How much concrete does a floor need?
A floor concrete calculator multiplies the floor's length by its width to get the area, then by the slab thickness to get the volume of concrete. If you choose a nominal mix ratio, it also estimates how many 50 kg bags of cement and how much sand and coarse aggregate you'd need to mix that volume on site.
The formulas
- Floor area = Length × Width
- Concrete volume = Area × Thickness
- Dry material volume = Concrete volume × 1.54 × (1 + wastage %)
- Each material = Dry volume × its share of the ratio (for 1:2:4, cement is 1/7, sand 2/7, aggregate 4/7)
- Cement bags = Cement volume × 1,440 kg/m³ ÷ 50 kg
Length and width default to meters and thickness to millimeters, but each accepts mm, cm, m, ft or in. Volume is shown in m³ and ft³ to three decimal places. The default wastage is 5%, and the material estimate is off unless you pick a ratio.
Worked example
A garage floor 10 m × 8 m, 100 mm thick, mixed on site at 1:2:4, with 5% wastage:
- Area: 10 × 8 = 80.000 m².
- Volume: 80 × 0.1 = 8.000 m³ (282.517 ft³).
- Dry volume: 8 × 1.54 = 12.32 m³, plus 5% = 12.936 m³.
- Cement: 12.936 × 1/7 = 1.848 m³, × 1,440 ÷ 50 = 53.22 bags.
- Sand: 12.936 × 2/7 = 3.696 m³.
- Aggregate: 12.936 × 4/7 = 7.392 m³.
In feet and inches: a 20 ft × 12 ft slab, 4 in thick, is 80.000 ft³ (2.265 m³). Divide cubic feet by 27 for cubic yards, the unit most US ready-mix suppliers use, which gives about 2.96 yd³.
Materials per cubic meter by mix ratio
These figures come straight from the calculator for 1 m³ of concrete with 0% wastage. Multiply by your volume, then add wastage.
| Mix (cement:sand:aggregate) | Label in the calculator | Cement (50 kg bags) | Sand (m³) | Aggregate (m³) |
|---|---|---|---|---|
| 1:1.5:3 | Higher strength | 8.06 | 0.420 | 0.840 |
| 1:2:4 | General use | 6.34 | 0.440 | 0.880 |
| 1:3:6 | Mass concrete | 4.44 | 0.462 | 0.924 |
These are nominal volume-batched mixes. They don't guarantee a particular compressive strength, which depends on the materials, the water content and curing. Where a strength grade is specified, use a designed mix or ready-mix ordered to that grade.
Steps for estimating a floor slab
- Measure the inside of the formwork, or the area between walls for an internal floor.
- Decide the slab thickness from your drawings or local building rules.
- Check the sub-base is level. Low spots make the slab thicker and use more concrete than the nominal thickness suggests, so measure depth in several places and use an average.
- For ready-mix, take the volume and add a margin for spillage and uneven ground. For site mixing, choose the ratio and let the calculator add wastage to the materials.
- Round cement up to whole bags.
Limitations
- Assumes a flat slab of uniform thickness. Thickened edges, internal beams and ramps need to be added separately.
- Doesn't include the sub-base, damp-proof membrane, reinforcement mesh or water.
- The 1.54 factor, 1,440 kg/m³ cement density and 50 kg bag are fixed estimating conventions in this calculator.
Slab thickness, reinforcement and mix strength for a structural or heavily loaded floor should follow your local building code and be confirmed by a qualified engineer.
Related calculators
- Concrete calculator: volume and materials for any rectangular pour.
- RCC slab calculator: suspended reinforced slabs rather than ground-bearing floors.
- Tile calculator: tiles to finish the floor once it's cured.
- Volume converter: convert m³, ft³ and cubic yards.
Last updated