---
title: "Concrete calculator — free, runs in your browser"
description: "Concrete volume for a slab, footing or post hole, in cubic metres and bags. Free, no sign-up, and nothing you enter leaves your browser."
url: https://atoneplace.net/tools/concrete-calculator/
site: "At One Place"
---

# Concrete calculator

Concrete volume for a slab, footing or post hole, in cubic metres and bags.

## How it works

Calculates the volume of concrete for a rectangular slab, a footing or a cylindrical post hole, then converts it to bags of dry premix.

The bag conversion is the part worth checking. A bag of premix yields far less than its dry weight suggests, because water and compaction reduce the volume \u2014 a 25 kg bag makes roughly 0.011 m³, so a modest 2 m × 1 m × 100 mm slab needs about eighteen bags. This is why mixing by hand stops being sensible somewhere around half a cubic metre; past that, ready-mix delivered is usually both cheaper and better.

Concrete weighs about 2,400 kg per cubic metre, so that same small slab is close to half a tonne. Formwork and any surface it sits on need to be up to it.

Add five to ten per cent for spillage, uneven substrate and over-dig. Running out mid-pour creates a cold joint, which is a permanent weakness rather than a cosmetic one.

> This tool runs entirely in your browser. Nothing you enter is sent to a server, logged or stored, and the page keeps working with the network disconnected.

## Common questions

### How many bags of concrete do I need per cubic metre?

Roughly 90 bags of 25 kg premix, since each yields about 0.011 m³. That is the point at which ready-mix becomes the sensible option.

### How much does concrete weigh?

About 2,400 kg per cubic metre, or 2.4 tonnes. A 100 mm slab weighs roughly 240 kg per square metre.

### Why not run out and buy more mid-pour?

Because concrete poured against a partly set surface forms a cold joint, which is a permanent structural weakness rather than a visible blemish.

## Related pages

- [Percentage calculator](https://atoneplace.net/tools/percentage-calculator/)
- [Loan and EMI calculator](https://atoneplace.net/tools/loan-calculator/)
- [BMI calculator](https://atoneplace.net/tools/bmi-calculator/)
- [Tip calculator](https://atoneplace.net/tools/tip-calculator/)
- [Age calculator](https://atoneplace.net/tools/age-calculator/)
- [Paint calculator](https://atoneplace.net/tools/paint-calculator/)
- [Compound interest calculator](https://atoneplace.net/tools/compound-interest-calculator/)
- [Calorie and BMR calculator](https://atoneplace.net/tools/calorie-calculator/)
- [Dog age calculator](https://atoneplace.net/tools/dog-age-calculator/)
- [Discount calculator](https://atoneplace.net/tools/discount-calculator/)

## Sources

- Typical range compiled from manufacturer specifications — At One Place (/about/)