# How to calculate concrete for a slab, beam, column or footing

> Work out concrete volume from the dimensions, add a waste allowance, then split the dry volume by the mix to get cement, sand, aggregate and water.

- By: Editorial Team, Somali Construction Review (https://somaliconstructionreview.com/about)
- Published: 27 September 2026
- Format: Practical guide (general practice)
- Publication: Somali Construction Review (https://somaliconstructionreview.com)
- Subject: Materials
- Tags: concrete, materials, calculation, slab, mix-design
- Web version: https://somaliconstructionreview.com/blog/how-to-calculate-concrete

## Key points

- Concrete is calculated in three stages: volume from the dimensions, volume plus a waste allowance, then dry volume split by the mix ratio into cement, sand, aggregate and water.
- The site's concrete calculator at /concrete-calculator runs these stages for a slab, beam, column or pad footing and returns bags of cement, sand, aggregate and water.
- A nominal volume mix is site practice, not a design; structural concrete follows the engineer's specification and is checked with cube tests.

Concrete is calculated in three stages. First work out the volume of the member from its dimensions. Then add a waste allowance so the pour is not short. Then convert the wet volume to dry volume and split it by the mix ratio into cement, sand, aggregate and water. A ground slab 5 m long, 4 m wide and 150 mm thick at 1 : 2 : 4 with a five percent waste allowance takes 3.0 m³ of concrete, 20 bags of 50 kg cement, 1.386 m³ of sand, 2.772 m³ of aggregate and 499 litres of water.

## How do you work out the volume of a slab, beam, column or footing?

Match the formula to the shape, then multiply by how many you are casting. A slab is length times width times thickness. A beam or a pad footing is length times width times depth. A column is width times depth times height, or for a round column pi divided by four, times the diameter squared, times the height. Add the volumes of every member in the pour before you order anything. A ground slab 5 m long, 4 m wide and 150 mm thick comes to 3.0 m³. Openings, sloping soffits, thickenings and haunches are not in that figure unless you add them, so read the drawing and list them yourself. The [concrete calculator](/concrete-calculator) takes these dimensions for a slab, beam, column or pad footing and works the volume out for you.

## Why does the dry volume factor change the answer?

Loose dry cement, sand and aggregate take more room than the finished concrete, because water fills the voids between the particles. That is why the wet volume is multiplied by a dry volume factor before the mix is split. The calculator as set up uses 1.54, and about 1.52 to 1.57 is widely used on site as a rule of thumb rather than a requirement. The waste allowance is a separate input and covers spillage, over-excavated bases and uneven formwork. Many sites allow two to ten percent, and the right figure depends on the work and the crew. In the worked example, 3.0 m³ with a five percent waste allowance becomes 3.15 m³, and 3.15 times 1.54 gives 4.851 m³ of dry material to split.

## How do you split the dry volume into cement, sand, aggregate and water?

Add the parts of the mix together, then give each material its share of the dry volume. In 1 : 2 : 4 there are 7 parts, so cement takes 1/7 of 4.851 m³, which is 0.693 m³. Sand takes 2/7, which is 1.386 m³, and aggregate takes 4/7, which is 2.772 m³. Convert cement to bags by multiplying its volume by its bulk density of 1,440 kg per cubic metre, then dividing by the bag weight and rounding up: 0.693 times 1,440 gives 997.9 kg, or 19.96 bags of 50 kg, so buy 20. Water is the cement weight times the water-cement ratio; at 0.5 that is 499 litres, since 1 kg of water is 1 litre. Every one of those figures is an input you can change: the waste allowance, the dry volume factor, the cement bulk density, the bag weight and the water-cement ratio. Lower water gives stronger, more durable concrete if the mix can still be placed and compacted, and the specification or the mix design sets the maximum.

> **Warning:** A nominal volume mix such as 1 : 3 : 6 for blinding, 1 : 2 : 4 as a general site mix, or 1 : 1.5 : 3 where more cement is specified does not guarantee a strength. The strength a structural member needs is set by its engineer as a designed mix or a strength class and checked with cube tests. Treat every calculator figure as an estimate for planning and ordering.

## What to do next

Write down the dimensions and count of every member in the pour, decide the waste allowance with the site team, and run the figures through the [concrete calculator](/concrete-calculator). Then check the cement bag weight and the mix and water-cement ratio against the project specification before ordering, and book cube tests for any structural concrete.

## Questions

### Does the calculator replace a mix design?

No. It estimates quantities by the nominal volume-batching method used on site, using inputs you can change. It does not replace the engineer's specification or a mix design, and structural concrete is checked with cube tests.

### What waste allowance should I use?

It depends on the work and the crew. Many sites allow two to ten percent to cover spillage, over-excavated bases and uneven formwork, and you set the figure as an input.

### How much water do I add?

Water is the cement weight times the water-cement ratio. At a ratio of 0.5 with 997.9 kg of cement that is 499 litres, since 1 kg of water is 1 litre. The specification or the mix design sets the maximum ratio.

## Sources

- [Concrete calculator (Somali Construction Review)](https://somaliconstructionreview.com/concrete-calculator)