Concrete Bag Calculator

Inputs

Length (m)

Width (m)

Thickness (mm)

Bag Size

Approximate wet-mix yield per bag. Check the bag label — yields vary by mix.

Waste / Spillage (%)

For estimation purposes only. Terms of Use

Result

—

bags of concrete needed

Volume—

Bags (no waste)—

Total mix weight—

How to Calculate Concrete Bags

Find the volume: multiply length × width × thickness (converted to metres).
Add your waste allowance: multiply the volume by 1 + waste% (5–10% is typical for bag work).
Divide by the bag yield: a 30 kg bag makes roughly 0.014 m³, a 40 kg bag about 0.019 m³. Round up to the nearest full bag.

Formula Used

Volume = Length × Width × Thickness
Bags = Volume × (1 + Waste %) ÷ Bag Yield

Worked Example

A 3m × 1.2m slab at 100mm thick with 5% waste using 40 kg bags:

Volume = 3 × 1.2 × 0.1 = 0.36 m³
With 5% waste = 0.378 m³
Bags = 0.378 ÷ 0.019 = 20 bags (rounded up)

When to Use This Calculator

Use this for small pours where ready-mix trucks don't make sense — footings, pads, walkways, and repairs — or when ordering bagged mix from a supplier.

When Not to Use This Calculator

For slabs or footings over roughly 1 m³, ready-mix concrete is usually cheaper and more consistent than bags. Always confirm bag yields with the mix manufacturer's label.

Frequently Asked Questions

How many 40 kg bags are in a cubic metre? ▾

About 53 bags — a 40 kg bag yields roughly 0.019 m³ of mixed concrete. Confirm with your specific mix, since yields vary.

Is bagged concrete cheaper than ready-mix? ▾

For very small quantities, yes. Above about 1 m³, ready-mix is usually cheaper per cubic metre and gives more consistent results.