MixTally
Volumetric Material Takeoff3D Spatial Engine

Concrete Slab Calculator

Compute cubic yards, cubic feet, pre-mix bags, and supplier orders with real-time 3D dimensional verification and transparent mathematical formulas.

Units:
Slab DimensionsUS Customary (Feet / Inches)
Project Presets:
20 ft
ft
10 ft
ft
Standard sidewalk / patio
4 in
in
Additional quantity to account for placement losses, uneven forms and ordering margin.
10%
0% (Exact CAD)10% (Trade Standard)20% (Rough Ground)
Live Parametric 3D SlabDimensions: 20×10 ft, 4 in thick
3D Slab Dimension Inspector
Orbit: Left Click / Touch DragZoom: Scroll / Pinch
Current slab geometry: length 20 feet, width 10 feet, thickness 4 inches. Total required concrete volume is 2.72 cubic yards.
Left-click / touch to rotate 360° · Scroll / pinch to zoomDimensions update automatically

Estimated Concrete Required

Rational precision with trade-calibrated over-order margin

ASTM Ready-Mix & Pre-Mix Equivalents
CONCRETE REQUIREDPrimary Takeoff
2.72 yd³
Base volume:2.47 yd³
Planning allowance:10% (+0.25 yd³)
Ordering quantity:2.72 yd³
Ready-Mix Deliveries:~0.3 Trucks (~9 yd³ drum)
Base Geometric Volume
66.67 ft³
2.47 yd³ net

Theoretical geometric volume from entered dimensions before accounting for planning allowances.

Over-Order Allowance (10%):+6.67 ft³ (0.25 yd³)
Pre-Mix Dry Bag Alternative
123 Bags (80 lb)
or 163 Bags (60 lb)

If mixing dry concrete bags on site with a portable drum mixer or wheelbarrow.

Yield Factors:80lb = 0.60 ft³ · 60lb = 0.45 ft³
Technical Workflow DiagramConcrete Slab Takeoff
DimensionsL × W × Depth
Base VolumeNet ft³ / m³
Order Total+ Waste % → yd³ / Trucks

Transparent Calculation Breakdown

Step 1: Convert Thickness
4 in ÷ 12 = 0.333 ft

Thickness must be in the same dimensional units as length and width before computing volume.

Step 2: Net Base Volume
20 × 10 × 0.333 = 66.67 ft³

66.67 ft³ ÷ 27 = 2.47 cubic yards net.

Step 3: Add Over-Order (10%)
66.67 × 1.10 = 73.33 ft³

73.33 ft³ ÷ 27 = 2.72 cubic yards to order.

Practical Field Note: Ground subgrades are never laser-flat. Minor depressions, formwork deflection under wet hydrostatic pressure, and residual concrete stuck inside the delivery truck chute routinely consume 5% to 10% more volume than theoretical drawings. Always order with adequate margin.

Optional Material Cost Estimate

Provide your local supplier quote to calculate estimated material costs

Check the box above to enter your supplier's per-yard (or per-cubic-meter) quote and compute material cost.

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Concrete Slab Calculator Guide

Whether pouring a residential patio, sidewalk, equipment pad, or detached garage slab, calculating estimated concrete volume is fundamental to avoiding costly cold joints or excess disposal fees. This guide outlines standard geometric formulas, typical trade waste assumptions, and practical batching tips.

How to Calculate Concrete for a Slab

Concrete is measured and sold in cubic yards (yd³) in the United States and cubic meters (m³) internationally. To calculate the volume required for a rectangular slab:

  1. Measure the Length and Width in feet (or meters).
  2. Determine the Slab Thickness in inches (or centimeters).
  3. Convert Thickness to Feet by dividing by 12 (or to meters by dividing by 100).
  4. Multiply Length × Width × Thickness to find the cubic feet (or cubic meters).
  5. Divide Cubic Feet by 27 to convert into cubic yards.
  6. Add a Waste Allowance of 10% to account for subgrade unevenness and form flexure.

Concrete Volume Formula

Imperial Formula (Cubic Yards)
Volume (yd³) = [ Length (ft) × Width (ft) × (Thickness (in) ÷ 12) ] ÷ 27 × (1 + Waste %)
Metric Formula (Cubic Meters)
Volume (m³) = Length (m) × Width (m) × (Thickness (cm) ÷ 100) × (1 + Waste %)

Worked Example

Suppose you are pouring a backyard patio measuring 20 feet long by 10 feet wide with a standard 4-inch depth:

1. Convert thickness: 4 in ÷ 12 = 0.333 ft
2. Calculate net cubic feet: 20 ft × 10 ft × 0.333 ft = 66.67 ft³
3. Convert to cubic yards: 66.67 ft³ ÷ 27 = 2.47 yd³ net
4. Add 10% waste: 2.47 yd³ × 1.10 = 2.72 yd³ final order

For bulk delivery, you would round this order up to 2.75 or 3.0 cubic yards. For bagged concrete, 73.33 ft³ ÷ 0.60 ft³ requires 123 eighty-pound bags.

How Much Extra Concrete Should I Order?

Running short of concrete during a pour is a contractor's worst nightmare—it forces an emergency delivery and risks a weak, unsightly "cold joint" where new wet concrete cures against already-stiffened concrete. Standard trade guidelines recommend:

  • Standard Slabs on Flat Ground: Order 10% extra.
  • Deep Footings or Irregular Excavations: Order 12% to 15% extra due to soil sloughing and trench variations.
  • Smooth Pre-formed Raised Decks: Order 5% extra.

Important Considerations

Minimum Ready-Mix Surcharges

Most ready-mix concrete suppliers require a minimum order (typically 4 to 5 cubic yards). Orders below this threshold incur "short-load" delivery fees.

Subgrade Compaction

A 4-inch compacted gravel base (crushed stone #57 or road base) prevents subgrade settlement, frost heaving, and uncontrolled slab cracking.

Frequently Asked Questions

How many 80 lb bags of concrete equal one cubic yard?

One 80-pound bag yields approximately 0.60 cubic feet. Since one cubic yard contains 27 cubic feet, it mathematically requires approximately 45 bags of 80 lb concrete (or 60 bags of 60 lb concrete) per cubic yard based on this planning assumption.

How thick should a concrete slab be?

Sidewalks, walkways, and patios typically require 4 inches (10 cm). Vehicle driveways, garage slabs, and heavy storage pads require a minimum of 5 to 6 inches (13 to 15 cm) with steel rebar or wire mesh reinforcement.

What does a cubic yard of concrete weigh?

Standard normal-weight concrete weighs approximately 4,050 lbs (about 2 tons or 1,840 kg) per cubic yard when wet.

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