A concrete calculator works out how much concrete a slab needs, in cubic yards, from its length, width and thickness. A 10 × 10 ft slab 4 inches thick needs 1.235 cubic yards exactly, or about 1.36 cubic yards and 62 bags of 80 lb mix with a 10% waste allowance.
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How to use the concrete calculator
- Enter the length and width in feet.
- Enter the thickness in inches.
- Set the waste allowance (10% by default) and choose a bag size.
- Read the cubic yards and the number of bags.
What formula does the concrete calculator use?
- Volume (cubic feet) = length (ft) × width (ft) × thickness (in) ÷ 12
- Cubic yards = cubic feet ÷ 27
- 80 lb bags = cubic feet ÷ 0.60; 60 lb bags = cubic feet ÷ 0.45 (round up)
The concrete calculator returns the volume of a rectangular slab in cubic yards, the unit ready-mix suppliers use, adds the waste allowance you choose (10% by default) and counts the premix bags needed for 40, 50, 60, 80 or 90 lb bags. Thickness is entered in inches and converted to feet.
Concrete calculator worked example
A 10 × 10 ft slab 4 inches thick holds 10 × 10 × 4 ÷ 12 = 33.33 cubic feet, which is 33.33 ÷ 27 = 1.235 cubic yards. QUIKRETE’s data sheet for its Concrete Mix lists a yield of about 0.60 cubic feet per 80 lb bag, so the slab needs 33.33 ÷ 0.60 = 55.6, or 56 bags, before any allowance for waste.
| Slab | Thickness | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 10 × 10 ft | 4 in | 1.23 | 56 | 75 |
| 12 × 12 ft | 4 in | 1.78 | 80 | 107 |
| 20 × 20 ft | 4 in | 4.94 | 223 | 297 |
| 10 × 10 ft | 6 in | 1.85 | 84 | 112 |
| 24 × 24 ft | 6 in | 10.67 | 480 | 640 |
| 3 × 40 ft | 4 in | 1.48 | 67 | 89 |
Add a waste allowance
The concrete calculator shows both the exact geometric volume and the total with your waste allowance. Real slabs use more because the ground is uneven, forms bow outward and some concrete is spilled or left in the mixer. Many contractors order 5% to 10% extra; with the default 10% the 10 × 10 ft example becomes 1.358 cubic yards, or 62 bags of 80 lb mix. Set the allowance to 0 to see the exact volume alone. Running short in the middle of a pour leaves a weak joint, so round up.
Bags or ready-mix?
- Bags: practical for small jobs such as post footings, pads and steps. About 45 bags of 80 lb make one cubic yard (27 ÷ 0.60), which is a lot of mixing.
- Ready-mix truck: usually better above roughly one cubic yard. Many suppliers have a minimum order or charge a short-load fee for small amounts, so ask before ordering.
- Thickness: 4 inches is common for patios and walkways; driveways and slabs that carry heavier loads are usually thicker. Follow local building codes and your designer’s specification.
Other shapes
For a round slab or footing, use π × radius² in square feet instead of length × width, then multiply by the depth in feet. A round pad 8 ft across and 4 inches thick is π × 4² × 0.333 ≈ 16.76 cubic feet, about 0.62 cubic yards. For L-shaped areas, split the area into rectangles, run the concrete calculator for each and add the results. For a column, a 12-inch-diameter tube 4 ft deep holds π × 0.5² × 4 ≈ 3.14 cubic feet, about six 80 lb bags.
Metric units
In metric, multiply length × width × thickness in meters to get cubic meters directly. One cubic yard is about 0.765 cubic meters, and a 10 cm slab of 3 × 3 m holds 0.9 m³.
Before the pour
Prepare the base before you buy concrete. Remove grass, roots and soft soil, then compact a layer of gravel so the slab does not settle unevenly. Set forms level, or with a slight slope away from buildings so water drains, and measure the depth inside the forms at several points. If the base is low in places, the slab will be thicker there and will use more concrete than the concrete calculator predicts, so fix low spots first or increase your waste allowance.
Check the weather and plan for help. Fresh concrete should not freeze, and hot, windy days dry the surface too quickly. One person can mix a few bags, but a slab needing 50 or more bags is a big job; a mixer and several helpers, or a ready-mix truck, keep the pour continuous. Once it is placed, keep the surface damp for several days so it cures and gains strength rather than drying out.
Related converters and calculators
Related calculators:
- Gravel calculator: gravel
- Mulch calculator: mulch
- Square footage calculator: area
- Volume calculator: volume
Outdoor projects: see the paver calculator, the retaining wall calculator and the fence cost calculator.
Sources
QUIKRETE Concrete Mix No. 1101 product data sheet.
Questions about this calculator
How much concrete for a 10 × 10 slab 4 inches thick?
About 1.235 cubic yards, or 56 bags of 80 lb.
How many 80 lb bags make a yard?
About 45, at 0.60 cubic feet per bag.
How much extra concrete should I order?
Typically 5% to 10%.
How thick should a patio be?
4 inches is common; follow local codes.
Does the tool add waste?
Yes, 10% by default; you can change it or set it to 0.
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
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