How many bags of concrete do I need for a slab?

A standard 20kg bag of concrete mix yields approximately 0.01 m³. For a 20 m² slab at 100 mm thick (2 m³), you'd need roughly 200 bags. Always add 5–10% for wastage.

How do I calculate concrete volume for a slab?

Multiply length (m) × width (m) × thickness (m). For a 5 m × 4 m slab at 0.1 m thick: 5 × 4 × 0.1 = 2 m³. Add 5–10% for wastage to ensure you order enough.

How thick should a concrete slab be for a shed?

A standard shed slab should be 100 mm thick for light garden sheds. For larger sheds or those storing heavy equipment, use 125–150 mm with reinforcement such as F82 mesh or steel rebar.

Concrete Slab Calculator — Free m³ & Bag Estimator

🧱 Concrete Slab Volume Calculator

Length (mm)

Width (mm)

Thickness (mm)

Concrete Volume Needed

0 m³

Volume in litres

0 L

20 kg bags needed

Add 5% wastage

0 m³

Total to order (incl. wastage)

0 m³

How to Calculate Concrete for a Slab

Calculating concrete volume is straightforward: Length (m) × Width (m) × Thickness (m) = Volume (m³). This calculator converts your millimetre inputs to metres automatically and returns the volume in cubic metres and litres.

For example: a slab measuring 5,000 mm × 4,000 mm × 100 mm = 5 m × 4 m × 0.1 m = 2.0 m³. That's 2,000 litres of concrete or roughly 200 bags of 20 kg pre-mix.

Pro tip: Always order 5–10% more concrete than calculated to account for uneven ground, spillage and variations in slab depth. This calculator includes a 5% wastage adjustment by default.

Typical Slab Thicknesses by Application

Application	Recommended Thickness
Garden shed / pathway	75–100 mm
Patio / home slab	100 mm
Garage / carport	125–150 mm
Driveway (light vehicles)	125 mm
Driveway (heavy vehicles)	150–200 mm
Commercial / industrial	150–250 mm

Worked Examples

Example 1 — Standard Patio Slab (5 m × 4 m × 100 mm)

Item	Value
Length × Width × Thickness	5,000 × 4,000 × 100 mm
Volume	2.00 m³
20 kg bags needed	200 bags
With 5% wastage	2.10 m³ / 210 bags

Example 2 — Single Garage Slab (6 m × 3 m × 125 mm)

Item	Value
Length × Width × Thickness	6,000 × 3,000 × 125 mm
Volume	2.25 m³
20 kg bags needed	225 bags
With 5% wastage	2.36 m³ / 236 bags

Frequently Asked Questions

How many 20 kg bags make 1 m³ of concrete?▾

Approximately 100 bags of 20 kg pre-mix concrete are needed for 1 m³, as each bag yields roughly 0.01 m³. Actual yield varies by brand and mix — always check the bag label and consider ordering a few extra bags.

Should I use bagged concrete or order a truck?▾

For volumes under 1 m³ (100 bags), bagged concrete is usually more practical and cost-effective. For larger pours, ready-mix concrete delivered by truck is cheaper per cubic metre and ensures consistent quality. Most trucks carry 4–8 m³.

Does this calculator include reinforcement?▾

No. Reinforcement (F82 mesh, SL82 mesh, or steel rebar) is a separate cost. As a guide, slab mesh costs around $8–$15 per m² and rebar costs $5–$12 per linear metre, depending on size and specifications.

What concrete strength do I need for a residential slab?▾

For most residential slabs (patios, sheds, garages), a 20 MPa mix with a slump of 80–100 mm is standard. For driveways and areas subject to vehicle loads, use 25 MPa or higher. Always specify the application when ordering from a readymix supplier.

Disclaimer: Concrete bag yields vary by manufacturer. This calculator provides estimates based on a standard 20 kg bag yielding 0.01 m³. Always verify with your supplier and consult a structural engineer for load-bearing applications.

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