Concrete Slab Calculator — Patio, Shed Base & Driveway

About the concrete slab calculator

A slab is the most common concrete project, and also the easiest to get slightly wrong: a thickness of 4 in instead of 3.5 in changes the volume by more than 14 %. This calculator focuses on rectangular slabs and adds the numbers you need when ordering: coverage per cubic yard (or cubic metre) at your thickness, and bag counts at your bag size and at the other common sizes.

How to use it

Measure the inside of your formwork for length and width.
Enter the finished slab thickness. Check several points if the sub-base is not perfectly level and use the average.
Keep 10 % waste for most jobs, or adjust to suit your site.
Enter your bag size (and its printed yield if you have it) and local prices to compare options.

Formula

Slab area

A = length × width

Slab volume

V = A × thickness

With waste

V_order = V × (1 + waste % ÷ 100)

Coverage

area per unit volume = 1 ÷ thickness

Coverage tells you how much slab one cubic yard or one cubic metre covers at your chosen thickness. At 4 in, 1 yd³ covers 81 ft²; at 100 mm, 1 m³ covers 10 m².
Thickness is entered in inches (imperial) or centimetres (metric) and converted to metres internally.

Worked example: A 12 ft × 12 ft shed base, 4 in thick

Area = 12 × 12 = 144 ft²
Volume = 144 × (4 ÷ 12) = 48 ft³ = 1.78 yd³
With 10 % waste: 52.8 ft³ = 1.96 yd³
80 lb bags (0.60 ft³): 52.8 ÷ 0.60 = 88 bags; 60 lb bags (0.45 ft³): 117.3 → 118 bags

Result: About 2 yd³ of ready-mix, or 88 × 80 lb bags.

Assumptions and limitations

The slab is a simple rectangle of constant thickness. Thickened edges, footings or haunches must be added separately (use the Concrete Calculator for each part).
Bag yields are typical estimates unless you enter your product's printed yield.
Sub-base (gravel), vapour barrier, reinforcement and formwork are not included — see the Gravel Calculator for the base layer.

Frequently asked questions

How thick should a concrete slab be?

Around 4 in (100 mm) is common for patios and paths, while driveways and slabs carrying vehicles are usually thicker. Required thickness depends on loads, soil and local rules, so follow your plans or code.

How many square feet does a yard of concrete cover?

At 4 in thick, one cubic yard covers 81 ft². At 6 in it covers 54 ft². The calculator shows this coverage for your thickness.

Is it cheaper to mix bags myself?

For very small slabs, bags avoid delivery minimums. For larger slabs, ready-mix usually costs less per cubic yard and saves a great deal of labour. Enter both prices to compare.

Do I need a gravel base under the slab?

Many slabs are placed on a compacted granular sub-base, but requirements vary by soil and project. Your plans or local code should specify it; the Gravel Calculator can estimate the stone.

Sources and further reading

Results are estimates for planning purposes. See our disclaimer for details.