Concrete Calculator
Estimate concrete for a slab or footing, round post holes or solid steps in cubic yards, cubic feet and m³, with waste, bag count at your bag’s yield and cost.
Interactive calculator · Method CONCRETE-VOLUME-1.0 · Last reviewed: 2026-11-04
Calculate your scenario
Slab = length × width (ft) × thickness (in) ÷ 12.
Your inputs
Default worked example. Edit the inputs to calculate your scenario.
Default worked example
10 × 10 ft × 4 in ÷ 12 = 33.33 cu ft × (1 + 10%) = 36.67 cu ft = 1.358 cu yd.
How to use the calculator
Pick slab, post holes or steps, fill in the dimensions for that shape, then a waste allowance, the yield printed on your bag and a price per bag or per cubic yard.
What the result means
A 10 × 10 ft slab 4 in thick is 33.33 cu ft; with 10% waste that is about 1.358 cu yd, or 62 bags at 0.6 cu ft each.
Formula and units
Slab = length × width (ft) × thickness (in) ÷ 12. Post holes = count × (π × diameter² ÷ 4 − post width²) × depth, all in inches, ÷ 1,728. Solid steps are stacked blocks: step k is k × rise tall, so volume = width (ft) × run × rise (in) × (1 + 2 + … + n) ÷ 144. Add waste; cubic yards = cu ft ÷ 27; m³ = cu ft × 0.0283168; bags = cu ft ÷ bag yield, rounded up.
Worked examples
10 × 10 ft slab, 4 in thick, 80 lb bags: Slab length: 10 ft; Slab width: 10 ft; Slab thickness: 4 in; Hole or column diameter: 10 in; Hole or column depth: 36 in; Number of holes or columns: 4 holes; Post width set in each hole (0 if none): 3.5 in (square side); Number of steps: 3 steps; Step rise (height of each step): 7 in; Step run (tread depth): 11 in; Step width: 4 ft; Waste and spillage allowance: 10 %; Bag yield from the bag label (example 0.6 cu ft for 80 lb): 0.6 cu ft per bag; Price (example $6.50 per bag; your quote): 6.5 $ per bag or cu yd; What are you pouring?: slab; Price basis: bag.
Example result: Concrete needed: 1.358 cu yd; Cubic feet: 36.67 cu ft; Cubic meters: 1.038 m³; Bags at entered yield: 62 bags of 0.6 cu ft; Estimated cost: $403.00.
Three solid steps 4 ft wide, ready-mix per yard: Slab length: 10 ft; Slab width: 10 ft; Slab thickness: 4 in; Hole or column diameter: 10 in; Hole or column depth: 36 in; Number of holes or columns: 4 holes; Post width set in each hole (0 if none): 3.5 in (square side); Number of steps: 3 steps; Step rise (height of each step): 7 in; Step run (tread depth): 11 in; Step width: 4 ft; Waste and spillage allowance: 5 %; Bag yield from the bag label (example 0.6 cu ft for 80 lb): 0.6 cu ft per bag; Price (example $6.50 per bag; your quote): 160 $ per bag or cu yd; What are you pouring?: steps; Price basis: yard.
Example result: Concrete needed: 0.499 cu yd; Cubic feet: 13.48 cu ft; Cubic meters: 0.382 m³; Bags at entered yield: 23 bags of 0.6 cu ft; Estimated cost: $79.85.
Limitations and common mistakes
Quantity only: no mix design, strength, reinforcement, base preparation, frost depth, load or footing approval. The bag yield is printed on each bag and differs by product and weight, so the 0.6 cu ft default is an example. For a round post, enter 0.886 × its diameter as the width (same cross-section). Steps assume a solid block on grade with no landing; add a landing as a slab.
Dividing cubic feet by 9 or forgetting the ÷ 12 for inches of thickness; another is assuming an 80 lb bag makes a full cubic foot.
Sources and assumptions
Method CONCRETE-VOLUME-1.0; last formula review 2026-11-04. Sources and their supported claims. Calculation methodology.