Concrete Calculator: Cubic Yards and Bags
Multiply length × width × thickness, with all three in feet, then divide by 27 for cubic yards. A 10 × 10 ft slab 4 inches thick is 33.3 cubic feet; with 10% extra that's 1.36 cubic yards, or 62 bags of 80 lb premix.
Concrete needed
1.36 cu yd
In cubic feet
36.67 cu ft
Or in premixed bags
40 lb bags
123
60 lb bags
82
80 lb bags
62
Over about 1 cubic yard, ordering ready-mix concrete by the truck is usually cheaper and easier than mixing bags.
Show the math
- 1
Convert the thickness to feet
4 in ÷ 12 = 0.3333 ft
All three measurements need the same unit before multiplying.
= 0.3333 ft
Read the steps as text
- Convert the thickness to feet. 4 in ÷ 12 = 0.3333 ft All three measurements need the same unit before multiplying.
- Volume = length × width × thickness. 10 × 10 × 0.3333 = 33.333 cu ft
- Add 10% for waste and spillage. 33.333 × 1.1 = 36.667 cu ft Uneven ground, spills and form bulges mean you always need a bit more than the exact volume.
- Convert to cubic yards. 36.667 cu ft ÷ 27 = 1.358 cu yd A cubic yard is 3 × 3 × 3 = 27 cubic feet. Ready-mix trucks sell by the cubic yard.
- Bags of premixed concrete. 36.667 ÷ 0.6 cu ft per 80 lb bag = 61.11 → 62 bags Always round up: a part bag still has to be bought. Check the yield printed on your bag, as mixes vary.
How the volume is calculated
Concrete is sold by volume, so the first job is to find the volume of the space you're filling. For a slab, footing or wall, that's length × width × thickness. Thickness is usually measured in inches, so it's divided by 12 to convert to feet before multiplying. For a round column or post hole, the volume is π × radius² × height.
Ready-mix concrete is ordered in cubic yards. A cubic yard is a cube 3 feet on each side, so it holds 3 × 3 × 3 = 27 cubic feet; dividing cubic feet by 27 gives cubic yards. In metric, concrete is ordered in cubic meters directly.
Waste allowance and bags
Always order a little more than the exact volume. Ground that isn't perfectly level, forms that bow outward and spillage all use extra concrete, and running short mid-pour is far worse than having some left over. 5 to 10% extra is typical; use more for uneven ground.
Premixed bags list their yield on the bag. The standard figures are about 0.30 cubic feet for a 40 lb bag, 0.45 for 60 lb and 0.60 for 80 lb. The bag count is always rounded up, because you can't buy part of a bag. For more than about a cubic yard, ordering ready-mix from a truck is usually cheaper and far less work.
Frequently asked questions
- How many bags of concrete do I need for a 10×10 slab?
- For a 4-inch slab, about 56 bags of 80 lb concrete for the exact 33.3 cubic feet, or 62 bags with 10% extra for waste. With 60 lb bags it's 82, including the 10%.
- How thick should a concrete slab be?
- 4 inches is standard for patios, walkways and shed floors. Driveways and garage floors for cars are usually 5 to 6 inches. Follow local building codes and an engineer's advice for structural work.
- How many cubic feet are in a cubic yard?
- 27. A cubic yard is 3 feet × 3 feet × 3 feet.
- Should I use bags or order ready-mix?
- Bags make sense for small jobs like post holes and small pads. Above about 1 cubic yard (roughly 45 bags of 80 lb), ready-mix delivery is usually cheaper and much easier.