Concrete Slab Materials Calculator
Get a complete material list for your slab project — concrete, gravel base, rebar or wire mesh, and vapor barrier — all from one calculator.
Slab Material Calculator
// Complete Material List
Understanding Slab Material Requirements
Gravel Base
A properly compacted gravel base (also called a subbase) is critical for slab longevity. It improves drainage, prevents frost heave, and creates a stable, even surface for the concrete. Use crushed stone (3/4" or 1" crushed gravel, also called "road base") rather than pea gravel or river rock — angular stone compacts better. The gravel volume in this calculator includes a 15% compaction factor since loose gravel compacts significantly when tamped.
Wire Mesh vs. Rebar
Wire mesh (6×6 W1.4×W1.4 welded wire reinforcement) is the standard choice for residential patios and walkways. It's inexpensive, easy to handle, and helps control cracking. Position it on wire chairs or small stones to keep it in the lower third of the slab — not flat on the ground where it provides no reinforcement benefit.
Rebar provides stronger reinforcement and is recommended for driveways, garage floors, and slabs subject to vehicle loads or frost heave. #3 rebar (3/8" diameter) on an 18-inch grid is common for driveways; #4 rebar (1/2" diameter) for garage floors and heavier applications.
Vapor Barrier
A 6-mil polyethylene vapor barrier placed directly on the gravel base (under the concrete) prevents ground moisture from migrating up through the slab. Required by code for all interior slabs in most jurisdictions. Overlap seams by 12 inches and tape them. Extend the barrier up the inside of the forms and trim after the slab is poured.
Expansion Joints
Isolation joints (also called expansion joints) are installed at the perimeter of the slab where it meets existing structures like house foundations, garage walls, or existing concrete. Use 1/2" asphalt-impregnated expansion joint material. The perimeter linear footage is the outside measurement of your slab.
Frequently Asked Questions
- How much gravel do I need under a concrete slab?A 4-inch compacted gravel base is standard for residential slabs. Calculate: L × W × (4"/12) ÷ 27 × 1.15 (compaction factor) = cubic yards. A 20×20 slab needs about 2.0 yards of gravel for a 4-inch base.
- Do I need a vapor barrier under a concrete slab?Yes, for all interior slabs — garage floors, basements, and house slabs. 6-mil poly sheeting is the standard specification. Exterior patio slabs don't require it but benefit from one in wet climates.
- How much wire mesh do I need for a 20×20 slab?A 20×20 slab = 400 sq ft. Wire mesh typically comes in 5×150 ft rolls (750 sq ft) or 5×10 sheets (50 sq ft). Add 10% for overlaps — you need about 440 sq ft of mesh. That's roughly 9 sheets or less than one roll.