Material estimator

Concrete Slab Calculator

Estimate cubic yards, cubic feet, bag counts, and cost for concrete slabs, patios, walkways, and shed pads.

Reviewed for formula logic and buying assumptions on May 21, 2026.

How This Calculator Works

Use the slab calculator for flat pours where the main variables are length, width, thickness, quantity, and overage.

Formula

Slab cubic feet = length x width x thickness in feet. Cubic yards = cubic feet / 27.

Assumptions

Residential patios and walkways are often 4 inches thick, while driveways and load-bearing slabs may need thicker sections and reinforcement. Always verify project requirements before ordering.

Measurement Checklist

Measure the inside dimensions of the slab forms after they are squared and braced.
Use the designed slab thickness, not the thinnest edge of a sloped or uneven pour.
Add separate volume for thickened edges, curbs, steps, or turndowns.
Compare bagged mix to ready-mix once the estimate is near one cubic yard.

Practical Examples

A 20 ft by 10 ft slab at 4 inches thick is 66.7 ft^3 before waste, or 2.47 yd^3. With 10% overage, plan around 2.72 yd^3.
A 10 ft by 12 ft patio at 4 inches thick with 10% overage needs about 1.63 yd^3, which is already worth comparing against ready-mix delivery.
A 4 ft by 20 ft walkway at 4 inches thick with 10% overage needs about 1.09 yd^3, or about 49 bags of 80 lb mix.
A 24 ft by 24 ft driveway section at 5 inches thick with 5% overage needs about 9.33 yd^3, so bagged concrete is not a practical plan.

Before You Buy

Buying Guidance

Use this for the flat slab volume, then calculate thickened edges or footings separately and add them to the order.
Patios and walkways often use 4 inches, but driveways, poor soil, and structural loads can require a different design.
If the result is multiple cubic yards, get ready-mix pricing instead of assuming bags are the cheaper route.

Waste Rules

Use 5% overage for professional forms and an even base. Use 10% for DIY forms, small pours, and rough excavation.
For slabs with slope or an uneven gravel base, use the average thickness rather than the thinnest edge.

Common Mistakes

Do not ignore thickened edges. They can add enough volume to change both bag count and ready-mix minimums.
Do not wait until the truck or bags arrive to solve access, reinforcement, screed boards, or finishing tools.

Plan the Rest of the Job

Common Questions

What thickness should I use?

Many patios and walkways use 4 inches. Vehicle loads, poor soil, and structural work can require more.

Should I include thickened edges?

Yes. Calculate thickened edges separately as footings, then add that volume to the slab total.

Why is the bag count so high?

Bagged concrete is convenient for small pours, but slabs consume volume quickly. Cubic yards are the better planning unit for larger slabs.