How to calculate how much concrete you need
Concrete is ordered by volume. Ready-mix trucks deliver it by the cubic yard, and premixed bags are labeled with the volume they make once mixed. So every estimate starts the same way: measure the length and width of the slab, decide on its thickness, and multiply.
Cubic feet = length (ft) × width (ft) × thickness (ft)
Cubic yards = cubic feet ÷ 27
Bags = cubic feet ÷ bag yield (0.30, 0.45 or 0.60 cu ft)
Thickness is usually given in inches, so divide it by 12 first: 4 inches is 0.333 ft. For a round slab or a column, choose Circle and enter the height of the column as the thickness. For a triangle, the area is base × height ÷ 2.
Then add a margin. Forms bow a little, the ground is never perfectly flat, and some concrete is always lost in the mixer or the wheelbarrow. Adding 5% to 10% is standard practice; this calculator uses 10% by default. Running short in the middle of a pour leaves a cold joint, which is much worse than a few spare bags.
Worked example: a 12 × 12 ft patio slab
Say you are pouring a 12 ft by 12 ft patio, 4 inches thick.
- Area: 12 × 12 = 144 sq ft
- Thickness: 4 ÷ 12 = 0.333 ft
- Volume: 144 × 0.333 = 48 cubic feet
- Cubic yards: 48 ÷ 27 = 1.778 cubic yards
- With 10% extra: 52.8 cubic feet, or 1.956 cubic yards
- In 80 lb bags (0.60 cu ft each): 52.8 ÷ 0.60 = 88 bags
Without the margin you would need exactly 80 bags of 80 lb. At about 150 lb per cubic foot, the poured slab weighs around 3.6 tons, which is why jobs this size are usually easier with a ready-mix delivery or a rented mixer. The gravel base under the slab is a separate material: work it out with the gravel calculator.
How many bags of concrete are in a cubic yard?
Common premixed bags in the US are labeled with their yield: a 40 lb bag makes about 0.30 cubic feet, a 60 lb bag about 0.45 cubic feet and an 80 lb bag about 0.60 cubic feet. A cubic yard is 27 cubic feet, so:
| Bag size | Yield per bag | Bags per cubic yard | Bags per 10 × 10 ft slab, 4 in |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 112 |
| 60 lb | 0.45 cu ft | 60 | 75 |
| 80 lb | 0.60 cu ft | 45 | 56 |
Yields can differ slightly between brands and mixes, so check the label on the bag you buy. For anything over about one cubic yard, compare the price of bags with a ready-mix delivery or a short load; many suppliers have a minimum order.
How thick should a concrete slab be?
As a general guide, walkways and patios are commonly poured 4 inches thick, while driveways and slabs that carry vehicles are often 5 to 6 inches. Footings and post holes depend on the load and on your local frost depth. Always follow your local building code and the instructions for your project. The table shows how much concrete each thickness takes.
| Thickness | Cubic feet | Cubic yards | Sq ft covered by 1 cubic yard |
|---|---|---|---|
| 4 in | 33.3 | 1.235 | 81 |
| 5 in | 41.7 | 1.543 | 64.8 |
| 6 in | 50 | 1.852 | 54 |
| 8 in | 66.7 | 2.469 | 40.5 |
Laying a patio with pavers instead of poured concrete? Use the paver calculator. To measure an odd-shaped area first, try the square footage calculator, and for the raw volume of any material there is the cubic yard calculator.
Frequently asked questions
How much concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 × 10 ft slab takes 33.3 cubic feet, or 1.23 cubic yards. That is about 56 bags of 80 lb, 75 bags of 60 lb or 112 bags of 40 lb, before adding 5% to 10% extra.
How many 80 lb bags of concrete make a yard?
About 45. An 80 lb bag yields roughly 0.60 cubic feet and a cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45 bags.
How much concrete for a post hole?
Choose Circle, enter the hole diameter and use the depth as the thickness. A 12 inch wide hole that is 4 ft deep holds about 3.14 cubic feet, or six 80 lb bags once rounded up. Subtract a little for the post itself if you like.
How much does a cubic yard of concrete weigh?
Normal concrete weighs about 150 lb per cubic foot, so a cubic yard weighs roughly 4,050 lb, or just over 2 short tons. The calculator uses this value for its weight estimate.
Should I order extra concrete?
Yes. Most contractors add 5% to 10% to cover uneven ground, form movement and spillage. Running out in the middle of a pour creates a weak joint, while a few spare bags can be returned or kept for repairs.
More free calculators
These calculators give estimates for planning only. Densities, bag sizes and coverage vary by supplier, so confirm quantities before you order.