Concrete Slab Calculator
Calculate concrete for slab
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Concrete Slab Calculator is built for calculating concrete for slab — fast, free, and private. Enter Length (m), Width (m) and Thickness (m) in any order — Concrete Slab recalculates on every edit. In Concrete Slab the working sits beside the answer. Anyone comparing options in Construction can use Concrete Slab to compare scenarios. Nothing is uploaded while Concrete Slab works, and the figures stay put. Concrete Slab sits in the Construction section next to how many bags of concrete do i need calculator and concrete volume calculator in cubic yards. Concrete Slab is great for comparing scenarios. Change one value and watch. Concrete Slab stays bookmarked for next time.
What the defaults give
A slab 5 m long, 3 m wide and 150 mm thick needs 2.25 m³ of concrete.
That is about 90 bags of 40 kg mix at the calculator's yield, before any allowance for spillage or an uneven base.
The formula
Volume = length × width × thickness, all in metres
Bags = volume × 40 (40 kg bags at the calculator's assumed yield)
Thickness is the trap. Plans are often marked in millimetres, so 150 mm is 0.15 m; entering 150 gives a slab 1,000 times too large. Convert before multiplying.
Worked examples
5 × 3 × 0.15 m → 2.25 m³, about 90 bags
4 × 5 × 0.10 m → 2.00 m³, about 80 bags
3 × 2 × 0.15 m → 0.90 m³, about 36 bags
10 × 8 × 0.12 m → 9.60 m³, about 384 bags
6 × 4 × 0.10 m → 2.40 m³, about 96 bags
Bags versus ready-mix
Bags make sense up to roughly 1 m³, or about 40 bags, which one or two people can mix and place before it sets. Beyond that the labour and the risk of cold joints climb quickly, and a ready-mix truck is usually cheaper per cubic metre and far more consistent.
The bag count uses the site's own yield assumption. Real yields vary by product and by how much water you add, so check the bag label: a 40 kg bag of general-purpose mix typically fills 0.02–0.025 m³. Buy at least two extra bags for a small pour.
What the volume does not cover
The figure is the finished slab only. It excludes the sub-base of compacted hardcore, the sand blinding, formwork, reinforcement mesh, and any thickening at the edges. A garage slab with a 300 mm thick perimeter beam needs noticeably more concrete than the flat area suggests.
Add 5–10% for an uneven subgrade, and more if the ground falls away. A slab that runs 20 mm thick over 30 m² is an extra 0.6 m³, which is a quarter of this default pour.
Order ready-mix in whole cubic metres with a small surplus, and have somewhere to dump the leftover rather than letting it set in the chute.
Frequently Asked Questions
How much concrete for a 5 m by 3 m slab?
2.25 m³ at 150 mm thick, or about 90 bags of 40 kg mix. Multiply 5 × 3 × 0.15 to get the volume.
How many 40 kg bags make a cubic metre?
The calculator assumes 40 bags per cubic metre. Real yield depends on the mix and water content, so check the bag label before ordering.
Should I order ready-mix or use bags?
Bags are practical up to about 1 m³. Above that, ready-mix is faster, more consistent and usually cheaper per cubic metre.
How much extra concrete should I order?
5–10% above the calculated volume covers spillage and an uneven base. Add more if the subgrade slopes or the slab has thickened edges.
Does this include the sub-base?
No. The volume is for the concrete slab only, not the hardcore, sand blinding, mesh or formwork underneath and around it.
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