Use this guide to plan a concrete slab for a patio at 4 inches thick, with wire mesh reinforcement and a broom finish. For a different patio size, use the Concrete Slab Calculator as the tool for custom sizes, and the Concrete Slab Cost Calculator for a cost estimate.
Recommended patio slab setup
Choose a 4-inch patio slab on compacted ground, with wire mesh in the middle of the slab and a broom finish for grip. Slope the slab slightly away from the house, and cut control joints so cracks form where you plan them.
How much concrete for a patio slab at 4 inches thick
Volume in cubic feet = length (ft) × width (ft) × 1/3. Divide by 27 for cubic yards. The “order” column adds a 10% allowance, and bag counts use 80-pound bags at 0.60 cubic feet per bag, rounded up.
| Patio slab size | Area | Base volume | Order with 10% allowance | 80-pound bags |
|---|---|---|---|---|
| 12 × 14 ft | 168 sq ft | 2.07 cubic yards | 2.28 cubic yards | 103 bags |
| 14 × 18 ft | 252 sq ft | 3.11 cubic yards | 3.42 cubic yards | 154 bags |
| 16 × 20 ft | 320 sq ft | 3.95 cubic yards | 4.35 cubic yards | 196 bags |
Worked example: a 15 × 18 ft patio slab
Area = 15 × 18 = 270 sq ft. Volume = 270 × 1/3 = 90.00 cubic feet = 3.33 cubic yards. With a 10% allowance: 3.33 × 1.10 = 3.67 cubic yards, which is 99.00 cubic feet. At 0.60 cubic feet per bag, that is 165 bags. For a slab this size, ready-mix concrete is usually more practical than bags.
Method note
Measure the finished slab area and use the full 4-inch thickness. The 10% allowance covers formwork variation and spillage; it does not cover a thicker edge or an uneven sub-base, which need to be added separately.
Patio slab questions
Why wire mesh in a patio slab?
Wire mesh helps hold the slab together if it cracks and spreads loads across the slab. It does not make a thin slab into a structural slab.
What is a broom finish?
A broom is dragged lightly across the fresh concrete to give a textured, non-slip surface. It suits outdoor patios better than a smooth trowel finish.
How far apart should control joints be?
As a guide, space joints about 2 to 3 times the slab thickness in feet, and keep panels roughly square. For a 4-inch patio slab, that is about every 8 to 12 feet.
Should I order bags or ready-mix?
Small slabs can use bags. For most patio slabs, ready-mix ordered in cubic yards is faster and more consistent; use the Concrete Slab Cost Calculator to compare the cost.
For custom patio sizes, use the Concrete Slab Calculator.