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Concrete Calculator

Calculate concrete volume in cubic yards, cubic feet, and 60-lb/80-lb pre-mixed bags for slabs, footings, and Sonotube columns with excavation waste buffer.

Instant results
Real-time calculation
Accurate & reliable
Standard SI formulas
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Length (L)Width (W)Thickness (T)
Technical Dimension Schematic100% Vector
2. Dimensions & Thickness
Quick Thickness:
Excavation Loss & Waste Buffer+10%
Project Cost Estimator (Optional)
Concrete Volume SchematicLength (L)Width (W)Thickness (T)

Cubic Yards = (L × W × Thickness) ÷ 27 • 1 yd³ = 45 × 80-lb Bags

Total Concrete Needed (+10% buffer)
2.72 yd³
Equals 73.3 ft³ or 2.08 m³
Raw Volume: 2.47 yd³
Estimated Cost: $380.25
Most Common
Standard 80-lb Bags
123 bags

0.60 cu ft yield per bag (45 bags per cubic yard).

Lighter 60-lb Bags
163 bags

0.45 cu ft yield per bag (60 bags per cubic yard).

Ready-Mix Truck Load
Less than 1 truckload (2.7 yd³ of 10 yd³ truck)

Standard transit mixer truck capacity is ~10 cubic yards.

Mixing Water & Structural Cured Weight

Mixing Water:
~92 gal (349 L)
Cured Weight (lbs):
10,633 lbs
Cured Weight (Tons):
5.3 US tons
Metric Equivalent:
2.08 m³

Mathematical Formula & Step-by-Step Derivation

SI Standard Math
General Formula:
Cubic Yards = (Length × Width × Thickness_inches ÷ 12) ÷ 27 × (1 + Waste%)
Live Calculation With Your Input Values:
11. Raw Geometric Volume
Formula: Volume = Length × Width × Thickness
Substituted: Raw Volume = 66.67 ft³ (2.47 cubic yards)
Step Result: 2.47 yd³

Direct mathematical volume before excavation loss or spillage buffer.

22. Safety & Excavation Buffer (+10%)
Formula: Total Volume = Raw Volume × (1 + 10%)
Substituted: 2.47 yd³ × 1.10
Step Result: 2.72 yd³ (73.3 ft³)

Contractor standard safety margin to prevent coming up short on site.

33. Pre-Mixed Concrete Bag Requirements
Formula: Bags (80 lb) = Volume (ft³) ÷ 0.60 ft³/bag | Bags (60 lb) = Volume (ft³) ÷ 0.45 ft³/bag
Substituted: 73.3 ft³ ÷ 0.60 = 123 bags (80 lb) | ÷ 0.45 = 163 bags (60 lb)
Step Result: 123 × 80-lb bags OR 163 × 60-lb bags

Standard Home Depot / Lowe's bagged concrete mix yield counts.

Concrete Engineering Formulas & Bag Yield Math
Slab / Patio Formula (Yards):yd³ = (L × W × Thick_in ÷ 12) ÷ 27
Column / Sonotube Formula:yd³ = (π × (D_in ÷ 24)² × Depth_ft) ÷ 27
80-lb Pre-Mix Bag Yield:Bags = Cubic Feet ÷ 0.60
60-lb Pre-Mix Bag Yield:Bags = Cubic Feet ÷ 0.45

*When measurements are in metric units, 1 cubic meter (m³) = 1.30795 cubic yards or 35.3147 cubic feet.

Worked Contractor Example: 10 × 12 ft Patio Slab (4″ Thick with 10% Waste)

Problem: Calculate cubic yards, 80-lb bag requirements, and cost for a 10 × 12 ft backyard patio slab poured at standard 4-inch depth.

1. Thickness in feet: 4 inches ÷ 12 = 0.3333 ft

2. Raw Cubic Feet: 10 ft × 12 ft × 0.3333 ft = 40.0 cu ft

3. Raw Cubic Yards: 40.0 cu ft ÷ 27 ≈ 1.48 cubic yards

4. Add 10% Waste Buffer: 1.48 yd³ × 1.10 = 1.63 cubic yards (44.0 cu ft)

5. 80-lb Bags Needed: 44.0 cu ft ÷ 0.60 cu ft/bag = 73.3 → Round up to 74 bags

6. 60-lb Bags Needed: 44.0 cu ft ÷ 0.45 cu ft/bag = 97.7 → Round up to 98 bags

7. Cured Weight: 44.0 cu ft × 145 lbs/cu ft ≈ 6,380 lbs (~3.2 US tons)

8. Material Cost at $6.50 per 80-lb bag: 74 bags × $6.50 ≈ $481.00

Common Use Cases

Bagged Concrete vs. Ready-Mix Truck Delivery: If your project requires less than 1 cubic yard (under 45 bags), buying pre-mixed bags from a home improvement store and mixing in a wheelbarrow or portable drum mixer is usually cheapest. For projects over 2 to 3 cubic yards (90+ bags), ordering a ready-mix truck delivery saves immense physical labor and prevents cold joints.

Standard Slab Thickness Guidelines:
• Sidewalks, residential walkways & garden sheds: 4 inches (10 cm)
• Driveways for standard cars & light trucks: 4 to 5 inches (10–12 cm)
• Heavy truck parking, RV pads & garage floors: 6 inches (15 cm) with rebar reinforcement.

Related Tools: For landscaping soil, topsoil and gravel, visit our Cubic Yard Calculator, for boxed freight, use our CBM Calculator, or calculate rectangular prisms with our Rectangular Prism Calculator.

Complete Guide: How to Calculate Concrete in Cubic Yards & Pre-Mixed Bags

Concrete is sold in two primary ways: by the cubic yard (yd³) via bulk ready-mix transit mixer trucks, or in individual pre-mixed bags (60 lb and 80 lb) from home improvement retailers like Home Depot and Lowe's. An accurate calculation prevents either running out mid-pour (creating cold joints) or paying costly truck return fees for excess concrete.

  1. Convert All Dimensions to Feet: If slab thickness or column diameter is measured in inches, divide by 12 to convert to feet (e.g., 4 inches = 4 ÷ 12 = 0.333 ft).
  2. Calculate Cubic Feet (ft³): Multiply Length (ft) × Width (ft) × Thickness (ft). For cylindrical columns, compute π × (Radius in ft)² × Depth (ft) × Quantity.
  3. Convert Cubic Feet to Cubic Yards (yd³): Divide your total cubic footage by 27 (since 1 cubic yard = 3 × 3 × 3 = 27 cubic feet).
  4. Add 10% Safety Buffer for Excavation & Spillage: Sub-grades are rarely 100% laser-level, wooden form boards deflect under wet hydrostatic pressure, and gravel compaction causes loss. Always add +10% overage.
  5. Determine Bag Count:
    • 80-lb bags: Divide total cubic feet by 0.60 (45 bags = 1 cu yd).
    • 60-lb bags: Divide total cubic feet by 0.45 (60 bags = 1 cu yd).

Frequently Asked Questions

How many 80-lb bags of concrete are in a cubic yard?
There are exactly 45 standard 80-lb bags of concrete in one cubic yard. Each 80-lb bag yields approximately 0.60 cubic feet of mixed concrete (27 cu ft ÷ 0.60 cu ft/bag = 45 bags).
How many 60-lb bags of concrete in a cubic yard?
There are 60 standard 60-lb bags of concrete in one cubic yard. Each 60-lb bag yields approximately 0.45 cubic feet of mixed concrete (27 cu ft ÷ 0.45 cu ft/bag = 60 bags).
How many bags of concrete do I need for a 10x10 slab?
A 10x10 ft slab poured at standard 4-inch thickness requires 1.23 cubic yards (33.3 cu ft). With a recommended 10% safety buffer for uneven ground (1.36 cu yd / 36.7 cu ft), you need 62 bags of 80-lb concrete or 82 bags of 60-lb concrete.
How do I calculate cubic yards of concrete?
Multiply Length (ft) × Width (ft) × Thickness (inches ÷ 12) to get total cubic feet, then divide by 27. For example, a 20 × 10 ft slab at 4 inches thick: 20 × 10 × (4 / 12) = 66.67 cu ft ÷ 27 = 2.47 cubic yards.
Why should I add 10% waste to my concrete calculation?
Excavated ground is rarely perfectly flat, wooden forms push outward slightly under wet concrete weight, and material is left inside wheelbarrows or pumps. A 10% buffer ensures you do not come up short before finishing.
How much does a cubic yard of concrete weigh?
Standard cured concrete weighs approximately 3,900 to 4,050 pounds per cubic yard (about 145 to 150 lbs per cubic foot, or 2,300 to 2,400 kg per cubic meter).
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