Concrete Slab Calculator
Size the concrete for a patio, driveway, garage floor, shed base or walkway.
Enter the slab dimensions and thickness and get cubic yards, cubic metres, bag counts, approximate weight and an optional cost.
The calculator handles rectangles, circles and triangles, so odd-shaped patios can be worked out in pieces and added together.
How to use the concrete slab calculator
Enter the slab length, width and thickness.
Add a waste allowance — 5% on good ground, 10% on rough.
Read cubic yards, bags and weight.
Why use the Build Tally concrete slab calculator
Patio to driveway
Works for any flat pour from a stepping stone to a garage floor.
Weight included
See the approximate tonnage for delivery planning.
Bag equivalents
Five bag sizes including 20 kg and 25 kg metric bags.
Free and private
Nothing you enter is uploaded.
Standard slab thicknesses
Most residential slabs fall into a narrow range. Walkways and patios: 4 inches. Standard car driveway: 4 inches on a well-compacted 4-inch gravel base. Driveways taking a heavy pickup, RV or delivery truck: 5–6 inches, often with reinforcement. Garage floors: 4–6 inches. Shed bases: 4 inches is ample for a garden shed, 6 for a workshop with machinery.
What goes under the slab matters more than the slab
A 4-inch slab on a properly compacted base outlasts a 6-inch slab on soft ground. Typical build-up is 4 inches of compacted crushed stone, a vapour barrier if the slab is inside or will be finished, then the concrete. Skipping compaction is the most common cause of cracked slabs, and no amount of extra thickness compensates for it.
Control joints and cracking
Concrete shrinks as it cures and will crack — the question is only whether the cracks are where you chose. Control joints cut about a quarter of the slab depth, spaced in feet at roughly 2–3 times the thickness in inches (so 8–12 feet for a 4-inch slab), give the shrinkage somewhere to go. Cut them within 6–18 hours of the pour.
Read the guide: How Much Concrete Do I Need? A Straight Answer for Slabs, Footings and Posts
Frequently asked questions
How many yards of concrete for a 20x20 slab?
A 20 × 20 ft slab at 4 inches is about 4.94 cubic yards. At 6 inches it is 7.41 cubic yards. Add 5–10% for waste.
How thick should a driveway slab be?
Four inches for cars on a compacted base; five to six inches for heavier vehicles, usually with reinforcement.
Do I need rebar or mesh in a slab?
Most residential slabs use either welded wire mesh or rebar to control cracking. Requirements vary by code and use — check locally.
How long before I can walk on a new slab?
Usually 24–48 hours for foot traffic, seven days before driving on it, and 28 days to reach full design strength.
What base do I need under a slab?
Typically 4 inches of compacted crushed stone, plus a vapour barrier for interior or finished slabs.
Can I pour a slab in sections?
Yes, with proper construction joints. Calculate each section separately and add them.
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