Concrete Slab Calculator
Enter the length, width and thickness of your flatwork and this concrete slab calculator returns the volume in cubic yards and cubic meters, bag counts for 40, 60 and 80 pound mixes, the slab weight, and a material cost estimate. It works for patios, driveways, garage floors, sidewalks, shed pads and basement floors.
Concrete Slab Calculator
A slab spreads a thin layer over a large footprint, so thickness is the smallest number but it controls the volume. Half an inch of extra depth across a 400 square foot floor is roughly 0.6 cubic yards of concrete you did not plan for. Getting the depth right matters more than getting the length and width right.
How to calculate concrete for a slab
The formula is length times width times thickness, with all three in the same unit. The result is cubic feet. Divide by 27 to get cubic yards, the unit ready mix plants deliver in.
The common error is leaving thickness in inches while length and width are in feet. A 4 inch slab is 0.333 feet. Enter it that way, or set the thickness field to inches and let the tool convert.
Worked example, a 20 by 15 patio
Take a patio that is 20 feet long, 15 feet wide and 4 inches thick. The footprint is 20 times 15, or 300 square feet. Convert the thickness: 4 divided by 12 is 0.333 feet. Multiply: 300 times 0.333 equals 100 cubic feet. Divide by 27 and you get 3.70 cubic yards. That assumes a perfectly flat base, which never happens, so add 10 percent to reach 4.07 yards and order 4.25. If you poured this from 80 pound bags instead, 100 cubic feet at 0.60 per bag is about 167 bags, which is clear evidence that anything above one yard is faster as a ready mix delivery.
Slab thickness, strength and reinforcement
Thickness is set by the load the slab carries, not by preference. The table below gives typical thickness by use, the concrete strength usually specified, the common reinforcement, and the concrete needed per 100 square feet of floor.
| Application | Thickness | Strength | Reinforcement | Cu yd per 100 sq ft |
|---|---|---|---|---|
| Walkway, garden path | 3.5 to 4 in | 3,000 psi | Fiber or 6×6 mesh | 1.08 to 1.23 |
| Patio | 4 in | 3,000 to 3,500 psi | 6×6 welded wire mesh | 1.23 |
| Residential driveway | 4 to 5 in | 3,500 to 4,000 psi | No. 3 rebar at 18 in | 1.23 to 1.54 |
| Garage or shop floor | 5 to 6 in | 4,000 psi | No. 4 rebar at 16 in | 1.54 to 1.85 |
| Light warehouse floor | 6 to 8 in | 4,000 psi or more | Engineered rebar mat | 1.85 to 2.47 |
Reinforcement changes how a slab fails, not how much concrete it needs. Mesh or rebar holds cracks tight and lets a slab bridge small soft spots in the base. The concrete volume stays the same, so budget the steel separately. Thickness, rebar size and joint spacing for load bearing work should follow the structural drawings.
Common slab sizes at a glance
These figures are a quick sanity check against the calculator. All use a 4 inch slab and 80 pound bags before waste.
| Slab size | Cubic yards | 80 lb bags |
|---|---|---|
| 8 × 10 | 0.99 | 49 |
| 10 × 10 | 1.23 | 56 |
| 10 × 12 | 1.48 | 67 |
| 12 × 12 | 1.78 | 80 |
| 20 × 20 | 4.94 | 222 |
| 24 × 24 (garage) | 7.11 | 320 |
What a concrete slab costs
Slab price has two parts: the concrete itself and the labor to form, pour and finish it. Ready mix is sold by the cubic yard and set regionally, so two local quotes beat any national figure. Installed cost per square foot depends on thickness, finish and site access. A broom finished 4 inch patio sits at the low end; a stamped, colored or polished slab costs more because of the finishing work, not the concrete. The calculator estimates material cost from a price per yard or per bag that you enter. For a full installed price, get a contractor quote that includes base prep, forms, reinforcement and finishing.
Gravel base under a slab
A slab needs a compacted base. Use 4 to 6 inches of crushed stone, not pea gravel, and compact it in 2 inch lifts before pouring. The base improves drainage, limits moisture wicking up through the slab, and reduces cracking from soil movement. To size the stone, run the slab footprint through the calculator with the thickness set to your base depth.
Mistakes that leave a slab pour short
- Ignoring dips in the base. A sagging subgrade quietly adds tenths of a yard. Order toward the 10 percent figure when the base is hand graded.
- Forgetting a thickened edge or footing. A plain length by width by thickness volume leaves out a turned down perimeter. Add that concrete separately.
- Mixing units. Thickness in inches while the field reads centimeters throws the result off by about 2.5 times. Check the unit on each field.
- Ordering the exact computed volume. A short load means a cold joint. Carry 5 to 10 percent and round up to the plant delivery increment.
Related calculators
- Footing calculator for the footing under a thickened slab edge
- Column calculator for piers and posts next to a slab
- Bags calculator to convert the slab volume into an exact bag count
- How to calculate concrete guide for the full method
Frequently asked questions
How thick should a concrete slab be?
Patios and walkways are typically 4 inches. Driveways run 4 to 5 inches, garage floors 5 to 6 inches, and light commercial floors 6 to 8 inches. Thickness follows the load, so defer to the structural drawings for anything load bearing.
How many cubic yards do I need for a 20 by 20 slab?
A 20 by 20 slab is 400 square feet. At 4 inches that is about 4.94 cubic yards; at 6 inches it rises to 7.41. Add 5 to 10 percent and order toward 5.25 or 8 yards.
How many bags of concrete for a 10 by 10 slab?
About 56 bags of 80 pound mix or 74 bags of 60 pound, before waste. At 1.23 cubic yards this is right at the point where ready mix is worth pricing.
Do I need rebar or wire mesh in a slab?
Patios often use fiber or a single layer of 6 by 6 mesh. Driveways add No. 3 rebar at about 18 inches, and garage floors No. 4 rebar at 16 inches. Reinforcement controls cracking but does not change the concrete volume.
How long before I can use a new slab?
You can usually walk on it after 24 to 48 hours and drive on a driveway after about 7 days. Full design strength takes about 28 days. Keep the surface moist for the first week to limit shrinkage cracks.
Does this calculator include a thickened edge or footing?
No. It computes a uniform thickness slab. If your slab has an integral footing poured at the same time, add that footing volume with the footing calculator.
References
- Quikrete, Concrete Mix 1101 data sheet (bag yields, 4,000 psi at 28 days): https://www.quikrete.com/PDFs/DATA_SHEET-Concrete Mix 1101.pdf
- American Concrete Institute, ACI 360R Guide to Design of Slabs on Ground: https://www.concrete.org
- Portland Cement Association, Curing in Construction: https://cement.org/Learn/concrete-technology/concrete-construction/curing-in-construction
- ASTM C94, Standard Specification for Ready Mixed Concrete: https://www.astm.org/c0094_c0094m-22.html
Methodology: volume uses length times width times thickness in a common unit, divided by 27 for cubic yards; bag counts use published manufacturer yields. Estimates are for planning. Confirm slab thickness, joint spacing and reinforcement with a licensed engineer or local code. Recommended: attribute this page to a named author and add a review date.