Concrete Calculator — Cubic Yards & Bag Count
Ordering concrete is the classic two-way street: order too little and you're pouring a cold joint; order too much and you're paying to haul it away. This calculator gets the number right before you call the ready-mix plant or load the wheelbarrow.
Enter the slab dimensions — length, width, depth — and get cubic yards for the truck order, plus bag counts if you're mixing by hand.
Concrete needed
1.23 yds
60 lb bags
75
80 lb bags
56
Order 10% extra to cover waste and uneven forms. For a 4" slab: length × width × 0.33 ÷ 27 = cubic yards.
The formula
Cubic yards = (Length ft × Width ft × Depth in ÷ 12) ÷ 27. Order 10% extra to cover waste, uneven forms, and spillage. A 60-lb bag yields ~0.45 cu ft; an 80-lb bag ~0.6 cu ft.
Pro tips
- →Always add 10% — a short pour mid-slab is a disaster; a half-yard extra is cheap.
- →For a 4-inch slab: length × width × 0.33 gives cubic feet, then ÷ 27 for yards.
- →Slabs thicker than 4" (driveways, footings) change the math fast — recalculate, don't guess.
Frequently Asked Questions
How many cubic yards for a 10×10 slab at 4 inches?+
(10 × 10 × 0.33) ÷ 27 = 1.23 cubic yards — order about 1.35 with the 10% waste rule.
How many 80-lb bags do I need for a yard of concrete?+
One cubic yard is 27 cu ft. An 80-lb bag makes 0.6 cu ft, so you need 45 bags per yard.
What does a yard of concrete cover?+
One cubic yard covers 81 sq ft at 4 inches thick, or 65 sq ft at 5 inches.
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