How many cubic yards of concrete do I need for a 12x20 slab?

A 12ft × 20ft slab at 4 inches thick requires 2.96 cubic yards. With a recommended 10% waste buffer, you should order 3.3 cubic yards.

How many 80 lb bags of concrete make a cubic yard?

It takes exactly 45 bags of 80 lb premix concrete to make 1 cubic yard. If using 60 lb bags, you need 60 bags per cubic yard.

What thickness should a concrete slab be?

4 inches is standard for walkways, patios, and residential shed pads. 5 to 6 inches is recommended for driveways, heavy vehicle parking, and hot tubs.

Do I need rebar or wire mesh for a 4-inch concrete patio slab?

Installing #4 rebar in an 18" grid or 6x6 welded wire fabric prevents shrinkage cracks from separating and prevents uneven vertical settling over time.

How thick should the gravel sub-base be under a slab?

A compacted gravel sub-base should be at least 4 inches deep using 3/4" minus crushed stone, increased to 6-8 inches on expansive clay soils.

Concrete Slab Calculator – Yards, Premix Bags & Rebar Grid

1. Slab Shape & Dimensions

Length (feet)

Width (feet)

Slab Thickness (inches) Standard: 4" (10 cm)

Rebar Grid Spacing

Crushed Gravel Base

in depth

2. Pricing & Delivery Option

Ready-Mix Concrete ($/cu yd)

Gravel Base ($/ton)

Required Concrete Volume

3.26 cu yd

Includes 10% waste (Base: 2.96 cu yd • 2.27 m³)

$937 Est.

13,203 lbs total

Premix Bags (80 lb)

147 bags

Or 196 bags (60 lb) / 235 bags (50 lb)

Surface Area

240.0 sq ft

4 in thickness

Sub-Base Crushed Gravel

4.14 tons

2.96 cu yds compacted base (4" depth)

Reinforcement Steel

42 bars (10ft #4)

360 total linear feet of rebar

Slab Construction Profile4" Concrete + 4" Gravel

Concrete Slab Formula & Ready-Mix Buying Guide

How to Calculate Cubic Yards

Concrete is ordered and sold by the cubic yard (27 cubic feet). To calculate cubic yards for any rectangular slab:

Volume (Cu Yds) = [Length (ft) × Width (ft) × Thickness (in / 12)] ÷ 27
Example: 20 ft × 12 ft × 4 in = [20 × 12 × 0.333] ÷ 27 = 2.96 cu yds
With 10% Waste Buffer: 2.96 × 1.10 = 3.26 cu yds

Ready-Mix Truck vs Premix Bags: When to Switch?

- Under 1 Cubic Yard (~45 bags of 80 lb): Hand-mixing with a portable electric cement mixer is economical for small walkways, shed pads, or fence posts.
- Over 1.5 – 2 Cubic Yards: It is almost always cheaper, faster, and structurally stronger to order ready-mix concrete delivered directly from a batch plant mixer truck.

Frequently Asked Questions (FAQ)

A 12ft × 20ft rectangular slab at a standard 4-inch depth requires 2.96 cubic yards (80 cubic feet) of concrete. With an industry-recommended 10% waste buffer for uneven grade excavation and spillage, you should order 3.3 cubic yards from a ready-mix supplier.

It takes exactly 45 bags of 80 lb premix concrete to yield 1 cubic yard (each 80 lb bag yields 0.60 cu ft; 27 ÷ 0.60 = 45 bags). If you are using 60 lb bags, you will need 60 bags per cubic yard (each 60 lb bag yields 0.45 cu ft).

4 inches (10 cm) is the standard thickness for residential walkways, patios, and lightweight backyard storage shed pads. 5 to 6 inches (13 to 15 cm) with steel rebar reinforcement is recommended for driveways, heavy pickup truck parking, RV pads, and hot tubs.

While concrete possesses tremendous compressive strength, it has low tensile strength. Installing #4 rebar (1/2") in an 18" grid or 6×6 welded wire fabric placed on chairs in the middle third of the slab prevents cracking, shrinkage separation, and uneven vertical settling over time.

A compacted gravel sub-base should be at least 4 inches deep (using 3/4" minus crushed stone or road base aggregate). On expansive clay soils or poorly draining ground, increase gravel depth to 6–8 inches to ensure solid subgrade compaction and rapid water drainage beneath the slab.