Concrete Slab Calculator
Size a concrete slab: cubic yards, bags, weight and cost for patios, driveways, sheds and garage floors.
What you need
1.96cu yd
Concrete needed — Order this much ready-mix, waste allowance included.
- Same amount in cubic feet
- 52.8 cu ft
- Bags needed
- 88
- Approximate weight when cured
- 3.96 US tons
Rounded up. Mixing more than about 20 bags by hand is a long day.
Estimates for planning. Confirm quantities with your supplier before ordering — see the assumptions below for the figures used.
How to use the concrete slab calculator
A slab is the most common concrete pour a homeowner takes on, and the one where thickness is most often guessed. Enter your dimensions and slab thickness to get the volume, bag count and estimated cost — plus guidance below on how thick it actually needs to be.
What this calculator assumes
Every estimate depends on figures that vary by supplier and conditions. These are the ones used here, so you can sanity-check the result against your own quote.
- Bag yields are the manufacturer's published figures: 0.30 cu ft for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb and 0.60 for 80 lb.
- Cured concrete is taken as 150 lb per cubic foot (about 4,050 lb per cubic yard).
- A waste allowance is included in the volume shown. Ready-mix suppliers usually have a minimum order and charge for short loads.
- The calculation covers concrete only. A slab also needs a compacted base — usually 4 to 6 inches of crushed stone — which you can size with the gravel base calculator.
- Reinforcement, vapour barrier and control joints are not included in the volume.
Frequently asked questions
How thick does a concrete slab need to be?
Four inches handles foot traffic, patio furniture and a garden shed. Go to 5 inches for a car driveway and 6 inches with rebar for a garage floor, RV pad or anything with truck traffic. Thickened edges — a 12 inch deep perimeter beam — do more to prevent cracking than uniform extra depth.
Do I need gravel under a concrete slab?
Yes, in almost every case. Four to six inches of compacted crushed stone gives you uniform support, breaks the capillary path for ground moisture, and lets water drain away rather than freezing under the slab. Concrete poured straight onto clay cracks.
Does a slab need rebar or mesh?
Fibre mesh in the mix controls surface cracking and is enough for a small patio. Steel — rebar on chairs at mid-depth, or welded wire mesh — is what holds the slab together once it has cracked, and is worth it for driveways, garage floors and any slab over about 10 feet in a dimension.
How many 80 lb bags of concrete are in a cubic yard?
Forty-five. An 80 lb bag yields 0.60 cubic feet and a cubic yard is 27 cubic feet. That is around 3,600 pounds of bags to mix by hand — past roughly 1 cubic yard, ready-mix or a rented mixer stops being optional.
Should I order ready-mix or mix bags myself?
Bags make sense up to about half a cubic yard — post holes, small pads, repairs. Beyond that, ready-mix is cheaper per yard, far faster, and gives a consistent mix with no cold joints. Most suppliers have a 1 yard minimum plus a short-load fee.
How much extra concrete should I order?
Ten percent is the usual allowance. Subgrade is never perfectly flat, forms bow slightly, and running short mid-pour forces a cold joint you cannot undo. If your slab is deep or the subgrade is rough, go to 15%.
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