Concrete calculator
You need a volume you can order — cubic yards for a ready-mix truck, or bag counts if you are mixing on a driveway. Thickness in inches fools people: a 10×10 slab at 4 inches is not “100 square feet of concrete,” it is about 1.2 yards before waste. Pick a shape, enter length / width / thickness (or diameter / depth for holes and columns), set a waste percent, and read yards plus bags. This page does not design rebar, mix design, or PSI. It answers “how much do I buy so I am not short when the truck is already pouring.”
+10% so you don't come up short
Estimate only — not a contractor bid. Confirm bag yields and local codes before you pour or order.
When to use
Patio or shed pad shopping list; comparing bag-mix vs a small ready-mix order; checking a contractor’s yardage against your tape measure; teaching a helper why 4 inches matters more than “it looks flat.”
When not to
Structural footings under a house, stamped engineering, or “what strength mix for a driveway” — that is a mix / code question. For bag-only shopping with a fixed yardage already known, use the concrete bag calculator. For a rectangular slab with a simpler form, the slab calculator is tighter.
Assumptions
Density and bag yields use common bagged concrete approximations: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 40 lb ≈ 0.30 cu ft (check your bag — brands differ). Default waste 10%. Ready-mix shown in cubic yards (27 cu ft = 1 yd³). No air-entrainment or slump math.
Worked examples
Example 1
Slab 10 ft × 10 ft × 4 in Waste 10% 80 lb bags
→ ≈ 1.23 yd³ net → ≈ 1.36 yd³ to order; ≈ 61 bags of 80 lb
Under ~1–1.5 yd³ many people still bag-mix; above that, call ready-mix.
Example 2
Three post holes, 12 in diameter × 24 in deep Waste 10%
→ ≈ 0.17 yd³ ordered; bag count for 60/80 lb shown
Irregular holes waste more — bump waste if the auger wandered.
How it calculates
Volume from the shape you picked, convert to cubic yards, apply waste, then divide by bag yield and round up.
- Choose Slab, Footing/Pad, Column/Hole, or Bags from yards.
- Enter dimensions — thickness in inches, not feet.
- Pick 40 / 60 / 80 lb bags and set waste on the tape slider.
- Read ordered yards and bag count on the ticket; confirm yield on the bag you buy.
cu ft = geometry (slab: L × W × thickness_ft; cylinder: π r² h) yd³ = cu ft ÷ 27 order = volume × (1 + waste%) bags = ceil(order cu ft ÷ yield_per_bag)
Common traps
- Thickness in inches, then convert. Entering “4” as feet is a classic disaster.
- Bag yield on the bag wins over any website table if they disagree.
- Short loads and Saturday fees can make tiny ready-mix orders more expensive than bags — the calculator does not price trucks.
- Hot days and long wheelbarrow runs eat workable time; order a little extra rather than remixing half-set piles.
Next tool
Need only bag math from a yardage you already trust? → concrete bag calculator.
Pure “how much do I need” wording? → how much concrete.
Slab-shaped patio with fewer shape options? → concrete slab calculator.
FAQ
Multiply length × width × thickness (in feet) for cubic feet, divide by 27 for yards, then add waste. This page does that and converts to bags.
Related
Estimate only — not a contractor bid or engineered takeoff. Confirm bag yields and local codes before you pour or order.