Concrete Slab & Yardage Calculator: Cubic Yards & Bags

Concrete Slab & Yardage Calculator

Calculate exact ready-mix concrete yardage (cubic yards, cubic feet, and cubic meters), 80 lb / 60 lb / 40 lb pre-mixed bag counts, slab weight, and estimated material cost. Includes rectangular and circular slab geometries, customizable waste allowance, and a live 3D isometric slab diagram.

Slab Dimensions & Specs

Common Slab Presets:

Length (L)

ft in yd m

Width (W)

ft in yd m

Outer Diameter (D)

ft in yd m

Slab Thickness / Depth (T)

in ft cm

Number of Slabs (Qty)

slabs

Waste, Spillage & Subgrade Allowance 0% — Exact Net Theoretical Volume Only 5% — Smooth, Level Compacted Subbase 10% — Standard Contractor Buffer (Recommended) 15% — Uneven Subgrade or Deep Trenching

Ready-Mix Price ($/yd³)

$/yd³

80 lb Premix Price ($/bag)

$/bag

Concrete Order Summary

Total Concrete to Order (Includes 10% Waste)

3.26 Cubic Yards

Exact Net Volume: 2.96 yd³ (80.00 ft³) + 0.30 yd³ Waste

3D Slab Profile: 12 ft × 20 ft × 4 in Area: 240.00 sq ft

Cubic Feet

88.00 ft³

Cubic Meters

2.49 m³

Total Weight

13,200 lbs (6.60 t)

Pre-Mixed Concrete Bags Needed Rounded Up to Whole Bags

80 lb Bags (0.60 ft³)

147 bags

60 lb Bags (0.45 ft³)

196 bags

40 lb Bags (0.30 ft³)

294 bags

Est. Ready-Mix Truck Cost

$537.78

Est. 80 lb Premix Bag Cost

$955.50

Step-by-Step Concrete Yardage Calculation

1. 1. Surface Area: 12.00 ft × 20.00 ft × 1 slab = 240.00 sq ft.
2. 2. Net Concrete Volume: 240.00 sq ft × (4.00 in ÷ 12 in/ft) = 80.00 cubic feet. Dividing by 27 ft³/yd³ gives 2.96 cubic yards net.
3. 3. Waste-Adjusted Order Volume (+10%): 80.00 ft³ × 1.10 = 88.00 ft³ = 3.26 cubic yards (2.49 m³).
4. 4. Pre-Mixed Bag Equivalents: 88.00 ft³ ÷ 0.60 ft³/bag = 147 bags (80 lb), or 196 bags (60 lb), or 294 bags (40 lb).

Detailed Slab Volume, Bag & Weight Breakdown

Material / Engineering Metric	Net Theoretical Value (0% Waste)	Order Value (With Selected Waste)
Total Pour Surface Area	240.00 sq ft	240.00 sq ft
Volume in Cubic Yards (yd³)	2.96 yd³	3.26 yd³
Volume in Cubic Feet (ft³)	80.00 ft³	88.00 ft³
Volume in Cubic Meters (m³)	2.27 m³	2.49 m³
80 lb Pre-Mix Bags (0.60 ft³ yield)	134 bags	147 bags
60 lb Pre-Mix Bags (0.45 ft³ yield)	178 bags	196 bags
40 lb Pre-Mix Bags (0.30 ft³ yield)	267 bags	294 bags
Estimated Concrete Weight (150 lbs/ft³)	12,000 lbs (6.00 US tons)	13,200 lbs (6.60 US tons)

Concrete Slab & Yardage Formulas

Rectangular Slab Volume (ft³) = Length (ft) × Width (ft) × [Thickness (in) ÷ 12] × Qty
Circular Slab Volume (ft³) = π × [Diameter (ft) ÷ 2]² × [Thickness (in) ÷ 12] × Qty
Cubic Yards (yd³) = Cubic Feet (ft³) ÷ 27
Order Volume with Waste = Net Volume × (1 + Waste % ÷ 100)
80 lb Bags = ⌈ft³ ÷ 0.60⌉ | 60 lb Bags = ⌈ft³ ÷ 0.45⌉ | 40 lb Bags = ⌈ft³ ÷ 0.30⌉

Standard Assumptions: Pre-mixed bag yields follow standard commercial concrete yields (1 cubic yard = 27 ft³ = forty-five 80 lb bags, sixty 60 lb bags, or ninety 40 lb bags). Normal-weight Portland cement concrete is calculated at 150 lbs per cubic foot (4,050 lbs per cubic yard).

Illustrative Concrete Slab Example

Example — 12 ft × 20 ft Single-Car Driveway at 4 Inches Thick (Illustrative):

• Step 1 (Area & Depth in Feet): 12 ft × 20 ft = 240 sq ft. Convert 4 inches to feet: 4 ÷ 12 = 0.3333 ft.
• Step 2 (Net Cubic Feet & Yards): 240 sq ft × 0.3333 ft = 80.00 ft³. Divide by 27 ft³/yd³ = 2.96 yd³ net.
• Step 3 (Add 10% Waste Allowance): 2.963 yd³ × 1.10 = 3.26 yd³ (88.00 ft³), or 147 bags of 80 lb concrete mix.

How to Use the Concrete Slab & Yardage Calculator

1. Select Slab Geometry: Choose Rectangular Slab / Patio (Length × Width) or Circular Slab / Round Pad (Outer Diameter), or click a preset chip for standard patio, driveway, or garage dimensions.
2. Enter Dimensions & Units: Enter your measurements and select the matching unit (ft, in, yd, m, or cm) for each field.
3. Set Waste Buffer & Local Unit Prices: Keep the recommended 10% contractor buffer to account for subgrade variations, and optionally adjust local ready-mix ($/yd³) and 80 lb bag prices.
4. Copy or Print Your Estimate: Review the cubic yardage, bag counts, and weight table, then click Copy Result or Print / Save PDF for your supplier order.

Frequently Asked Questions

How many 80 lb or 60 lb bags of concrete are in 1 cubic yard?

One cubic yard equals 27 cubic feet. Because an 80 lb bag yields 0.60 ft³, it takes 45 bags of 80 lb concrete (27 ÷ 0.60) to make 1 cubic yard. A 60 lb bag yields 0.45 ft³, requiring 60 bags of 60 lb concrete per cubic yard, and a 40 lb bag (0.30 ft³) requires 90 bags per cubic yard.

Why should I add a 10% waste allowance when ordering ready-mix concrete?

Even a 0.5-inch dip in the compacted gravel subbase across a 4-inch slab increases actual concrete volume by 12.5%. Adding a 10% buffer prevents running short mid-pour and avoiding costly short-load delivery fees for a second truck.

When is it cheaper to order a ready-mix concrete truck instead of buying bags?

For pours exceeding 1.5 to 2 cubic yards (68 to 90 bags of 80 lb concrete), ready-mix delivery is typically more cost-effective and saves hours of manual mixing while ensuring a uniform, monolithic pour.

How thick should a residential concrete slab be?

Standard residential sidewalks, patios, and passenger-car driveways are typically poured at 4 inches thick. Garage floors, RV pads, and heavy pickup driveways are commonly poured at 5 to 6 inches thick.