Concrete Calculator – Formula & Examples

About the Concrete Calculator

Getting a concrete order right matters: too little and the pour has a cold joint, too much and you pay for waste. This calculator works out the volume for the shape you are pouring, adds a waste allowance, and then breaks that volume down into cement bags, sand and aggregate for the mix ratio you choose.

Also known as: concrete volume calculator, how much concrete do i need, cubic meters of concrete, ready mix calculator, cement concrete calculator.

How to use this calculator

Choose the shape you are pouring.
Enter the finished dimensions in metres.
Pick the mix ratio — M20 (1:1.5:3) suits most domestic slabs and footings.
Order to the nearest quarter cubic metre above the figure shown.

Concrete formula

Volume = length × width × depth (× π/4 × d² for round)

Worked example

Using the values pre-filled in the calculator above:

What are you pouring?: Slab or footpath
Length = 5 m
Width = 4 m
Thickness or depth = 0.1 m
How many? = 1
Mix ratio: 1 : 1.5 : 3 (M20) — 20 MPa — slabs and footings
Waste allowance = 5 %
Price per m³ (optional) = 140

the calculator returns:

Concrete required: 2.1 m³
Without waste: 2 m³
In cubic yards: 2.747
Cement bags (50 kg): 17

Working

5 × 4 × 0.1 = 2 m³ each
× 1 and +5% waste = 2.1 m³

Tips & common mistakes

Always round up. Running out mid-pour creates a weak cold joint.
A 50 kg cement bag is about 0.035 m³ of loose powder.

Frequently asked questions

How many bags of cement in 1 m³ of concrete?

For a 1:1.5:3 (M20) mix, roughly 8 bags of 50 kg cement per cubic metre, plus 0.42 m³ sand and 0.84 m³ aggregate.

How thick should a concrete slab be?

A footpath is usually 75–100 mm, a domestic driveway 100–150 mm, and a garage floor 150 mm with reinforcement.

How long before I can walk on it?

Normally 24–48 hours for foot traffic, 7 days for light loads and 28 days for full design strength.