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ConcreteVerified Formulas

Concrete Slab Calculator

Calculate volume, bag counts, and rebar grid footage for concrete slabs and patios.

Concrete Slab Calculator Interactive Tool

Interactive Calculator

Enter your project dimensions below. Results update instantly.

4" for walkways & garage floors; 5-6" for heavy trucks
typical value, change to match your product10% standard buffer

Bag Yield Specifications

typical value, change to match your productStandard 80-lb premix yields 0.60 cu ft
typical value, change to match your productStandard 60-lb premix yields 0.45 cu ft
typical value, change to match your productStandard 40-lb premix yields 0.30 cu ft

Results

Base Volume

7.11 cu yd

192 cu ft (exact dimensions)
Concrete to Order

7.82 cu yd

Includes +10% buffer (211.2 cu ft)
80-lb Bags (Premix)

352

At 0.6 cu ft/bag
60-lb Bags (Premix)

470

At 0.45 cu ft/bag
40-lb Bags (Premix)

704

At 0.3 cu ft/bag
Slab Surface Area

576 sq ft

24 ft × 24 ft footprint
Rebar Grid Estimate

~816 lin ft

#4 rebar tied at 18" on-center

Slab Volume & Rebar Formulas

Slab cu yd = (Length (ft) × Width (ft) × (Thickness (in) ÷ 12)) ÷ 27 × (1 + Waste %) | Rebar Grid = Rows × Length + Cols × Width

Formula Variables:

  • 4-inch Slab:Standard for walkways, residential patios, and shed foundations.
  • 6-inch Slab:Standard for residential driveways and garage floors bearing vehicle weights.
  • Rebar Grid:#4 (1/2") rebar spaced at 18" on-center in both directions.

Excavate 4 inches deeper than slab thickness to install a compacted crushed stone sub-base.

Step-by-Step Worked Example

Example: 20 × 24 ft two-car garage slab at 6 inches thick

Pouring a 480 sq ft garage floor slab engineered for heavy vehicle parking with 10% waste allowance.

Dimensions:20 ft × 24 ft (480 sq ft)
Thickness:6 inches
Waste:10%
  1. Calculate cubic feet: 480 sq ft × (6 in ÷ 12 in/ft) = 240 cu ft.
  2. Convert to base cubic yards: 240 ÷ 27 = 8.89 cu yd.
  3. Add 10% waste margin: 8.89 × 1.10 = 9.78 cu yd.
  4. Rebar grid estimation (18" grid): 17 lengthwise bars (24 ft) + 14 crosswise bars (20 ft) = 688 linear feet.
Order 10.0 cubic yards of 4,000 PSI ready-mix concrete and ~690 linear feet of #4 rebar.

Input Explanations & Measurement Guidelines

Slab Length & Width

Typical: 12 × 16 ft

Internal dimensions of the wooden formwork.

Slab Thickness

Typical: 4 or 6 inches

4" for patios/sheds; 5"–6" for driveways and vehicle parking.

Assumptions & Limitations

  • Assumes flat, uniform depth across the entire slab footprint.
  • Slabs thicker at edges (thickened edge or monolithic footings) require additional trench concrete volume.

Safety & Professional Notes

  • Install control joints cut to 1/4 the slab depth every 8 to 10 feet within 24 hours of pouring to control shrinkage cracks.
Recommended Guide · 8 min read

How to Calculate Concrete Volume: Slabs, Footings & Bag Counts

Concrete cannot be returned once ordered or mixed. Underestimating volume leads to disastrous cold joints, while overestimating wastes hundreds of dollars. Learn how to calculate precise concrete volume.

Read Guide

Frequently Asked Questions

How thick should a concrete slab be?

4 inches is standard for sidewalks, shed pads, and back patios. Driveways and garages holding passenger cars or trucks require 5 to 6 inches with rebar reinforcement.

Why does my concrete slab need a gravel base?

A 4-inch compacted gravel base allows water to drain freely away from the underside of the slab, preventing frost heave from cracking the slab in winter.

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