How it is calculated
Volume is length × width × thickness for a slab, or total length × width × depth for a footing, plus extra for spillage and an uneven base. An 80 lb bag of concrete mix yields about 0.6 cu ft, so a cubic yard takes about 45 bags. To mix your own, dry materials are about 1.54 times the finished volume because they compact when water is added.
Cubic yards = length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27 × (1 + extra)Bags of 80 lb ≈ cubic feet ÷ 0.6Dry volume = volume × 1.54, split by cement : sand : gravelExample
A 10 × 10 ft slab 4 in thick needs 1.3 cubic yards with 5 % extra: 58 bags of 80 lb concrete mix. Mixed 1:2:4 yourself, that is about 690 lb of Portland cement (8 bags of 94 lb), 0.57 yd³ of sand and 1.14 yd³ of gravel.
Tips from practice
- Above about 1 cubic yard, ready-mix from a truck is usually cheaper and stronger than bags.
- Keep fresh concrete moist for the first week so it does not crack.
- Mix ratios suit small jobs. Structural concrete follows your plans and local code.
Questions and answers
How many 80 lb bags in a yard of concrete?
About 45 bags. A 60 lb bag yields 0.45 cu ft, so about 60 of them.
How thick should a patio slab be?
Patios and walkways are usually 4 in. Driveways and garage floors are 5–6 in with reinforcement.
How many cubic yards for a 10 × 10 slab?
At 4 in thick, 1.23 cubic yards before extra.
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