Concrete Calculator
Calculate concrete volume for slabs, footings and columns, with cement, sand and aggregate quantities.
Please note: Material estimates for planning only. Always confirm quantities against your drawings, local building codes and supplier specifications before ordering. Structural work requires a qualified engineer.
What the Concrete Calculator does
Concrete volume is length by width by thickness for a slab, or the cross-section by height for a column. Because dry cement, sand and aggregate occupy more space than the mixed concrete they produce, the dry quantities are scaled up by roughly 54% before splitting them by the mix ratio.
Formula
Slab volume = Length × Width × ThicknessCircular volume = π × (Diameter ÷ 2)² × HeightOrder volume = Net volume × (1 + Waste %)Dry volume = Order volume × 1.54, split by the mix ratioCement bags = Dry cement volume × 1,440 kg/m³ ÷ Bag size
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Element | one of 4 options | Yes | — |
| Length | number | In some modes | For steps, this is the tread depth of one step. Accepts more than 0. Shown in 3 of the 4 modes. |
| Length unit | one of 6 options | In some modes | Shown in 3 of the 4 modes. |
| Width | number | In some modes | Accepts more than 0. Shown in 3 of the 4 modes. |
| Width unit | one of 6 options | In some modes | Shown in 3 of the 4 modes. |
| Thickness or depth | number | In some modes | For steps, this is the rise of one step. Accepts more than 0. Shown Element is Slab or footing (rectangular), or Element is Steps. |
| Thickness or depth unit | one of 5 options | In some modes | Shown Element is Slab or footing (rectangular), or Element is Steps. |
| Diameter | number | In some modes | Accepts more than 0. Shown Element is Circular column or pier. |
| Diameter unit | one of 5 options | In some modes | Shown Element is Circular column or pier. |
| Height | number | In some modes | Accepts more than 0. Shown Element is Column (rectangular), or Element is Circular column or pier. |
| Height unit | one of 6 options | In some modes | Shown Element is Column (rectangular), or Element is Circular column or pier. |
| Number of elements | number | Yes | Accepts 1 or more. |
| Number of steps | number | In some modes | Accepts 1 or more. Shown Element is Steps. |
| Concrete mix | one of 4 options | Yes | — |
| Waste allowance | % | Yes | Extra material for cuts, breakage and offcuts. 5–10% is typical, more for complex layouts. |
| Cement bag size | kg | Yes | Accepts 1 or more. |
| Ready-mix price per m³ | selected currency | Optional | Accepts 0 or more. |
| Currency | one of 10 options | Optional | — |
How to use it
- Choose Element and Concrete mix.
- Enter Number of elements, Waste allowance and Cement bag size.
- Fill in the remaining inputs the form shows for your choice.
- Optionally add Ready-mix price per m³.
- Select Calculate.
Worked example
A 5 m × 4 m slab, 100 mm thick, M20 mix (1:1.5:3), 10% waste, 50 kg bags.
- Length
- 5 m
- Width
- 4 m
- Thickness
- 100 mm
- Mix
- 1:1.5:3
- Waste
- 10
Net 2 m³, order 2.2 m³. Dry volume 3.388 m³ → cement 0.616 m³ = 887 kg = 18 bags, sand 0.924 m³, aggregate 1.848 m³.
Frequently asked questions
Why is the dry volume larger than the concrete volume?
Dry sand and aggregate contain air voids that fill with cement paste and water during mixing. The 1.54 factor is the standard allowance for that compaction.
How much waste should I allow?
5–10% for a simple slab. Increase it for complex shapes, pump losses, or uneven subgrade that soaks up extra depth.
Should I mix on site or order ready-mix?
Below roughly 1 m³ site mixing is usually practical. Above that, ready-mix is cheaper, faster and far more consistent.