How thick should a concrete slab be?

Standard thicknesses by application: 3–4 inches for sidewalks and interior floors, 4 inches for patios, 6 inches for driveways and areas with vehicle traffic, 8–12 inches for slabs supporting heavy equipment. Always check local building codes for your specific application.

How many bags of concrete make a cubic yard?

One cubic yard = 27 cubic feet. A 60 lb bag of pre-mix concrete yields about 0.45 cubic feet, so you need approximately 60 bags per yard. An 80 lb bag yields about 0.60 cubic feet, requiring approximately 45 bags per yard. For slabs over 1 cubic yard, ordering ready-mix concrete by the truck is almost always more economical.

What is the difference between concrete and cement?

Cement is one ingredient in concrete. Concrete is a mixture of cement (about 10–15%), sand (fine aggregate, about 30%), gravel or crushed stone (coarse aggregate, about 45%), and water (about 15–20%). Cement acts as the glue that binds everything together when mixed with water and it hardens through a chemical process called hydration.

Do I need rebar in a concrete slab?

Rebar is recommended for driveways, garage floors, and any slab that will bear vehicle loads. For patios and sidewalks, wire mesh reinforcement is often sufficient. Rebar should be placed in the lower third of the slab, 1.5–2 inches from the bottom, on plastic chairs to hold it off the ground.

How long does concrete take to cure?

Concrete is walkable in 24–48 hours, reaches 70% strength in 7 days, and achieves full design strength (28-day strength) in 28 days. Do not drive vehicles on a new driveway slab for at least 7 days, and avoid heavy loads for 28 days. Cure concrete by keeping it moist (covered with plastic or wet burlap) for at least 7 days.

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Concrete Slab Calculator

Calculate cubic yards and bag count for any concrete slab project.

Results

Cubic Yards—

Bags Needed—

Cubic Feet—

Estimated Cost (bags)—

How to Calculate Concrete for a Slab

The concrete slab calculator uses a simple formula — length × width × thickness — to find the volume of concrete you need. The result in cubic yards tells you how much ready-mix to order; the bag count tells you how many pre-mix bags to buy for smaller projects.

The Formula

Volume (cubic feet) = length (ft) × width (ft) × thickness (ft). Convert inches to feet by dividing by 12. For a 20×10 patio at 4 inches thick: 20 × 10 × (4÷12) = 20 × 10 × 0.333 = 66.7 cubic feet. Divide by 27 to convert to cubic yards: 66.7 ÷ 27 = 2.47 cubic yards. Always add 5–10% for waste and spillage — order 2.6–2.7 yards.

Bags vs. Ready-Mix

An 80 lb bag of pre-mix concrete yields 0.60 cubic feet. For our 66.7 cu ft example: 66.7 ÷ 0.60 = 112 bags. At $6–$8 per 80 lb bag, that's $672–$896 in materials plus significant mixing labor. A cubic yard of ready-mix concrete costs $120–$180 delivered. For 2.47 yards: $296–$445 — roughly half the material cost with no mixing. Use bags only for projects under 1 cubic yard (small steps, posts, repairs).

Thickness Guide by Application

3–4 inches: Sidewalks, interior floors, patios with foot traffic only. 4–5 inches: Garage floors, light vehicle areas with reinforcement. 6 inches: Driveways, any slab bearing vehicle weight. 8–12 inches: Structural foundations, slabs for heavy equipment. Local building codes may specify minimums — always verify before pouring.

Order 5–10% extra: Ready-mix trucks measure volume, but your forms, sub-base, and spillage all consume some of that volume. Short pours are a disaster — you cannot cold-joint fresh concrete without visible seams and weak spots. Always over-order slightly.

Sub-Base Preparation

A properly prepared sub-base is as important as the concrete itself. Excavate to the depth of your slab plus 4–6 inches of compactable gravel (road base). Compact the gravel with a plate compactor. This prevents settling and cracking. Poor sub-base is the #1 cause of cracked driveways and patios within the first 5 years.

Reinforcement

For driveways and garage floors, use #4 rebar on 12-inch spacing, supported 1.5 inches from the bottom on plastic chairs. For patios and sidewalks, 6×6-W1.4×W1.4 welded wire mesh is sufficient. Place reinforcement before pouring — it must be in the lower third of the slab to control cracking from bending loads.

Control Joints

Cut control joints (also called expansion joints) every 8–10 feet in each direction for a 4-inch slab. Joint depth should be 1/4 of slab thickness — 1 inch for a 4-inch slab. Control joints give the concrete a planned place to crack as it shrinks during curing. Without them, random surface cracks appear within months.

Frequently Asked Questions

4 inches for patios and sidewalks, 5–6 inches for garages and driveways with vehicle traffic, and 8–12 inches for heavy equipment or structural slabs. Always check local building codes.

An 80 lb bag yields 0.60 cu ft, so you need about 45 bags per cubic yard. A 60 lb bag yields 0.45 cu ft, requiring about 60 bags per yard. For projects over 1 yard, ready-mix is cheaper.

Cement is a powder ingredient. Concrete is the finished material made by mixing cement, sand, gravel, and water. "Cement driveway" is a common misnomer — driveways are made of concrete.

Yes for driveways and garage floors bearing vehicle loads. Wire mesh reinforcement is sufficient for patios and sidewalks. Rebar belongs in the lower third of the slab, 1.5–2 inches from the bottom.

Walkable in 24–48 hours, 70% strength in 7 days, full strength in 28 days. Keep new slabs moist for at least 7 days. Do not drive on a new driveway for at least 7 days.