Concrete Slab Calculator

Calculate the exact amount of concrete needed for any slab project — patios, garage floors, driveways, pool decks, and foundation slabs. Enter your slab length, width, and thickness, and our calculator returns cubic yards and 80 lb bag counts with a 10% overage already factored in. Getting the right thickness is critical: under-specifying depth is the most common cause of slab cracking.

Enter Your Measurements

Slab Length(ft)

Slab Width(ft)

Thickness(in)

Results

Concrete Needed

4.94

cubic yards

With 10% Overage

5.43

cubic yards

Slab Area

400

sq ft

Volume

133.3

cubic feet

80 lb Bags

223

bags

Cost Estimate

Cost per cubic yard

per cubic yard

Estimated Cost

$740.74

Formula

Cubic Yards = (Length × Width × Thickness ÷ 12) ÷ 27

How to Use This Calculator

How to Use

1
Measure the full length of the slab area in feet, including any bump-outs or extensions.
2
Measure the width in feet — for L-shaped or irregular slabs, split the area into rectangles and use this calculator for each section.
3
Enter the thickness in inches: 4 inches is standard for patios and walkways, 6 inches for driveways and garage floors, 8 or more for heavy equipment areas.
4
Review results showing cubic yards with overage and the number of 80 lb bags if you're mixing by hand for a smaller section.

Frequently Asked Questions

How thick should a concrete slab be?

Standard patios, walkways, and interior floors use 4-inch slabs — this is adequate for foot traffic and furniture loads. Driveways and garage floors should be at least 6 inches to handle vehicle weight without cracking. Pool decks are typically 4 inches. Foundation slabs and industrial floors range from 6 to 12 inches depending on the structural load. When in doubt, go thicker — the material cost difference between 4 and 6 inches is modest compared to the cost of slab repair.

Do I need rebar in my concrete slab?

For driveways, garage floors, and any slab that will bear vehicle weight, rebar is strongly recommended and often required by code. Use #4 (half-inch) rebar on 12-inch centers for residential slabs. For patios, walkways, and interior floors with light use, welded wire mesh or fiber reinforcement is usually sufficient and easier to install. Rebar prevents cracks from spreading — even well-mixed concrete will develop small cracks over time.

Do I need control joints in a concrete slab?

Yes — control joints are planned weak points that encourage any cracking to happen at predictable, straight lines rather than randomly across your slab. Cut control joints to one-quarter of the slab thickness, so a 4-inch slab gets 1-inch deep joints. Space joints no further apart than 2 to 3 times the slab thickness in feet — for a 4-inch slab, joints every 8 to 10 feet. Joints should be cut within 6 to 18 hours of pouring, before the concrete fully hardens.

How long should I wait before walking or driving on a new slab?

For foot traffic, wait a minimum of 24 hours, though 48 to 72 hours is safer and reduces the risk of surface damage. For vehicle traffic, wait at least 7 days before driving passenger cars over the slab. For heavy vehicles or full structural loads, wait the full 28-day cure period. Curing time can be slowed by cold weather and accelerated by warm, humid conditions. Keep the slab moist during the first week for best results.

How much does it cost to pour a concrete slab?

The cost to pour a concrete slab professionally is typically $6 to $12 per square foot, all-in. This includes excavation, gravel base, formwork, ready-mix concrete, labor, and finishing. A standard 10x10 patio slab runs $600 to $1,200 installed. A 20x20 ft garage slab typically costs $2,400 to $4,800. DIY pouring your own slab can cut costs significantly — expect to spend $3 to $5 per square foot for materials — but requires experience to finish properly.

In-Depth Guides

📖

How Much Concrete Do You Need? Complete Guide (2026)

Learn exactly how to calculate concrete for slabs, footings, and columns. Covers bag sizes, ready-mix vs bags, waste factors, and cost estimates.

📖

How Much Does Concrete Cost Per Cubic Yard in 2026?

Concrete pricing guide for 2026: ready-mix costs per yard, short-load fees, bagged concrete costs, and regional price variations. Get accurate estimates for your project.

📖

Concrete Mix Ratios Guide: Which Mix for Which Project

Standard concrete mix ratios by project type — slabs, footings, structural vs. non-structural applications — with PSI requirements and how to mix correctly.

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About This Calculator

This calculator uses the formula: Cubic Yards = (Length × Width × Thickness ÷ 12) ÷ 27. All calculations follow industry-standard methods. Results are estimates — always verify with a licensed professional for structural or code-compliant work.

Built and maintained by the CalcSmart team. Last updated March 2026.

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