Estimate cubic metres, bags, and materials for a rectangular slab pour.
Slab volume: 0.90 m³ (31.8 ft³)
Volume + waste: 0.99 m³
Cement: 216 kg
Sand: 480 kg
Gravel: 756 kg
Bag count (including waste):
This is a planning estimate. Structural slabs should follow the engineer's drawing for thickness, mix specification, and reinforcement.
Explore other useful tools
Overview hub covering slabs, footings, post holes, bags, and aggregate.
Cylinder-volume estimator for fence, deck, and mailbox post holes.
Estimate strip and pad footing concrete volumes and bag counts.
Convert between square metres, square feet, and other area units.
Convert between cubic metres, litres, cubic feet, and gallons.
Concrete slab volume is simply length × width × thickness. The tricky part is picking the right thickness, mix, and waste allowance for the job.
Reinforcement is out of scope for this estimate. Mesh sheets and rebar quantities depend on slab thickness, span, and load — get them from the project drawing or supplier guidance.
This concrete slab calculator estimates the volume of a rectangular slab and the cement, sand, gravel, and bag counts needed to pour it. It is part of our concrete calculators set, alongside dedicated post hole and footing tools.
Use it for patio slabs, shed bases, garage floors, and domestic driveways. For foundations, columns, and structural footings, use the concrete footing calculator instead — slab math and footing math are not interchangeable.
For a rectangular slab the formula is straightforward:
Volume (m³) = length (m) × width (m) × thickness (m)
Thickness is normally given in millimetres on UK projects and in inches on US projects. Convert before multiplying: 100 mm = 0.1 m, 4 in = 0.333 ft. The calculator above accepts the more natural units (mm or inches) and converts internally.
| Slab use | Typical thickness (metric) | Typical thickness (US) | Notes |
|---|---|---|---|
| Footpath / walkway | 75 mm | 3 in | Pedestrian only, well-compacted base |
| Patio | 100 mm | 4 in | General domestic patio loads |
| Shed base | 100 mm | 4 in | Light garden building, hardcore sub-base |
| Driveway | 150–200 mm | 6–8 in | Cars and light vans; thicker for heavier vehicles |
Bag counts come from dividing slab volume (including a waste allowance) by the yield per bag. Common yields:
Bag yields are manufacturer-specific — check the bag before ordering at quantity. For pours larger than about 1 m³, ready-mix delivery is usually cheaper and easier than bagged concrete.
1. Patio slab — 3 m × 3 m × 100 mm
2. Garage base — 6 m × 3 m × 150 mm
3. Driveway slab — 10 m × 4 m × 200 mm
This calculator estimates concrete volume only — it does not size reinforcement. For patios and small bases, reinforcement is often unnecessary if the subgrade is well-compacted. Driveways and garage floors normally use a single sheet of A142 or A193 fabric, set on chairs at mid-depth. Structural slabs come from the engineer's drawing, which will specify bar diameter, spacing, cover, and lap length.
Slab volume is length × width × thickness. For a 3 m × 3 m patio at 100 mm thickness, that is 3 × 3 × 0.1 = 0.9 m³. Add a 5–10% waste allowance for spillage, over-excavation, and finishing losses.
Footpaths and patios are usually 75–100 mm, shed bases 100 mm, garage floors 100–150 mm, and domestic driveways 150–200 mm. Heavier vehicle traffic, weak ground, and freeze-thaw exposure all push thickness higher.
Around 100 × 20 kg bags per cubic metre. A 3 m × 3 m × 100 mm patio (0.9 m³) is therefore roughly 90 × 20 kg bags. For 25 kg bags, divide m³ by 0.0125 — the same slab is about 72 × 25 kg bags.
A 1:2:3 mix (cement : sand : gravel) is widely used as a general-purpose slab mix. For driveways and slabs carrying heavier loads, a stronger 1:1.5:3 mix is common. Light-duty paths and patios can use 1:3:6.
Add 5–10% to a flat, well-prepared slab. Add closer to 10–15% if the subgrade is uneven, edges are rough, or the slab is being placed in awkward sections that increase spillage during the pour.
Patios and small bases often work without reinforcement when the slab is well-supported by a compacted base. Driveways, garage floors, and slabs carrying point loads typically use either a steel mesh sheet (e.g. A142 or A193) or a grid of rebar.
Pour each slab as a single placement where possible — cold joints between separately-poured sections are weaker than monolithic concrete. For very large slabs, plan control joints rather than separate pours, and have enough labour or ready-mix on site to keep the pour continuous.
No. A slab is a flat horizontal pour for a floor, patio, or base. A footing is a strip or pad beneath a wall or column that transfers structural loads to the ground. Use the footing calculator for foundations and footings.
Up to the hub: Concrete Calculator. Sideways: Concrete Post Hole Calculator and Concrete Footing Calculator. Supporting converters: Area, Volume, and Length.