Concrete Slab Calculator
Calculate everything you need for a concrete slab — concrete volume, rebar quantities, wire mesh, vapor barrier, and a full cost breakdown.
How to Plan a Concrete Slab
A well-built concrete slab requires more than just pouring concrete. Proper reinforcement with rebar and wire mesh prevents cracking, while a vapor barrier stops moisture from migrating up through the slab. This calculator accounts for all these materials so you can order everything at once.
Rebar Spacing Explained
Rebar is placed in a grid pattern within the slab. Common spacings are 12", 16", 18", and 24" on center (the distance between each bar). Tighter spacing provides more strength but costs more. For residential slabs, 18" on center with #4 rebar is a solid standard. Garage floors and driveways benefit from 12–16" spacing.
Do I Need a Vapor Barrier?
A vapor barrier (typically 6–10 mil polyethylene sheeting) is recommended for any slab that will have flooring installed over it, be in a conditioned space, or be built on soil with high moisture content. It prevents moisture from wicking up through the concrete, which can damage flooring and promote mold growth.
Wire Mesh vs. Rebar
Wire mesh (typically 6×6 W1.4/W1.4) helps control cracking in the slab surface. Rebar provides structural reinforcement. For most residential slabs, you want both — rebar for strength and wire mesh for crack control. Some contractors substitute fiber mesh additive for welded wire, but traditional wire mesh remains the most common approach.
Frequently Asked Questions
What thickness should my slab be?
Standard residential slabs are 4 inches thick. Garage floors should be 4–6 inches. Any slab supporting heavy equipment or vehicles over 10,000 lbs should be 6–8 inches with engineered reinforcement.
How much extra concrete should I order?
Order 5–10% more than calculated to account for waste, uneven subgrade, and spillage. For smaller pours using bags, round up to the nearest full bag.
What size rebar should I use?
For residential slabs, #4 rebar (1/2" diameter) is standard. #3 rebar (3/8") works for sidewalks and patios. #5 rebar (5/8") is used for structural footings and heavy-duty slabs.
How deep should the gravel base be?
A 4–6 inch layer of compacted gravel beneath the slab provides drainage and a stable base. In areas with poor drainage or expansive clay soils, 6–8 inches is better.
Can I pour the slab myself?
Small slabs (under 100 sqft) are manageable for experienced DIYers. Larger slabs require coordinating a concrete truck, having enough helpers to screed and finish the concrete before it sets, and proper form construction. For anything over 200 sqft, consider hiring a professional.
How long before I can use the slab?
Light foot traffic after 24–48 hours. Vehicle traffic after 7 days minimum. Full cure strength at 28 days. Keep the surface moist for the first 7 days for best results.
Related Calculators
- Concrete Calculator — simple volume calculator for any shape
- Concrete Driveway Cost Calculator — full driveway cost with finish options
- Concrete Block Calculator — CMU block walls
Estimates only. Consult a licensed professional for your specific project requirements.