Concrete Slab Calculator

What This Concrete Slab Calculator Is For

This calculator helps DIY builders and homeowners estimate concrete volume accurately for slab-on-grade projects.

It is designed for small residential construction where correct concrete quantities matter but full structural software is unnecessary.

• Concrete slab volume calculation
• Slabs with or without thickened edges
• Subbase (gravel) volume estimates
• Reinforcement (slab mesh and edge bars)
• Ready-mix and concrete bag estimates
• Metric (mm) and imperial (in) units
• DIY and small construction projects

How to calculate concrete for a slab

To calculate concrete volume correctly, you need to know the slab dimensions, thickness, and expected waste.

This calculator uses true geometric volume formulas instead of rough rules of thumb.

• Slab length
• Slab width
• Slab thickness
• Waste percentage
• Optional thickened edges
• Optional subbase (gravel)
• Optional reinforcement

All values can be adjusted, and results update instantly.

How much waste should I add?

A waste factor helps prevent running out of concrete due to uneven ground, formwork leakage, or material loss during pouring.

Typical waste values:

• 0–5% – very clean forms and flat ground
• 5–10% – standard residential slab work
• 10–15% – uneven subbase or complex formwork

You can set waste to 0% if you want an exact theoretical volume.

Common mistakes when estimating concrete for slabs

• Forgetting to include waste
• Mixing millimeters and inches
• Ignoring thickened slab edges
• Underestimating concrete bag quantities
• Ordering too little ready-mix concrete

Using a calculator helps avoid costly delays and re-orders.

Thickened edge slabs explained

Thickened slab edges are commonly used to increase load capacity along the perimeter of a slab.

They are often required for:

• Garages
• Sheds
• Small workshops
• Structures carrying wall loads

This calculator allows you to include thickened edges and calculates the additional concrete volume automatically.

Reinforcement in concrete slabs

Reinforcement helps control cracking and improves slab durability.

Depending on the project, slabs may include:

• Single or double layer slab mesh
• Perimeter reinforcement in thickened edges

This calculator provides reinforcement quantity estimates to support material planning.
Final reinforcement design should always follow local building codes.

FAQ

How thick should a concrete slab be?

Most residential slabs are 100–150 mm (4–6 inches) thick.

Slabs supporting vehicles or heavier loads may require additional thickness or reinforcement.

Does this calculator include gravel under the slab?

Yes. If enabled, the calculator estimates subbase (gravel) volume based on slab footprint.

Is the cost estimate accurate?

No. The cost estimate is a rough approximation only.

Concrete prices vary by region, supplier, delivery distance, and order size.

Can I use this calculator for shed or garage slabs?

Yes. This calculator works well for slabs under sheds, garages, pergolas, and similar small structures.

Planning a shed or pergola?

CraftFrame plans include foundation layouts, slab dimensions, reinforcement guidance, and step-by-step instructions to help you build correctly and efficiently.

For pile foundations and small footings, use our Pile Concrete Calculator to estimate concrete volume, waste, and material quantities accurately.