Concrete Slab Calculator
Enter the slab's length, width and thickness to get the concrete volume in cubic feet and cubic yards, the amount to order with waste, and the bag count.
Volume
40 cu ft
Volume
1.48 cu yd
Concrete to Order
1.63 cu yd
Bags Needed
74 bags
How it works
A slab is a rectangular box, so its volume is length × width × thickness. Length and width are in feet and thickness is in inches, so the thickness is divided by 12 first. That gives cubic feet; dividing by 27 converts to cubic yards, which is how ready-mix concrete is sold.
The waste factor adds a margin on top. Some concrete is always lost to spillage, a sub-grade that is not perfectly flat, and forms that bow outward under the weight of wet mix. Ordering exactly the calculated volume and running short part-way through a pour is far more expensive than a little left over.
The bag count converts the volume with waste into bags of pre-mixed concrete using the yield printed on the bag: about 0.45 cu ft from a 60 lb bag and 0.60 cu ft from an 80 lb bag. It is rounded up to whole bags. For anything much over a cubic yard, ready-mix delivered by truck is usually cheaper and far less work than mixing bags.
Formula
cubic feet = length × width × (thickness ÷ 12) cubic yards = cubic feet ÷ 27 cubic yards with waste = cubic yards × (1 + waste ÷ 100) bags = ⌈ cubic feet × (1 + waste ÷ 100) ÷ yield per bag ⌉ yield per bag: 60 lb ≈ 0.45 cu ft, 80 lb ≈ 0.60 cu ft
Example
A 10 ft × 12 ft patio slab 4 in thick is 10 × 12 × 4⁄12 = 40 cu ft, which is 40 ÷ 27 = 1.48 cu yd. Adding a 10% waste factor brings the amount to order to 1.63 cu yd.
In bags, 40 cu ft plus 10% is 44 cu ft. At 0.60 cu ft per 80 lb bag that is 73.3 bags, so you would buy 74. With 60 lb bags at 0.45 cu ft each it is 97.8, so 98 bags.
Assumptions and limitations
- The slab is a rectangle of uniform thickness. Thickened edges, footings, curbs and steps need to be calculated separately and added.
- Bag yields are the manufacturer's standard figures: 0.45 cu ft per 60 lb bag and 0.60 cu ft per 80 lb bag. Actual yield varies a little with the mix and how much water is added; check the bag you are buying.
- The waste factor covers spillage, uneven sub-grade and form bulge. 10% is a common allowance; a rough or poorly compacted base can take more.
- Ready-mix suppliers sell by the cubic yard, usually in quarter-yard steps with a minimum load. Round the figure up to what they will deliver.
- Rebar, wire mesh, vapour barrier and the gravel base under the slab are not included.
Frequently asked questions
Should I order ready-mix or buy bags?
Run the numbers for your slab and look at the bag count. A 10 ft × 12 ft slab at 4 in needs around 74 80 lb bags, each of which has to be carried, mixed and placed before it starts to set. Most people switch to ready-mix somewhere between half a yard and a yard; above that, bags are slower, harder on the back and often no cheaper.
How thick should the slab be?
4 in is the usual minimum for patios, walkways and shed bases on well-compacted ground. Driveways and anything that carries vehicles are normally 5–6 in. Local building codes and the soil under the slab decide the final answer, so check before pouring anything structural.
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