Concrete Slab Calculator: Thickness, Cubic Yards, Bags, and Waste

Quick Answer

For a slab, multiply length by width by thickness in feet, divide cubic feet by 27 to get cubic yards, then add 5% to 15% waste. Four inches is common for patios and walkways, while driveways often need 5 to 6 inches depending on loads and local requirements.

Use the Concrete Calculator to adjust the dimensions, waste factor, coverage rate, and local unit price for your exact project.

Search Intent This Answers

People searching for a concrete slab calculator usually need a fast answer for slab thickness, cubic yards, bag count, and how much extra concrete to order.

Core Formula

Cubic yards = length(ft) x width(ft) x thickness(ft) / 27

Step-by-Step Estimate

1. Measure slab length and width in feet.
2. Convert thickness from inches to feet.
3. Multiply length x width x thickness to get cubic feet.
4. Divide by 27 to get cubic yards.
5. Add waste and round up to the supplier ordering unit.

What to Check Before Buying

• Confirm slab thickness with local code or project plans.
• Include thickened edges, footings, or turn-downs if present.
• Choose bags only for small pours; use ready-mix for larger slabs.
• Plan base gravel, forms, stakes, reinforcement, and curing supplies.

Common Estimating Mistakes

• Multiplying feet by inches without converting thickness.
• Forgetting waste for uneven subgrade.
• Ignoring delivery minimums or short-load fees.
• Estimating the slab but not the base preparation materials.

Related Calculators and Guides

• Concrete Calculator - calculate cubic yards and bag counts from your exact slab dimensions
• Concrete Bags vs Ready-Mix - compare bagged concrete with delivery for your pour size
• Paver Calculator - compare a poured slab with a paver patio option

FAQ

How thick should a concrete slab be?

Many patios and walkways use 4 inches. Driveways commonly use 5 to 6 inches, but soil, load, reinforcement, and local code matter.

How much waste should I add for a slab?

Use 5% for simple forms and 10% to 15% for uneven ground, irregular edges, or first-time work.

Should I order exact cubic yards?

No. Round up to the supplier ordering increment so you do not run short during the pour.

Bottom Line

Start with the formula above, run the Concrete Calculator, then round up to the nearest practical bag, box, bundle, panel, roll, or delivery unit. Treat the result as a planning estimate and verify final quantities against site conditions, local code, and manufacturer instructions.