Key Takeaways
- Volume is length times width times thickness, with thickness converted to feet
- Four inches is standard for patios and walkways; garage floors usually need more
- A 10 percent waste allowance covers spillage and an uneven subgrade
- Bagged concrete works for small slabs; ready-mix is more economical past about 1 cubic yard
Cubic yard is the standard unit ready-mix concrete is sold by, equal to 27 cubic feet.
A concrete slab is the simplest concrete shape to estimate, but the one detail that trips people up is thickness. Length and width are usually measured in feet without a second thought; thickness is a few inches and gets entered as feet by mistake, which inflates the volume by a factor of twelve.
Work out the volume
Multiply length by width to get the area in square feet, then multiply by the thickness in feet (inches divided by 12). A 10 by 10 foot slab at 4 inches thick covers 100 square feet and needs 100 x (4/12) = 33.3 cubic feet, or 1.24 cubic yards.
Add a waste allowance
Concrete spills, a subgrade is never perfectly level, and a truck chute or a wheelbarrow run always leaves some behind. Ten percent is a reasonable allowance for a straightforward rectangular slab; a slab with cutouts or an irregular shape can need more. Applied to the example above, 1.24 cubic yards becomes about 1.36 cubic yards.
Bags or ready-mix
Below about a cubic yard, bagged concrete mix is usually the more practical choice: a 60 lb bag yields roughly 0.45 cubic feet of concrete, so the 4 inch, 10 by 10 foot slab above needs about 82 bags. Past a cubic yard or so, ready-mix delivered by truck is typically both cheaper per yard and far less labor, since mixing 80-plus bags by hand or with a small mixer is a full day’s work on its own.
Check thickness against the job, not a default
Four inches suits a patio, a walkway or a shed floor. A garage floor or any slab carrying vehicle weight typically needs more, often 5 to 6 inches with reinforcement, and always needs to meet local code rather than a general default. Check with your local building department or a structural professional before pouring anything load-bearing.
Use the concrete slab calculator to run your own dimensions through this formula, including bag count and estimated cost. For smaller, repeated equipment pads (an AC pad or a generator base) see the concrete pad calculator instead, and for a driveway-scale pour see the concrete driveway calculator, which adds control joint spacing and truck load counts to the same volume math.