Building Codes Around the World on Chippy Tools

Concrete Calculations: The Complete On-Site Guide

Thu, 23 Jul 2026 00:00:00 +0000

Concrete is just sums until the truck turns up. On the day, every question comes down to the same few sums: how much volume, how many bags, what mix, and how deep the post holes. This page is the on-site concrete calculator, all in one place — each method worked out by hand, in metric. No rules to look up: a cubic metre is a cubic metre in every country.

Concrete for Post Holes: Volume Per Hole & Bags Needed

Thu, 23 Jul 2026 00:00:00 +0000

The method

A post hole is just a cylinder with a post standing in the middle of it. The concrete you buy fills everything the post doesn’t. So:

Concrete per hole = hole volume − post volume.

Hole volume is a cylinder: π × radius² × depth.
Post volume is the part of the post sitting inside the hole — its width times its width times how deep it’s buried.
Take one off the other, then multiply by the number of holes.

That’s the whole job. The only place people slip up is forgetting the post takes up space, or using the wrong radius — the hole size is usually given as the width across (the diameter), so halve it before you square it.

How Many Bags of Concrete Per m³? (20kg, 25kg, 40kg)

Thu, 23 Jul 2026 00:00:00 +0000

The method (weight over density)

There’s no code number here — concrete works the same in every country. The whole thing comes down to one idea: a bag of pre-mix makes about its own weight divided by the weight of a cubic metre of set concrete.

A cubic metre of set concrete weighs about 2200 kg. So a 20 kg bag makes about:

20 kg ÷ 2200 kg/m³ ≈ 0.009 m³

Flip that around and you get bags per cubic metre:

How to Calculate Concrete Volume for a Slab

Thu, 23 Jul 2026 00:00:00 +0000

The requirement

Concrete is sold by volume — the cubic metre (m³). So before you order a truck or count bags, you need one number: how much space are you filling?

For a slab, that space is a box. Length, width, and depth. Multiply the three together and you have your volume:

Volume = Length × Width × Depth

The only trick is units. Do the whole sum in metres and the answer comes out in cubic metres — nothing to convert. Slab thickness is nearly always given in millimetres, so that’s the one you change: divide by 1000. A 100 mm slab is 0.1 m deep. A 150 mm slab is 0.15 m.