Concrete Calculator
Concrete calculator for slabs, footings, round columns and stairs — cubic yards and premix bags.
Pick a shape
Premix bags needed (includes waste allowance)
How the volume is calculated
Every shape reduces to length × width × height, with results converted to cubic yards (÷27) and cubic metres (×0.0283) since ready-mix is ordered by the yard and bagged mix is priced per bag.
Slab / footing: cubic feet = length (ft) × width (ft) × thickness (ft). Example: a 10 × 10 ft slab, 4" thick — 10 × 10 × (4/12) = 33.3 ft³ = 1.24 yd³.
Round column / sonotube: cubic feet = π × (diameter/2)² × height (ft) × count. A 12" tube, 4 ft tall, poured 3 times: π × 0.5² × 4 × 3 ≈ 9.42 ft³.
Stairs: treated as stacked rectangular blocks — each step adds its own tread area times the growing stack height, so total volume ≈ width × run × rise × N(N+1)/2, where N is the step count. This is a solid-fill estimate for closed-riser steps; open-back or stringer-supported stairs use noticeably less concrete.
Converting volume to bags
Standard premix yields differ by bag size: a 40 lb bag yields about 0.30 ft³ of mixed concrete, a 60 lb bag about 0.45 ft³, and an 80 lb bag about 0.60 ft³. Divide your total volume (with waste added) by the bag yield and round up — for 33.3 ft³ plus 10% waste (36.7 ft³) that's 123 bags of 40 lb mix, 82 bags of 60 lb, or 62 bags of 80 lb.
Practical tips: always add 5–10% waste for spillage, subgrade irregularity and over-dig. For pours over roughly 1 cubic yard, ready-mix delivery is usually cheaper and far less labor than mixing dozens of bags by hand. Round column pours (sonotubes) often use more concrete than expected because tube diameters run slightly larger than nominal — measure the actual tube if precision matters.
Concrete Calculator: frequently asked questions
How do I calculate concrete for a slab?
Multiply length × width × thickness (converted to feet) to get cubic feet, then divide by 27 to get cubic yards. A 10 × 10 ft slab at 4" thick is 10 × 10 × 0.333 = 33.3 cubic feet, or about 1.24 cubic yards.
How many bags of concrete do I need?
A 60 lb bag yields about 0.45 cubic feet and an 80 lb bag about 0.60 cubic feet of mixed concrete. Divide your total cubic feet by the bag yield and round up. For 33.3 cubic feet that is 75 bags of 60 lb mix or 56 bags of 80 lb mix.
How much concrete does a sonotube need?
Volume = π × (diameter ÷ 2)² × height, in feet, multiplied by how many tubes you are pouring. A 12" diameter tube 4 ft tall holds about 3.14 cubic feet, so three of them need about 9.4 cubic feet.
Why should I add a waste percentage?
Uneven subgrade, form leakage and over-excavation typically use 5–10% more concrete than the theoretical volume, so contractors add a waste allowance before ordering ready-mix or buying bags.
How thick should a concrete slab be?
A typical shed or patio slab is 4" thick; driveways for cars are usually 5–6"; footings depend on local frost depth and load requirements, so check your building code.