How to use the slab and pad calculator
Enter the slab's length, width and thickness. Everything else on the page is optional detail that turns a yardage figure into a full order: a thickened edge if you're building a shed or garage slab, the gravel base, reinforcement and the vapor barrier. The plan drawing shows where the control joints should go, which is the detail most first-time slabs get wrong.
A "pad" is just a small slab: AC units, generators, hot tubs, trash-can stands. The maths is the same; if you only need the yardage for a simple shape, the concrete calculator is quicker.
The formulas
slab = length × width × thickness
thickened edge = perimeter strip × edge width × extra depth
cubic yards = (slab + edge) × (1 + waste) ÷ 27
joint spacing ≈ 2.5 × thickness (in), read as feet
The thickened edge runs around the whole perimeter. Corners are counted once, so the strip's area is the slab area minus the inner rectangle that isn't thickened.
Worked example: a 20 × 12 ft pad, 4 inches thick
- Slab: 20 × 12 × (4 ÷ 12) = 80 ft³.
- Add 10%: 88 ft³, which is 88 ÷ 27 = 3.26 yd³.
- Gravel base, 4 in: 20 × 12 × (4 ÷ 12) = 80 ft³ = 2.96 yd³, about 4.3 tons.
- Joints: 2.5 × 4 = 10 ft maximum spacing. Splitting each side once gives 10 × 6 ft panels, which are too long and narrow, so the calculator splits the length twice instead: six panels of about 6.7 × 6 ft.
Add a 12 in wide edge thickened by 8 in and the order grows by 40 ft³ (1.48 yd³) before waste, to about 4.9 yd³ in total. Edges matter.
Control joints, explained
Concrete shrinks as it cures and it will crack. Control joints are grooves cut or tooled about a quarter of the slab's depth, so the crack happens in a straight line at the bottom of the groove instead of wandering across the surface. Guidelines from the Portland Cement Association and ACI 302:
- Space joints no more than about 2 to 3 times the thickness in inches, in feet (8-12 ft for a 4 in slab).
- Keep panels close to square. Long narrow panels (longer than 1.5 times their width) crack across the middle.
- Cut saw joints within the first day, as soon as the surface won't ravel.
- Put a joint at every inside corner and where the slab meets a step or a post.
Base, barrier and reinforcement
- Gravel base: 4 in of compacted crushed stone is typical under patios and sheds. It gives an even, drained support, and the calculator includes it as tons of crusher run.
- Vapor barrier: 10-mil or thicker polyethylene under interior slabs, overlapped about 6 in at seams. Outdoors it's usually left out.
- Rebar or mesh: neither stops cracks; both hold cracks tight once they form. Rebar must sit in the middle of the slab on chairs, not on the ground. The calculator counts a grid with 3 in of cover at the edges and 20 ft stock bars.
Common mistakes
- Forgetting the edge. A 12 × 8 in thickened edge on a 20 × 12 ft, 4 in slab adds half again to the concrete.
- Measuring forms at nominal size. 2×4 forms give a 3½ in slab, 2×6 forms 5½ in.
- No slope. Outdoor slabs should fall about ⅛ to ¼ in per foot away from buildings.
Questions people ask
How much concrete do I need for a 12x12 slab?
At 4 inches thick a 12 × 12 ft slab is 48 cubic feet, or 1.78 cubic yards. With 10% extra that's 1.96 yards, which most people round up to 2 yards of ready-mix (or about 88 bags of 80 lb mix).
How thick should a concrete pad be?
4 inches for a patio, walkway, shed or AC unit pad. 5 to 6 inches for a driveway, garage floor or anything carrying vehicles, and for a hot tub pad check the tub maker's installation guide, which often calls for 4 to 6 inches with reinforcement.
How far apart should control joints be?
A common rule from the Portland Cement Association and ACI 302 is no more than 2 to 3 times the slab thickness in inches, read as feet. So a 4 inch slab gets joints every 8 to 12 feet, in panels as close to square as you can make them.
Do I need rebar or wire mesh in a slab?
Plain 4 inch patios and walks on good ground are often poured without it, relying on control joints. Rebar or mesh helps hold cracks tight and is usual for driveways, garage floors and slabs on poor soil. Follow your plans or local code where they apply.
What is a thickened edge on a slab?
A deeper strip of concrete around the perimeter, often 8 to 12 inches deep and 8 to 12 inches wide, that stiffens the edge where loads and frost hit hardest. Sheds and garages on slabs commonly use one. It can add a third or more to the concrete order.
Do I need a vapor barrier under a slab?
Inside a building, or under any slab that will get flooring, a 10-mil (or thicker) polyethylene vapor retarder under the slab is standard practice. For an outdoor patio it's usually skipped.
Sources and assumptions
- QUIKRETE Concrete Mix (No. 1101) data sheet: 80 lb bag yields 0.60 ft³
- Portland Cement Association and ACI 302.1R guidance: joint spacing in feet about 2-3 × slab thickness in inches
- Rebar unit weights per ASTM A615 (#3 0.376, #4 0.668 lb/ft). Gravel base at 1.45 tons/yd³ (Vulcan Materials).
- Thickened edge is modelled as a rectangular strip; tapered edges use a little less concrete.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.