What is the standard concrete thickness for a floor slab?

For a residential floor slab, 4 inches (10 cm) is common for light use. Driveways often use 5–6 inches. Structural slabs must be designed by an engineer.

Can I use this calculator for ready-mix concrete orders?

Yes. The volume result (with waste) gives you a reliable quantity to quote with your ready-mix supplier. Always round up to the nearest 0.25 m³.

Concrete Calculator

Q: How much concrete do I need for a slab?

Multiply length × width × thickness to get the volume in cubic meters or cubic feet. Always add 8–10% waste to account for spillage and irregular forms.

Q: How many bags of cement per cubic meter of concrete?

A standard mix (roughly 1:2:3 cement:sand:gravel) uses about 300 kg of cement per m³, which equals about 12 bags of 25 kg. For a richer mix, use more bags.

Calculate the concrete volume, cement bags, sand and gravel you need for a slab, footing or any rectangular pour.

What this calculates

Calculates concrete volume (net and with waste allowance), estimated cement bags for hand mixing, sand and gravel quantities for a standard mix.

Formula used

Volume = length × width × thickness. Volume with waste = volume × (1 + waste %). Cement bags = volume × 300 kg/m³ ÷ 25 kg per bag (rounded up).

Worked example

A slab of 5 m × 4 m × 10 cm (4 in) has a net volume of 2.00 m³. With 8% waste: 2.16 m³. That requires about 26 bags of cement (25 kg each), 1.19 m³ of sand and 1.73 m³ of gravel.

Typical concrete thicknesses

Residential floor slabs: 10–12 cm (4–5 in). Driveways: 12–15 cm (5–6 in). Sidewalks: 10 cm (4 in). Always consult an engineer for structural applications.

When not to use this calculator

Do not use for structural slabs, elevated slabs, retaining walls, or any application requiring engineered mix design or reinforcement detailing.

Frequently asked questions

How much concrete do I need for a slab?

Multiply length × width × thickness and add 8–10% for waste. The result is the volume to order from your ready-mix supplier.

How many bags of cement per m³?

A standard mix uses about 300 kg of cement per m³, which equals about 12 bags of 25 kg. This calculator uses that ratio for hand-mixed concrete.

What thickness is standard for a floor slab?

10 cm (4 in) for light residential use. Driveways usually require 12–15 cm (5–6 in).

Can I use this for a ready-mix concrete order?

Yes. Use the volume with waste as your order quantity. Round up to the nearest 0.25 m³.

Assumptions

Cement: 300 kg/m³ (standard mix, suitable for H-20 grade).
Sand: 0.55 m³ per m³ of concrete.
Gravel: 0.80 m³ per m³ of concrete.
Bag size: 25 kg (adjust the bag count if your bags differ).
Does not include reinforcement (rebar, mesh), formwork or curing compounds.

Technical warning

Driveways, structural elements and heavily loaded slabs require engineered mix design, proper compaction and professional oversight.

Common mistakes

Not compacting the sub-base before pouring.
Adding too much water, which weakens the mix.
Forgetting expansion joints for large slabs.