Concrete Slab Calculator
A 4-inch concrete slab requires 0.012 cubic yards per square foot. A 12 by 14 ft patio needs about 2.07 cubic yards or 96 sixty-pound bags. A standard 10 by 10 slab at the same thickness is 1.23 cubic yards or 74 bags.
Worked example: A 20 by 30 ft driveway at 5-inch thickness — (20 x 30 x 0.4167) / 27 = 9.26 cubic yards. With 10% waste: approximately 10.2 cubic yards.
Rebar: most residential slabs use number 3 or number 4 rebar at 18 inches on center. Driveways and garages need wire mesh or fiber reinforcement to handle vehicle loads without cracking.
Walk-on timing: concrete reaches 70% strength in 7 days. Foot traffic is safe after 24 to 48 hours. Vehicle traffic requires 7 days minimum. Full design strength arrives at 28 days.
Curing: keep the slab moist for the first 7 days to prevent shrinkage cracks. Cover with plastic sheeting or apply a curing compound immediately after finishing. Avoid letting the surface dry out during the first week — this is the most common cause of surface crazing on residential pours.
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Frequently Asked Questions
How long before I can walk on a fresh concrete slab?
You can walk on a concrete slab after 24 to 48 hours. Vehicle traffic requires 7 days minimum. Full strength is reached at 28 days.
What is the minimum thickness for a concrete driveway slab?
Residential driveways need at least 4 inches. For pickup trucks or heavy vehicles, use 5 to 6 inches and add rebar or wire mesh reinforcement.
Do I need rebar for a 4-inch patio slab?
Rebar is not required for a standard residential patio slab. Wire mesh (6x6 W1.4xW1.4) is the most common reinforcement for patios under 200 square feet. Larger slabs or areas with expansive soils benefit from number 3 or number 4 rebar at 18 inches on center.
Should I add a waste factor for my concrete order?
Yes. Add 5% for simple rectangular slabs and 10% for shaped pours or slabs with uneven subgrade. Pouring short is more costly than a small overorder.
How thick should a concrete slab be for a shed or garage?
4 inches is standard for garden sheds. A garage floor should be 4 to 5 inches for cars and 6 inches if you plan to store heavy equipment or do vehicle maintenance.
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