From concrete volume to a shopping list
Start with the shape and thickness to find the volume to fill. Then choose delivered concrete, premixed bags or a site mix. This changes what you buy, not the slab’s size.
How much concrete for a 20 m² slab?
For a uniform thickness, multiply area by thickness in metres. An irregular outline uses the area measured by the plan. Where the thickness changes substantially, calculate each section separately and add the volumes.
Volume (m³) = area (m²) × thickness (cm) ÷ 100
| Entered thickness | Volume |
|---|---|
| 10 cm | 2.00 m³ |
| 12 cm | 2.40 m³ |
| 15 cm | 3.00 m³ |
| 20 cm | 4.00 m³ |
How much extra concrete should you allow?
An allowance covers losses and small variations in the base. A 5 × 4 m slab at 12 cm is 2.40 m³. With an illustrative 7% allowance, the requirement becomes 2.568 m³, displayed as about 2.57 m³.
Choose the allowance using your measurements and placing conditions. It cannot correct a badly uneven base or an incorrect thickness. Do not add it again to quantities that already include it.
Volume with allowance = volume × (1 + allowance ÷ 100)
Concrete bags and cement bags are not interchangeable
A premixed concrete bag already contains the dry ingredients. Bag count depends on the fresh volume in litres given on the product sheet. The bag’s weight alone does not tell you its yield.
Conversion example: Sika lists about 13 L of fresh concrete per 25 kg bag of Sikacrete-08 SCC. At that specific yield, converting 1 m³ gives 77 bags before allowance. This calculation does not confirm that the product suits your project. Its sheet specifies a layer thickness of 25–100 mm: check its intended uses and enter the yield of the product you actually choose.
A cement bag contains only the binder, so sand, aggregate and water are still needed. With an assumed cement dose of 350 kg/m³ and 25 kg bags, 2.568 m³ needs 898.8 kg of cement: 36 whole bags.
Concrete bags = round up(volume in litres ÷ bag yield)
Understanding sand, aggregate and water quantities
In this tool, cement, sand and aggregate parts are calculated by weight. With 350 kg of cement per m³ and the example ratio 1:2:3, the estimate is 700 kg of sand and 1,050 kg of aggregate per m³. These are not bucket counts: equal volumes of different materials can weigh different amounts.
A water/cement ratio of 0.50 gives 175 L of water per m³ here. For 2.568 m³, these assumptions produce 1,797.6 kg of sand, 2,696.4 kg of aggregate and 449.4 L of water. Decimal values are accepted when adjusting the ratio.
These figures explain the calculator’s estimate. They are not a validated mix design for your slab. Sand moisture, aggregates and the specified concrete also matter: use the supplier’s mix design. The screed preset removes coarse aggregate; a screed does not automatically replace a structural slab.
Compare bags, site mixing and delivered concrete
Cement cost uses whole bags to buy. Sand and aggregate costs use their estimated weights and your price per tonne. Delivered concrete uses the order volume and price per m³. Compare quotations for the same volume.
Allow separately for anything you have not entered: delivery, a pump if needed, mixer hire, formwork, reinforcement and labour. Ingredient cost is not the price of a finished slab. For several cubic metres, compare access and handling time as well as the material price.
Before ordering and pouring
Confirm the volume and delivery arrangements with your supplier, including site access, truck position and how the concrete will reach the pour. For bagged concrete, use the yield and instructions for your chosen product.
Plan how to protect and cure the concrete after finishing; the method and duration depend on the mix and site conditions. Have the required thickness and reinforcement confirmed for your project: a quantity estimate does not determine them.
Mistakes that change your order
- Entering 12 m instead of 12 cm: 12 cm is 0.12 m.
- Confusing 36 cement bags with 36 premixed concrete bags.
- Using a bucket recipe in fields calculated by weight.
- Confusing concrete volume with material weight, or forgetting delivery.
Common questions before pouring
How thick should a patio slab be?
The calculator estimates quantities for your entered thickness; it does not select it. Soil, loads, the base and intended use must be considered. Confirm the design before ordering. The 12 cm example is not a recommendation for every patio.
How many cement bags for 1 m³?
Divide the cement dose by bag weight. At the assumed 350 kg/m³, that is 14 bags of 25 kg or 10 bags of 35 kg, before allowance. The required dose depends on the specified mix.
Can I reuse my terrace outline?
Yes. Use “Continue my project” from the terrace result, or select a saved zone. The shape is retained; you then enter the slab thickness and other missing details.
Does the total include reinforcement and delivery?
Not automatically. It includes the priced items you entered. Add any charges and supplies missing from the list to your overall budget.
Source for the bag-yield example
Sika’s Sikacrete-08 SCC product sheet gives a product-specific yield. Check the sheet for the material you buy. Sika — Sikacrete-08 SCC