🏗️Concrete Calculator
Calculate how much concrete you need for slabs, footings, columns, and circular pads in cubic yards, cubic feet, and 40/60/80 lb bags.
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Cubic Yards
1
You need 1.36 cubic yards (36.7 cu ft with 10% waste). That's 62 × 80lb bags.
Concrete Volume Summary
1.36
36.67
62
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Concrete Calculator: How Much Concrete Do I Need for a Slab, Footing, or Column
This concrete calculator answers the question every DIYer and contractor asks before a pour: how much concrete do I need? Enter your dimensions, pick your shape, and the calculator returns the volume in cubic yards, cubic feet, and cubic meters, then converts that to the number of bags required based on your selected bag size. Getting the quantity right before you buy saves a second trip to the hardware store or a costly second ready-mix delivery.
Concrete Calculator for a Slab in Cubic Yards
The standard volume unit for ordering poured concrete is the cubic yard. One cubic yard equals 27 cubic feet. For a rectangular slab, the formula is straightforward: multiply length by width by thickness, all in feet, to get cubic feet, then divide by 27 to convert to cubic yards.
For example, a 10-foot by 10-foot slab that is 4 inches thick requires the thickness to be converted to feet first: 4 inches divided by 12 equals 0.333 feet. The volume is 10 x 10 x 0.333 = 33.3 cubic feet, or about 1.23 cubic yards. A 20-foot by 20-foot garage floor at 4 inches thick works out to 4.9 cubic yards. A typical residential driveway of 600 square feet at 4 inches thick requires about 7.4 cubic yards.
Always add a waste factor before placing an order. Subgrade irregularities, over-excavation, and spillage during the pour mean the actual volume consumed almost always exceeds the theoretical calculation. The default 10 percent waste factor in this calculator is a solid baseline for most rectangular slabs. Use 15 percent for complex shapes, circular pads, or surfaces with uneven subgrades.
Standard slab thickness guidelines for common residential applications:
- Sidewalks and garden paths: 4 inches
- Residential driveways: 4 to 5 inches
- Garage floors: 4 inches minimum, 5 to 6 inches for heavy vehicles
- Patio slabs: 3.5 to 4 inches
- Foundation walls: 8 to 12 inches
- Footings: 12 inches deep minimum, and below the frost line in cold climates
How Many Bags of Concrete Do I Need Calculator
For small projects under roughly 1 cubic yard (27 cubic feet), bagged concrete mix such as Quikrete or Sakrete is the most practical option. Each bag size yields a fixed volume of mixed concrete:
- 40 lb bag: approximately 0.30 cubic feet
- 60 lb bag: approximately 0.45 cubic feet
- 80 lb bag: approximately 0.60 cubic feet
To find the number of bags needed, divide the total cubic feet (after adding your waste factor) by the coverage per bag, then round up to the next whole number. A 10-foot by 10-foot slab at 4 inches thick with a 10 percent waste factor requires about 36.6 cubic feet. Divided by 0.60 cubic feet per 80 lb bag, that is 61 bags. At roughly $6.50 per bag, the material cost is about $397 before tax.
For projects over 1 to 2 cubic yards, ordering ready-mix concrete from a transit mixer is more economical and far less labor intensive than mixing dozens of bags. Ready-mix is typically priced around $125 to $180 per cubic yard delivered, with a minimum order of 1 to 3 cubic yards depending on the supplier. At that scale, the per-cubic-foot cost is significantly lower than bagged concrete, and you avoid the physical effort of mixing batch after batch by hand or in a rented drum mixer.
If you go the bagged route, 80 lb bags offer the best value per cubic foot, though they are heavy. A 60 lb bag is more manageable for solo workers or anyone with physical limitations. The 40 lb option is the easiest to handle but requires the most bags and the most mixing time for the same volume.
Concrete Volume Calculator for Footings and Slabs
Footings and columns use different volume formulas than rectangular slabs. A cylindrical column or round footing uses the formula for a cylinder: volume equals pi multiplied by the radius squared multiplied by the height. For a 12-inch diameter column that is 3 feet deep, the radius is 0.5 feet, so the volume is 3.14159 x 0.25 x 3 = approximately 2.36 cubic feet. This calculator handles the geometry automatically once you select the cylindrical or circular shape and enter your dimensions.
Footings must extend below the frost line to prevent heaving from freeze-thaw cycles. Frost line depth varies by location, ranging from zero in southern states to over 4 feet in parts of Minnesota and the northern plains. Your local building department can confirm the required depth for your area. Building codes also typically specify minimum footing dimensions and concrete strength requirements for structural elements.
For reinforced concrete, use a mix rated at 3,000 PSI or higher. Bagged mixes list their compressive strength on the packaging. Standard residential slab concrete is typically 2,500 to 3,000 PSI. Heavy-load applications such as commercial driveways, machinery pads, or structural foundations call for 4,000 PSI or more. Consult your local building code or a structural engineer for specific requirements if your project is load-bearing or part of a permitted structure.
Frequently Asked Questions
How do I calculate how much concrete I need for a slab?
Multiply the length by the width by the thickness, all measured in feet (convert inches to feet by dividing by 12). That gives you cubic feet. Divide by 27 to get cubic yards. Add at least 10 percent for waste and subgrade irregularities. For example, a 12 x 14 foot slab at 4 inches thick is 12 x 14 x 0.333 = 55.9 cubic feet, or 2.07 cubic yards. With 10 percent waste, order 2.28 cubic yards.
How many 80 lb bags of concrete do I need per cubic yard?
Each 80 lb bag of concrete mix yields approximately 0.60 cubic feet of mixed concrete. One cubic yard equals 27 cubic feet. Divide 27 by 0.60 to get 45 bags per cubic yard. In practice, always round up and add a bag or two as a buffer for the waste factor. For a project requiring 1.5 cubic yards, you would need about 68 bags of 80 lb mix.
What is the difference between concrete and cement?
Cement is one ingredient in concrete. Concrete is a mixture of cement, sand (fine aggregate), gravel or crushed stone (coarse aggregate), and water. The cement acts as the binder that holds everything together through a chemical reaction called hydration. When you buy a bag of "concrete mix" like Quikrete 5000, it already contains cement, sand, and gravel pre-blended. You only add water. Cement alone, without aggregate, is used for grout, mortar, and specialty applications, not for structural pours.
How thick should a concrete slab be?
Thickness depends on the intended use. Sidewalks and patios typically require 4 inches. Driveways for standard passenger vehicles need 4 to 5 inches, while driveways for heavy trucks or RVs should be 5 to 6 inches. Garage floors are usually 4 inches for cars and 5 to 6 inches if heavy vehicles or equipment will be stored. Foundation slabs and footings follow building code requirements, which vary by location and structural load. Always check local codes for permitted structures.