JB PARCELConcrete Slab Calculator
Enter the dimensions and get the concrete: cubic yards before and after waste, what that costs delivered, and how many bags the same pour would take if you mixed it yourself.
How this calculator works
Volume is length times width times thickness, with the thickness converted from inches to feet, and a cubic yard is 27 cubic feet. That is the whole calculation, and it is also where the mistakes happen: the thickness is in different units from the other two sides, so an inch left unconverted is a twelve-fold error.
Add waste before you order. Concrete is consumed by an uneven subgrade, by spillage at the chute, and by the last wheelbarrow nobody wants to be short on. Five to ten percent is normal over a prepared base. Running out mid-pour is far more expensive than a little extra, because the joint between yesterday's concrete and today's is a permanent weakness.
Ready-mix is sold in quarter-yard steps, so the order figure rounds up to one. Below about a yard the plant's short-load fee can be larger than the concrete, which is the point where bags stop being the expensive option.
Footings use the same arithmetic with different numbers: the width is the footing width rather than the slab width, so a 60 foot run of 16 inch wide, 8 inch deep footing is 60 by 1.33 by 8 inches, which comes to just under two yards.
Worked example: a 24 by 24 foot garage slab
576 square feet at 4 inches thick, with a 10 percent waste allowance and concrete at $165 a yard.
| Volume | 192 cubic feet |
|---|---|
| Raw yards | 7.11 |
| Order | 8.00 yards |
| Concrete cost | $1,320 |
| Same pour in bags | 352 at 80 lb, or 470 at 60 lb |
The bag figures are why this slab is a delivery. Three hundred and fifty bags is a day of mixing for several people, and every batch boundary is a seam. Take the slab to 5 inches and the order goes to 10 yards: an inch of thickness is a quarter of the concrete.
Where the numbers come from
Volume is geometry, and the bag yields are the ones printed on 80 and 60 pound bags of standard concrete mix. Thickness, reinforcement, and base preparation are structural and local-code questions, and no figure here is a substitute for the building department that inspects the work.
Questions investors ask
How many cubic yards is a 24x24 slab?
At 4 inches thick, 7.11 cubic yards before waste, and 8 yards ordered once you allow 10 percent. At 5 inches it is 8.89 raw and 10 ordered. Thickness drives the number as hard as area does.
How many bags of concrete are in a yard?
Forty-five 80 pound bags, or sixty 60 pound bags, using the printed yields of 0.6 and 0.45 cubic feet. That is the arithmetic; the practical answer is that past about a yard, mixing bags costs more in time and in cold joints than the delivery saves.
How thick should a concrete slab be?
Four inches is the usual minimum for a patio, shed, or walkway. Five to six inches is normal where vehicles park, and thicker under a footing line or a heavy load. Thickness is a structural question your local code and soil answer, not a preference, so confirm it before you pour.
Do I need a permit for a slab?
Often yes, and it depends on size, location, and whether the slab supports a structure. Many jurisdictions exempt small at-grade pads and require a permit for anything a building sits on. Ask the building department that will inspect it; the exemption thresholds in the model codes are a starting point, not the local rule. See the rehab budget calculator for where permit and inspection costs land in a project.
How much does a yard of concrete cost?
It moves with the market and with how far the truck drives, so the price is an input on this page rather than a number baked into it. Ask two local plants, and ask what the short-load fee is below a yard and what the wait-time charge is after the first half hour on site.
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Screening a real property? Pull its appraised value, assessed value, acreage, land use, and last recorded sale off the JB PARCEL parcel map and drop the real numbers into this calculator instead of guessing. Coverage runs across every county we carry.