Concrete Slab Calculator
Work out everything a concrete slab needs — volume, bags or ready-mix, and reinforcement mesh sheets — from the slab size and thickness.
A slab is more than a volume of concrete: it needs the right thickness, reinforcement, and a sub-base beneath it. This calculator covers the slab itself — volume, bags, ready-mix and mesh — in one place.
For the gravel sub-base underneath, use the gravel calculator with the same length and width and a 4 in depth.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Measure the slab length and width, and choose the thickness.
- Set wastage at 10% unless the ground is very uneven.
- Enter a bag size for a bag count, or a ready-mix price per cubic yard for a delivered cost.
- Enter one mesh sheet's area to get the number of reinforcement sheets.
Worked example
| Slab | 20 ft × 15 ft × 4 in thick |
|---|---|
| Area | 20 × 15 = 300 ft² |
| Volume | 300 × (4 ÷ 12) = 100 ft³ = 3.70 yd³ |
| With 10% wastage | 100 × 1.10 = 110 ft³ = 4.07 yd³ |
| 80 lb bags | ⌈110 ÷ 0.60⌉ = 184 bags |
| Mesh (50 ft² sheets) | ⌈300 ÷ 50⌉ = 6 sheets |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
What is the difference between this and the concrete calculator?
This page is slab-specific: it adds the slab area, a ready-mix price per cubic yard, and reinforcement mesh sheets. The concrete calculator is the general volume-and-bags tool for any rectangular pour.
Do I need reinforcement mesh in a slab?
For most ground-bearing slabs, yes — mesh controls cracking as the concrete cures and moves. A 4 in patio slab usually takes one layer of mesh on chairs; structural slabs need an engineer's design.
How thick should my slab be?
4 in (100 mm) for patios, paths and shed bases. 4–6 in (100–150 mm) for driveways. Anything carrying heavy vehicles or a building needs a structural design, not a rule of thumb.
How long before I can walk or drive on a new slab?
Walk on it after 24–48 hours. Light use after 7 days. Full strength takes about 28 days — keep cars off a new driveway for at least 7 days, ideally 14.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.