Concrete Calculator

Calculate exactly how much concrete you need for slabs, footings, columns, and walls. Get results in cubic yards, cubic meters, number of bags, and estimated cost — with built-in waste factor.

Project Specifications

Total Volume

2.72 yd³

(incl. 10% waste)

Cubic Feet

73.33 ft³

Cubic Meters

2.08 m³

Estimated Cost

$380

Pre-Mixed Bag Estimates

123

80 lb bags

(0.6 ft³ per bag)

163

60 lb bags

(0.45 ft³ per bag)

245

40 lb bags

(0.3 ft³ per bag)

Net volume: 66.67 cu ft|Waste allowance: +6.67 cu ft (10%)|Estimated weight: 11,000 lbs (4,989 kg)

What Is a Concrete Calculator?

A concrete calculator estimates the volume of concrete required for a construction project based on the area's dimensions. It converts measurements into cubic yards (the standard ordering unit for ready-mix concrete in the United States) and provides equivalent values in cubic feet, cubic meters, and number of pre-mixed bags. Accurate concrete estimation is critical — ordering too little disrupts the pour and creates weak cold joints, while excessive overordering wastes money and creates disposal challenges for excess concrete.

How Is Concrete Volume Calculated?

Rectangular Slabs & Footings

For rectangular areas (driveways, patios, sidewalks, slabs, and trench footings), the formula is:

Volume (cu ft) = Length (ft) × Width (ft) × Depth (ft)

To convert to cubic yards: Volume (cu yd) = Volume (cu ft) ÷ 27

Worked Example — Driveway Slab

A 20-foot long, 10-foot wide driveway with 4 inches of concrete:

Volume = 20 × 10 × (4/12) = 20 × 10 × 0.333 = 66.67 cu ft = 66.67 ÷ 27 = 2.47 cubic yards

Adding 10% waste factor: 2.47 × 1.10 = 2.72 cubic yards. Order 3 yards from your ready-mix supplier (minimum order is typically 1 yard).

Round Columns (Sonotubes)

For cylindrical forms like deck post footings and sonotubes:

Volume = π × r² × h

Where r is the radius (half the diameter) and h is the height, all in the same unit.

Understanding Concrete Bags vs. Ready-Mix

OptionBest ForCost per YardNotes
80-lb bags< 0.5 cubic yards$200–28045 bags per yard; labor-intensive mixing
60-lb bags< 0.25 cubic yards$220–300Easier to handle; more bags needed
Ready-mix truck> 1 cubic yard$120–160Most economical for large pours
Short-load truck0.5–2 cubic yards$150–200Small truck; short-load fee may apply

Recommended Slab Thickness by Application

ApplicationMin. ThicknessRecommendedPSI Rating
Walkways & paths3.5 inches4 inches3,000 PSI
Patios3.5 inches4 inches3,000–3,500 PSI
Driveways (cars)4 inches5 inches3,500–4,000 PSI
Driveways (trucks)5 inches6 inches4,000+ PSI
Garage floors4 inches5–6 inches3,500–4,000 PSI
Footings8 inches12 inches3,000–4,500 PSI

Tips for Accurate Concrete Estimation

  • Measure twice, pour once: Verify all dimensions before placing your order. Digging inaccuracies are the #1 cause of shortfalls.
  • Account for subgrade variation: Soil compaction and grading are never perfect. A slab designed for 4 inches of concrete may actually need 4.5–5 inches in some spots.
  • Use the waste factor: 10% is standard for simple pours; use 15% for complex forms, slopes, or first-time pours.
  • Check local codes: Minimum thickness, rebar requirements, and soil prep vary by jurisdiction.
  • Time your pour: Concrete begins setting within 60–90 minutes. Ensure adequate labor and equipment are ready before the truck arrives.

Disclaimer: This calculator provides estimates for planning purposes. Actual requirements may differ based on site conditions, subgrade preparation, and local building codes. Always consult a licensed contractor for structural concrete projects.

Sources and References

  • American Concrete Institute. (2020). ACI 332R-20 — Guide to Residential Concrete Construction.
  • Portland Cement Association. (2021). Design and Control of Concrete Mixtures, 16th edition. PCA EB001.
  • National Ready Mixed Concrete Association. (2023). Concrete in Practice — What, Why and How? CIP Series. nrmca.org.

Frequently Asked Questions

How do I calculate how much concrete I need?
Measure the length, width, and depth (thickness) of the area to be filled. Multiply all three dimensions together to get the volume. If using feet and inches, convert the depth to feet first (divide inches by 12). The result is cubic feet. Divide cubic feet by 27 to get cubic yards, which is the standard unit for ordering ready-mix concrete. Our calculator automates all of this, including unit conversions, waste factor, and bag count estimation.
How many bags of concrete do I need?
A standard 80-lb (36 kg) bag of concrete yields approximately 0.6 cubic feet (0.017 cubic meters) of mixed concrete. A 60-lb bag yields about 0.45 cubic feet, and a 40-lb bag yields about 0.3 cubic feet. To find the number of bags needed, divide your total cubic feet by the yield per bag. For example, a 4×8-foot slab that is 4 inches thick requires 10.67 cubic feet ÷ 0.6 = approximately 18 bags (80-lb). Our calculator includes this automatically.
How much does concrete cost?
Ready-mix concrete typically costs $120–$160 per cubic yard for standard mixes, delivered. Prices vary by region, distance from the batch plant, order size, and concrete specification (higher PSI ratings cost more). Bagged concrete (80-lb bags from hardware stores) costs roughly $5–$7 per bag, which translates to $200–$280 per cubic yard — significantly more expensive for large projects. For orders over 3–4 cubic yards, ready-mix delivery is almost always more economical.
Should I order extra concrete for waste?
Yes. Industry standard practice is to order 5–10% more concrete than your calculated volume. This accounts for spillage, uneven subgrade (the ground beneath the concrete), over-excavation, and the concrete that remains in the mixer chute and forms. A 10% overage is standard for most residential projects. For complex forms or sloped subgrade, consider 15%. Running short is far more costly than a small overage — a partial pour that sets before additional concrete arrives creates a cold joint, which is a structural weakness.
What is the difference between cubic yards and cubic feet?
One cubic yard equals 27 cubic feet (3 feet × 3 feet × 3 feet). Cubic yards are the standard ordering unit for ready-mix concrete in the United States and Canada. Cubic feet are useful for smaller calculations and bag estimates. Cubic meters are the standard in most other countries. Our calculator provides results in all three units automatically.
What thickness should I use for a concrete slab?
Standard residential slab thickness depends on the application: driveways require 4–6 inches (6 inches for heavy vehicles), patios and walkways use 4 inches, garage floors need 4–6 inches with thickened edges, and footings typically require 8–12 inches depending on soil conditions and load requirements. Building codes vary by jurisdiction — always verify local requirements before pouring. The American Concrete Institute (ACI) provides detailed specifications in ACI 332R-20 for residential construction.

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