Concrete Slab Calculator

Pouring a flatwork concrete slab for a backyard shed, workshop, or ground-level extension requires matching slab thickness to expected loading conditi

Construction

Slab Length

Measured length in decimal feet

Slab Width

Measured width in decimal feet

Slab Thickness Preset

Waste & Subgrade Allowance

Covers subgrade settlement, spillage, and form spread (10% standard)

Reinforcement Schedule

Formula & Methodology

Converted Depth (ft) = Preset Thickness (in) ÷ 12 Net Volume (cu yd) = (Length ft × Width ft × Converted Depth ft) ÷ 27 Slab Concrete to Order = ⌈Net Volume × (1 + Waste % ÷ 100) ÷ 0.25⌉ × 0.25 Crushed Base (cu yd) = (Length ft × Width ft × 0.333 ft) ÷ 27 Formwork Perimeter (ft) = 2 × (Length ft + Width ft)

Slab thickness is selected via standard framing presets (such as 3.5 inches for nominal 2×4 form lumber or 4.0 inches for standard structural pads) and divided by 12 to yield decimal feet. Volume is calculated and rounded up to the nearest ready-mix supplier batch increment. Reinforcement grids, gravel sub-base, and vapor retarding membranes are estimated as separate companion line items and are never folded into concrete volume.

Slab Concrete to Order (Rounded Up)

1.75 cu yd

Overview

Pouring a flatwork concrete slab for a backyard shed, workshop, or ground-level extension requires matching slab thickness to expected loading conditions and preparing perimeter formwork and sub-base materials. This Concrete Slab Calculator computes necessary concrete volume using standard thickness presets while generating separate line-item estimates for crushed gravel, vapor barriers, and rebar.

Estimates are for material planning and budgeting purposes only. This calculator does not provide structural engineering, architectural design, or official building code compliance advice. Always verify measurements, local building codes, and permit requirements with a licensed contractor or local building authority before purchasing materials or commencing construction.

Worked Examples

Example 1: Standard 12 ft × 10 ft Shed Slab (4 in Thick with 10% Waste)

Inputs: Length = 12 ft, Width = 10 ft, Preset = 4.0 in, Waste = 10%, Reinforcement = #4 Rebar at 18" Grid

Convert thickness: 4 in ÷ 12 = 0.3333 ft.
Calculate net cubic feet: 12 ft × 10 ft × 0.3333 ft = 40.00 cu ft.
Convert to cubic yards: 40.00 cu ft ÷ 27 = 1.48 cu yd.
Add 10% waste reserve: 1.4815 cu yd × 1.10 = 1.63 cu yd.
Round UP to supplier 0.25 yd increment: 1.75 cu yd.
Calculate separate 4" gravel base: 120 sq ft × 0.333 ft ÷ 27 = 1.48 cu yd (approx. 2.22 tons).

Final Answer: 1.75 cu yd to order (1.48 net cu yd; 2.22 tons gravel base; 44 linear ft forms)

Example 2: Heavy Workshop Slab (20 ft × 15 ft at 6 in Thick)

Inputs: Length = 20 ft, Width = 15 ft, Preset = 6.0 in, Waste = 10%, Reinforcement = #4 Rebar at 12" Grid

Convert thickness: 6 in ÷ 12 = 0.50 ft.
Area: 20 × 15 = 300 sq ft; Net volume: 300 × 0.50 = 150.00 cu ft.
Cubic yards: 150.00 ÷ 27 = 5.56 cu yd.
Add 10% waste: 5.5556 × 1.10 = 6.11 cu yd.
Round UP to ready-mix increment: 6.25 cu yd.
Perimeter forming boards: 2 × (20 + 15) = 70 linear ft (2×6 lumber).

Final Answer: 6.25 cu yd to order (5.56 net cu yd + 0.56 waste reserve; 70 linear ft formwork)

Example 3: Sidewalk / Walkway Slab (30 ft × 4 ft at 3.5 in [Nominal 2×4])

Inputs: Length = 30 ft, Width = 4 ft, Preset = 3.5 in, Waste = 8%, Reinforcement = WWM

Convert thickness: 3.5 in ÷ 12 = 0.2917 ft.
Area: 30 × 4 = 120 sq ft; Net volume: 120 × 0.2917 = 35.00 cu ft.
Cubic yards: 35.00 ÷ 27 = 1.2963 cu yd.
Add 8% waste factor: 1.2963 × 1.08 = 1.40 cu yd.
Round UP to next 0.25 yd: 1.50 cu yd.

Final Answer: 1.50 cu yd to order (1.30 net cu yd; 68 linear ft 2×4 forms)

How to Use This Calculator

Enter the outer length and width of your planned concrete slab in feet.

Choose a slab thickness preset based on intended loading (4 inches is standard for storage sheds and garages).

Select your reinforcement schedule (rebar grid, welded wire mesh, or fiber).

Adjust the waste allowance (10% standard to account for slight subgrade excavation variance).

Review the concrete order yardage alongside separate planning quantities for gravel base, forming boards, and reinforcement.

Reference Table

Slab Footprint	Thickness Preset	Net Concrete	Ready-Mix Order (10% Waste)	Gravel Base (4" thick)
10 × 10 ft	3.5 in (Nominal 2×4)	1.08 cu yd	1.25 cu yd	1.85 tons
10 × 10 ft	4.0 in (Standard Pad)	1.23 cu yd	1.50 cu yd	1.85 tons
12 × 10 ft	4.0 in (Standard Shed)	1.48 cu yd	1.75 cu yd	2.22 tons
16 × 12 ft	4.0 in (Large Shed)	2.37 cu yd	2.75 cu yd	3.56 tons
20 × 20 ft	4.0 in (2-Car Garage)	4.94 cu yd	5.50 cu yd	7.41 tons
24 × 24 ft	5.0 in (Heavy Garage)	8.89 cu yd	10.00 cu yd	10.67 tons

Frequently Asked Questions

What is the difference between a 3.5-inch and a 4-inch slab?

A nominal 2×4 wood stud measures 3.5 inches wide, meaning forms built with standard 2×4 boards set on edge produce a 3.5-inch thick slab. A full 4-inch slab requires setting form boards on screed stakes or using specialized 2×6 forming lumber.

Do I need rebar or welded wire mesh in a shed slab?

Reinforcement does not prevent concrete from cracking initially, but it holds micro-cracks tightly together to maintain structural integrity and prevent differential settlement. #4 rebar on an 18-inch grid is standard for storage buildings.

Why is a compacted crushed gravel sub-base required under slabs?

A 4-inch base of compacted angular gravel (such as crushed 3/4-inch stone) provides uniform bearing support, prevents capillary moisture rise from damp soil, and allows proper drainage beneath the pad.

When should I use a vapor barrier under a concrete slab?

Install a 10-mil polyethylene vapor retarder whenever the slab will enclose an interior space, shed, or garage where stored tools or flooring could be damaged by moisture vapor diffusing through the concrete.

How far apart should control joints be cut into a concrete slab?

Control joints should be spaced at a distance in feet no greater than two to three times the slab thickness in inches. For a 4-inch slab, cut control joints every 8 to 10 feet in both directions to prevent random cracking.