Concrete Calculator · https://calcnaut.com/concrete-calculator/
Concrete Calculator
Work out how much concrete you need for any pour. Pick your shape, a slab or wall, a round column or footing, a circular tube, curb and gutter, or stairs, and enter the measurements to get cubic yards, bags, weight and estimated cost, with a waste allowance built in and the formula shown.
This is an estimate, not professional advice. Check your inputs and verify the result against your plans and local building code before you build or order. See terms and disclaimer.
How this calculator works
Choose the shape you are pouring, enter the measurements, and the result updates instantly with the concrete volume in cubic yards, the number of bags, the estimated weight and the cost. Each shape uses its own geometry, shown below, then the same steps: add a waste allowance, divide by 27 for cubic yards, round bags up, and round ready-mix up to the nearest quarter yard.
Slab / Wall
Volume (ft³) = Length(ft) × Width(ft) × (Thickness(in) ÷ 12) × PoursCubic yards = Volume × (1 + Waste%) ÷ 27 Weight = Volume × 150 lb/ft³Round column
Radius (ft) = (Diameter(in) ÷ 2) ÷ 12Volume (ft³) = π × Radius² × Depth(ft) × ColumnsCubic yards = Volume × (1 + Waste%) ÷ 27 Weight = Volume × 150 lb/ft³Tube / ring
Outer R, inner r (ft) = (Diameter(in) ÷ 2) ÷ 12Volume (ft³) = π × (R² − r²) × Height(ft) × TubesCubic yards = Volume × (1 + Waste%) ÷ 27 Weight = Volume × 150 lb/ft³Curb & gutter
Section (in²) = (Curb height × Curb depth) + (Gutter width × Flag thickness)Volume (ft³) = (Section ÷ 144) × Length(ft) × RunsCubic yards = Volume × (1 + Waste%) ÷ 27 Weight = Volume × 150 lb/ft³Stairs
Section (in²) = Run × Rise × Steps × (Steps + 1) ÷ 2Volume (ft³) = Width(ft) × (Section ÷ 144)Cubic yards = Volume × (1 + Waste%) ÷ 27 Weight = Volume × 150 lb/ft³Worked example
A 10 ft × 10 ft slab at 4 inches thick (Slab / Wall shape), with a 10% waste allowance. Inputs: Length 10 ft, Width 10 ft, Thickness 4 in, Number of pours 1, Waste allowance 10 %. Result: 1.36 yd³ (about 62 × 80 lb bags).
Concrete for common slabs
Slab shape at 4 inches thick with a 10% waste allowance. Tap a size to load it in the calculator above.
| Slab size | Concrete needed (yd³) | 80 lb bags | |
|---|---|---|---|
| 10 × 10 ft | 1.36 | 62 | Use → |
| 12 × 12 ft | 1.96 | 88 | Use → |
| 20 × 20 ft | 5.43 | 245 | Use → |
| 24 × 24 ft | 7.82 | 352 | Use → |
| 20 × 30 ft | 8.15 | 367 | Use → |
| 40 × 60 ft | 32.59 | 1,467 | Use → |
Method & assumptions
Concrete is sold by the cubic yard for ready-mix or by the bag for pre-mixed, so every estimate comes down to one thing: the volume of the pour. This calculator handles the shapes you actually pour. A slab, footing or wall is a rectangular box, length times width times thickness. A round column or pier is a cylinder, pi times the radius squared times the depth. A tube or ring is the area between two circles times the height. Curb and gutter is an L-shaped section run along its length. Stairs fill a stepped wedge, the run times the rise times the staircase series.
Whichever shape you choose, the steps after the volume are the same. We add your waste allowance, because no pour is perfectly formed and you cannot top up once the truck leaves, then divide by 27 to convert cubic feet to cubic yards. Bag counts use the printed yield, an 80 lb bag is about 0.60 cubic feet, a 60 lb bag 0.45, and are rounded up because you cannot buy part of a bag. Ready-mix is rounded up to the nearest quarter yard, which is how suppliers sell it. Above about one cubic yard, ready-mix is cheaper and far less work than mixing bags by hand, and the result tells you when that point is reached.
The estimated weight uses about 150 pounds per cubic foot for normal-weight concrete, which helps you check that forms, a trailer or a subgrade can carry the load. For the detail on any one shape, open its dedicated calculator: the slab calculator for floors and patios, the footing calculator for strip footings, the sonotube calculator for round forms, the curb and gutter calculator, and the stairs calculator. For odd shapes, split the pour into these basic shapes, run each one, and add the results. Always confirm structural sizes and reinforcement against your plans and local building code.
Pro tips and common mistakes
- Break odd shapes into basic ones. An L-shaped or stepped pour is just two or three rectangles. Calculate each piece, add them up, and you will not over or under order.
- Order slightly high. You cannot top up a pour once the truck leaves. The waste allowance and rounding up to a quarter yard cover spillage and an uneven base.
- Prep a compacted base. Pour over a firm, level, compacted base rather than soft soil. A good base keeps the volume accurate and stops cracking and settling later.
- Match thickness to the job. 4 inches suits patios and walkways, 5 to 6 inches driveways and slabs carrying vehicles. Too thin cracks under load.
- Reinforce structural pours. Driveways, footings, columns and stairs usually need rebar or mesh. Size it with our rebar calculator and follow local code for anything that carries load.
Frequently asked questions
How do I calculate how much concrete I need?
How many 80 lb bags of concrete are in a cubic yard?
How much does concrete weigh?
When should I order ready-mix instead of bags?
References
Related calculators
Related guides
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