BuildCalc Pro

Concrete Calculator

Calculate the volume of concrete needed for slabs, footings, and columns. Get results in cubic yards, cubic meters, or number of bags.

Written by the BuildCalc Pro Editorial TeamUpdated June 2026✓ Fact-checked

Everything You Need to Know About Concrete Estimation

Concrete is one of the most essential building materials in construction. Whether you are pouring a simple patio slab, building foundation footings, or setting posts, accurately estimating the amount of concrete you need is critical to avoid waste and unexpected costs.

Concrete is typically sold in cubic yards in the United States, and one cubic yard equals 27 cubic feet. For smaller projects, pre-mixed bags of 60 or 80 pounds are common. A standard 80-pound bag yields approximately 0.6 cubic feet of mixed concrete, while a 60-pound bag yields around 0.45 cubic feet.

When calculating concrete for a slab, multiply the length by width by depth (all in feet), then divide by 27 to convert to cubic yards. Always add 5-10% extra to account for uneven ground, spillage, and form irregularities. For round columns, use the formula: π × radius² × height.

Ready-mix concrete costs $120-$160 per cubic yard on average, while bagged concrete runs about $4-$6 per 80-pound bag. For projects larger than 1 cubic yard, ordering ready-mix from a truck is usually more economical and produces a more consistent result.

Proper preparation is essential for a successful concrete pour. Make sure your forms are level and well-braced, the sub-grade is compacted, and you have enough labor on hand since concrete sets quickly — typically within 30-60 minutes depending on weather conditions.

How to Use This Calculator

1. Select your measurement unit (Imperial or Metric). 2. Choose the shape type: Slab/Rectangle, Column/Cylinder, or Footing. 3. Enter the dimensions in the fields provided. 4. Set the quantity if you have multiple identical pieces. 5. Optionally enter a cost per unit for price estimation. 6. Click "Calculate" to see results. Tip: For slabs, enter length and width in feet and depth/thickness in inches. Always order 5-10% extra concrete to account for waste and spillage.

Key Formulas

Slab Volume

Length (ft) × Width (ft) × Depth (ft) ÷ 27 = Cubic Yards

Multiply all three dimensions in feet, then divide by 27 to convert cubic feet to cubic yards.

Round Column

π × Radius² (ft) × Height (ft) ÷ 27 = Cubic Yards

Use π (3.14159) times the radius squared times the height. Divide by 27 for cubic yards.

80-lb Bags Needed

Cubic Feet ÷ 0.6 = Number of 80-lb Bags

Each 80-lb bag yields approximately 0.6 cubic feet of mixed concrete.

60-lb Bags Needed

Cubic Feet ÷ 0.45 = Number of 60-lb Bags

Each 60-lb bag yields approximately 0.45 cubic feet of mixed concrete.

Concrete Bag Sizes & Coverage

Concrete bag sizes, yields, and best uses
Bag SizeYield (cu ft)Bags per CYCoverage at 4" ThickBest For
40 lb0.30900.90 sq ftSmall repairs, post holes
60 lb0.45601.35 sq ftMedium projects, easier to carry
80 lb0.60451.80 sq ftLarge projects, most cost-effective

Slab Thickness by Application

Recommended concrete slab thickness for common uses
ApplicationMin ThicknessRecommendedReinforcement
Walkway / Patio3 inches4 inchesWire mesh or fiber
Residential Slab4 inches4–6 inchesWire mesh, rebar for larger spans
Residential Driveway4 inches6 inches#4 rebar grid, 18" on center
Commercial Driveway6 inches8 inches#5 rebar grid, 12" on center
Heavy Equipment8 inches10–12 inchesEngineered reinforcement

Concrete Mix Types & Uses

Common concrete mix types, strength ratings, and applications
Mix TypePSI RatingBest ApplicationsSet Time
Fast-Setting3,000–4,000Quick repairs, post setting20–40 min
Standard Mix3,000–3,500Slabs, driveways, general24–48 hrs
High-Strength4,000–5,000Foundations, heavy loads48–72 hrs
Fiber-Reinforced3,000–4,000Crack-resistant slabs24–48 hrs

Frequently Asked Questions

How many bags of concrete do I need for a 10x10 slab?

For a 10x10 foot slab that is 4 inches deep, you need approximately 1.23 cubic yards of concrete. That equals roughly 56 bags of 80-pound concrete mix or 74 bags of 60-pound mix.

How much does a cubic yard of concrete cost?

Ready-mix concrete typically costs $120-$160 per cubic yard delivered, though prices vary by region. Pre-mixed bags cost about $4-$6 each for an 80-pound bag, which works out to roughly $250-$340 per cubic yard.

Should I add extra concrete to my order?

Yes, always order 5-10% more concrete than calculated. This accounts for spillage, uneven ground, and slight variations in form dimensions. Running short mid-pour is far more costly than having a small surplus.

What is the difference between concrete and cement?

Cement is one ingredient in concrete. Concrete is a mixture of cement (usually Portland cement), sand, gravel or crushed stone, and water. Cement acts as the binding agent that holds the aggregate together.