What the slab calculator does
A flat slab is the most common concrete pour around a home. Beyond the volume, a slab project needs to know its area (for sub-base, vapour barrier and reinforcement mesh) and its perimeter (for form boards). This calculator gives all three from one set of measurements.
For other shapes, like footings, columns or post holes, use the general concrete calculator.
How to use it
- Enter the slab length and width, measured to the inside of the forms.
- Pick a thickness from the list or choose Custom thickness.
- Adjust the waste allowance (10% by default).
- Optionally enter your supplier’s price per cubic yard or cubic metre.
Slab formulas
Area = Length × Width
Volume = Area × Thickness
Volume to order = Volume × (1 + Waste % ÷ 100)
Edge forms = 2 × (Length + Width)
In US units, convert thickness to feet (inches ÷ 12), multiply by the area in square feet, and divide by 27 for cubic yards. In metric, convert millimetres to metres (÷ 1,000) and multiply by the area in square metres.
Worked example: a two-car pad
A 20 ft × 20 ft slab, 4 inches thick, with 10% waste:
- Area: 20 × 20 = 400 sq ft
- Volume: 400 × (4 ÷ 12) = 133.3 ft³ = 4.94 yd³
- With 10% waste: 5.43 yd³
- Edge forms: 2 × (20 + 20) = 80 ft
- At 160 per cubic yard: 5.43 × 160 = 869.14
Metric: a 6 m × 6 m slab at 100 mm is 36 m² and 3.6 m³, or 3.96 m³ with 10% waste, with 24 m of edge forms. At 125 mm thick it rises to 4.5 m³ (4.95 m³ with waste).
Common slab thicknesses
The thickness a slab needs depends on its loads, the ground beneath it, reinforcement, climate and local building requirements. The figures below are commonly seen in residential guidance and are given only to help you check that an entry looks reasonable. They are not a specification.
| Use | Thickness often seen |
|---|---|
| Patios, paths, light shed bases | 4 in (about 100 mm) |
| Driveways and garage floors for cars | 4–6 in (about 100–150 mm) |
| Heavier vehicles, workshops, poor ground | Thicker, engineered design required |
Structural specifications must come from a qualified professional or the requirements of your local building authority, especially for garage floors, driveways and anything supporting a structure.
Understanding the results
- Concrete to order – volume including waste.
- Slab area – for estimating sub-base, membranes and mesh.
- Edge forms – total length of form boards around the perimeter.
- Estimated material cost – concrete only; excludes delivery, pumping, labour and reinforcement.
What can change the volume
- Uneven sub-base – every low spot is filled with concrete. Check depth across the slab with a string line.
- Thickened edges – many slabs are deeper at the perimeter; calculate the edge as a separate rectangular strip.
- Slope – slabs sloped for drainage keep the same thickness, so slope does not change volume, but an uneven base does.
Pouring with bags instead of ready-mix? Enter the volume in the concrete bag calculator. Estimate the stone sub-base with the gravel calculator. Our guide on how to calculate concrete volume has more examples.