under 3–4 yards). Bags make sense under 0.5 yd³ — roughly a 5' × 5' slab at 4" thick. Between 0.5 and 1 yd³, it is a judgment call based on your labor and how much mixing you want to do.
What is the right slab thickness?
Patios and walkways: 4". Driveways: 5–6". Garage floors: 4–5" with rebar or wire mesh. Footings: 8–12" depending on the local frost line. Always check local building codes.
Do I need rebar or wire mesh?
For anything larger than about 5' × 5', yes. Wire mesh works for slabs up to 4" thick; rebar at 12–18" on center is standard for driveways and structural slabs. Without reinforcement, concrete cracks as it shrinks.
How long before I can walk/drive on new concrete?
Walk: 24 hours. Drive a car: 7 days at minimum. Full design strength (3,000–4,000 psi): 28 days. Keep it moist for the first 5–7 days to prevent surface cracking.