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Concrete Calculator
Work out how much concrete you need for a slab, footing or round column in cubic yards, cubic meters and 40, 60 or 80 lb bags, with waste and cost.
Bag yields used: 40 lb ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³ of mixed concrete (typical values printed on premix bags - check yours).
How to use it
- Choose a rectangle (slab, patio, footing, path) or a round column.
- Enter the size. For slabs use length, width and thickness; for columns use diameter and height.
- Add 5-10% for spillage, uneven ground and over-excavation.
- Read cubic yards (for ready-mix trucks) or the number of premix bags for small jobs.
How it works
Volume (ft³) = length × width × thickness ÷ 12 for a slab, or π × (diameter ÷ 24)² × height for a round column. Divide by 27 for cubic yards. A 10 × 10 ft slab 4 in thick needs 33.3 ft³ = 1.23 yd³ before waste.
Ready-mix suppliers sell by the cubic yard (often with a minimum load); for under about 1 yd³, premix bags are usually easier. Typical patios and walkways are 4 in thick; driveways 4-6 in; check local code for footings.
FAQ
How many 80 lb bags are in a yard of concrete?
About 45 bags (27 ft³ ÷ 0.60 ft³ per bag). With 60 lb bags it is about 60, and with 40 lb bags about 90.
How thick should a concrete slab be?
4 inches for patios, sidewalks and sheds; 5-6 inches for driveways that carry heavier vehicles. Your local code or engineer has the final say.
How much extra concrete should I order?
5-10% extra is common to cover uneven subgrade, spillage and form bulging. Running short mid-pour is far more costly than a little leftover.
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