UnitConv

Concrete Calculator

Estimate concrete volume and bags for slabs and columns

Concrete volume
1.2
m³
Cubic yards
1.57
yd³
Cubic feet
42.38
ft³
With waste
1.26
m³
127bags of 20 kg
Volume needed1.26 m³
42%

Bar scaled to a 3 m³ reference (one mixer-truck load is roughly 6-8 m³).

Formula

Slab volume = length × width × thickness

Round column volume = π × (diameter / 2)² × height

All dimensions are converted to meters first, then multiplied. Waste is added as a percentage and bags are rounded up.

What is a Concrete Calculator?

A concrete calculator works out how much ready-mix concrete you need for a pour and how many bags of dry mix that takes. Enter the dimensions of a rectangular slab (length, width and thickness) or a round column (diameter and height) and the tool returns the volume in cubic meters, cubic yards and cubic feet. It also adds a waste allowance for spillage and uneven sub-base, and rounds the result up to whole bags for the size you select. Both metric (m, cm, kg) and imperial (ft, in, lb) units are supported, so it suits DIY footings, fence posts, patios and driveways alike.

How to Use

1. Choose metric or imperial units. 2. Pick a slab or a round column. 3. Enter the dimensions - length, width and thickness for a slab, or diameter and height for a column. 4. Set a waste percentage (5-10% is typical) and choose your bag size. 5. Read the volume and the number of bags you need to buy.

Formula & Definition

Slab volume = length × width × thickness Round column volume = π × (diameter / 2)² × height Volume with waste = volume × (1 + waste% / 100) Bags = round up( volume with waste / yield per bag ) For example, a slab 3 m × 4 m × 0.10 m is 1.2 m³; with 5% waste that is 1.26 m³, or about 70 bags of 80 lb mix.

Interpreting Results

The large figure is the raw concrete volume; the highlighted "with waste" value is what you should actually order, because some material is always lost to spillage, formwork gaps and an uneven base. One standard mixer-truck load is roughly 6-8 m³, so anything larger is usually cheaper as ready-mix delivery than as bagged product. Bag counts are rounded up - buy a little extra rather than running short mid-pour.

Frequently Asked Questions

How much concrete do I need? ▾

Multiply length × width × thickness for a slab, or use π × (radius)² × height for a column, then add 5-10% for waste. This calculator does it for you and converts to yards, feet and bags.

How many bags of concrete in a cubic yard? ▾

It depends on the bag yield: an 80 lb bag yields about 0.022 yd³, so roughly 45 bags fill a cubic yard; 60 lb bags need about 60, and 40 lb bags about 90.

Should I order bags or ready-mix? ▾

For small jobs (under about 1 m³ / 1 yd³) bagged mix is convenient. For larger pours, ready-mix delivered by truck is usually cheaper and far less work.

How much waste should I allow? ▾

Add 5-10% for spillage, formwork irregularities and an uneven sub-base. Use the higher end for rough ground or complex shapes.

What thickness should a slab be? ▾

Typical patios and paths are 100 mm (4 in); driveways and load-bearing slabs are often 150 mm (6 in) or more. Always follow local building codes.

This tool provides general estimates for planning only. Verify quantities against your supplier's figures and follow local building codes for structural work.