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Your project
Bags are rounded up, because concrete mix is sold in whole bags. The 10% waste allowance covers spillage and uneven ground.
Example: a 10 × 10 ft slab, 4 in thick → 33.33 cu ft (1.23 cu yd); with 10% waste, order 1.36 cu yd, or 62 bags of 80 lb mix.
Behind the numbers
How concrete volume is calculated
Multiply the length by the width and by the thickness in feet to get cubic feet, then divide by 27 to get cubic yards, because one cubic yard is 27 cubic feet. Add the waste allowance, then divide by the yield of one bag to get the number of bags, rounding up.
Cubic yards = cubic feet ÷ 27. Bags needed = ceil(volume with waste ÷ yield per bag). Worked example: 10 × 10 × (4 ÷ 12) = 33.33 cu ft; 33.33 ÷ 27 = 1.23 cu yd; with 10% waste, 36.67 cu ft ÷ 0.60 cu ft per 80 lb bag = 62 bags.
Yield of one bag of concrete mix
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb bag | 0.60 cu ft | 45 |
| 60 lb bag | 0.45 cu ft | 60 |
| 40 lb bag | 0.30 cu ft | 90 |
A few comparisons
Example concrete pours
| Project | Volume | Cubic yards | 80 lb bags (with 10% waste) |
|---|---|---|---|
| 10 × 10 ft slab, 4 in thick | 33.33 cu ft | 1.23 | 62 |
| 20 × 12 ft slab, 4 in thick | 80.00 cu ft | 2.96 | 147 |
| 30 ft footing, 12 in wide, 12 in deep | 30.00 cu ft | 1.11 | 55 |
| 20 ft wall, 8 ft high, 8 in thick | 106.67 cu ft | 3.95 | 196 |
Pour it right the first time.
Measure twice and enter the thickness in inches, not feet — a 4-inch slab entered as 4 feet would overstate the volume twelvefold.
Keep the 10% waste allowance on for slabs and footings, and raise it to 15% on uneven ground or around tree roots and rocks.
For pours over about 2 cubic yards, ready-mix concrete delivered by truck is usually cheaper than buying dozens of bags.
Order a little extra rather than a little less — a short load means a cold joint in the middle of your slab, which is far harder to fix than returning one spare bag.
Good to know
Your concrete questions, answered.
How many 80 lb bags of concrete are in a cubic yard?
One cubic yard is 27 cubic feet, and an 80 lb bag yields about 0.60 cubic feet, so a cubic yard takes 45 bags. For the example on this page, 1.23 cubic yards needs 56 bags before waste, or 62 bags with the 10% waste allowance.
How much concrete do I need for a 10 by 10 ft slab, 4 inches thick?
A 10 by 10 ft slab at 4 inches thick is 33.33 cubic feet, or 1.23 cubic yards. With a 10% waste allowance, order 1.36 cubic yards, which is 62 bags of 80 lb concrete mix, 82 bags of 60 lb mix, or 123 bags of 40 lb mix.
What is the 10% waste allowance for?
It covers spillage, uneven subgrade, and over-excavation, which are common on real pours. Use 10% for slabs and footings, and 15% for uneven ground or complex forms. The checkbox on this page adds the allowance to the volume before working out the bags.
How thick should a concrete slab be?
A 4-inch slab is standard for patios, sidewalks and light residential floors. Use 5 to 6 inches for driveways and garage floors that carry vehicles, and check local building codes before pouring a structural slab or footing.