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Concrete Slab Calculator — Yards, Bags and Rebar

Calculate concrete for a slab of any size or shape. Get cubic yards, bag counts, sub-base gravel, rebar length and an estimated cost as you type.

Add one row for every rectangle in the pour, and this concrete slab calculator totals the area, then sizes the gravel, steel and vapour barrier that go with it.

Slab sections

Pouring an L-shape or two separate pads? Add a section for each rectangle.

  • Section 1
in

Compacted gravel beneath the slab. Set to 0 to skip.

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$
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Enter your slab dimensions to see concrete, sub-base and rebar quantities.
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Concrete by Common Slab Size

Cubic yards and 80 lb bags for typical pours — tap any cell to load it into the form

Slab size4 in5 in6 in
10 × 10 ftshed or hot tub pad
12 × 12 ftpatio
12 × 20 ftsingle driveway
16 × 20 ftlarge patio
20 × 20 ftdouble garage
24 × 24 ftdouble garage, generous
30 × 40 ftworkshop or barn

What Goes Under and In a Slab

LayerTypical specWhy it is there
Compacted sub-gradeUndisturbed or tamped soil, no organic matter, graded to drainSoft or uneven soil settles unevenly, and a slab that loses support under one corner cracks across the span. Removing topsoil and roots and tamping fill in thin layers gives the gravel a firm bed to sit on.
Gravel sub-base4 in of compacted crushed stone (6 in for heavy loads or frost)It spreads the load, breaks the capillary path that draws groundwater upward and drains water away from the underside, which reduces frost heave. Crushed stone with fines locks together when compacted; round pea gravel does not.
Vapour barrier6 mil polyethylene, seams lapped 12 in and tapedConcrete is porous, and moisture vapour rising from the soil can lift flooring adhesive, rot stored boxes and leave the surface damp. The sheet also stops the dry gravel from pulling water out of the fresh mix.
Rebar or wire mesh#3 or #4 bar at 16 to 24 in centres, or 6×6 mesh, held mid-depth on chairsSteel does not prevent cracks. It holds them tightly closed so the two sides stay interlocked and level. It only works if it sits in the middle third of the slab, not lying on the gravel.
The slab4 in minimum; 5 to 6 in for vehicles; 3,000 to 4,000 psi mixThickness sets how much load the slab can bridge over small weak spots in the base. Higher psi resists freeze-thaw and surface wear, which is why exterior and garage pours specify a 4,000 psi, air-entrained mix.
Control jointsCut to ¼ of the thickness, spaced 24 to 36 × the thicknessConcrete shrinks as it cures, and it will crack somewhere. A joint is a deliberate weak line that makes the crack happen straight and hidden at the bottom of the groove instead of wandering across the surface.

How to calculate concrete for a slab

A slab is the simplest pour to calculate because it is a flat box. Multiply the area in square feet by the thickness in feet, then divide by 27 to get cubic yards. The step people miss is the thickness conversion: a 4 in slab is 4 ÷ 12 = 0.333 ft, a 5 in slab is 0.417 ft and a 6 in slab is 0.5 ft. Skipping that division overstates the volume by a factor of twelve.

Here is a full worked example for a garage floor. A slab measures 24 ft × 24 ft and is poured 6 in thick. The area is 576 sq ft, the volume is 576 × 0.5 ft = 288 cubic feet, and dividing by 27 gives 10.67 cubic yards. With a 10% allowance you order 11.73 cubic yards. Bagged, the same pour is 288 ÷ 0.60 = 480 bags of 80 lb, about 38,400 lb of dry mix to open, mix and place before the first batch stiffens. That is why a pour this size is always ordered as ready-mix.

For an L-shaped slab, split the outline into rectangles so that each corner of the L belongs to exactly one piece, and enter each as its own section. A 20 × 10 ft leg joined to a 10 × 12 ft leg is 200 + 120 = 320 sq ft. The rectangles must not overlap, or the shared corner is counted twice. The same method works for a T-shape, a pad with a step-out, or a driveway with a wider apron.

Sub-base comes next. Four inches of compacted gravel is the usual minimum because it spreads the load, drains water away and cuts frost movement. Under the 24 × 24 ft slab that is 576 × 0.333 ÷ 27 = 7.11 cubic yards compacted, or about 10 tons at 1.4 tons per cubic yard. Gravel compacts by roughly 20%, so order by the loose volume, which is around 8.5 cubic yards, to finish with a full 4 in after the plate compactor has run. If you are pouring over a basement, a deck or a balcony, check the weight of the slab per square foot first, because framing has to carry it. A slab also supports more than traffic: a block wall needs a footing sized for its weight, and the concrete block wall calculator shows how heavy that wall will be. A slab with a thickened or separate footed edge is calculated with the concrete footing calculator, which adds rebar and frost depth.

For reinforcement, a rebar grid at 16 in centres puts a bar every 16 in in both directions. Along one 24 ft side that is (24 × 12 ÷ 16) + 1 = 19 bars, each 24 ft long, and the same again the other way, giving 19 × 24 × 2 = 912 linear feet or 46 lengths of 20 ft bar. The number after the hash in a bar size is its diameter in eighths of an inch, so #4 bar is 4/8 = ½ in thick and #3 is ⅜ in. Overlap bars by 30 bar diameters, about 15 in for #4, and tie the crossings with wire.

Finally, plan for control joints. Space them at roughly 24 to 36 times the slab thickness, so a 6 in (0.5 ft) slab needs a joint every 12 to 18 ft in both directions, and a 4 in slab every 8 to 12 ft. Keep each panel close to square, because long thin panels crack between the joints. The perimeter figure above also tells you how many 8 ft form boards to order, and it deliberately over-counts any edge shared by two sections, which is the safe direction to be wrong when buying lumber.

How to use this calculator

  1. 01Enter the length and width of your slab, choosing a unit for each. Mixed units are fine.
  2. 02If the shape is an L, a T or two separate pads, tap + Add section and enter each rectangle on its own row, up to six.
  3. 03Set the thickness with the quick-pick buttons, then adjust the sub-base depth and the rebar spacing, or choose None for no rebar.
  4. 04Check the waste allowance and the price per cubic yard against your supplier. Both are editable.
  5. 05Read the concrete, sub-base, rebar and vapour barrier quantities, which update as you type, then confirm every figure with your supplier.

Common questions

How much concrete do I need for a 24x24 slab?
A 24 ft × 24 ft slab is 576 sq ft. At 4 in thick it needs 192 cubic feet (7.11 cubic yards), at 5 in it needs 240 cubic feet (8.89 cubic yards) and at 6 in it needs 288 cubic feet (10.67 cubic yards). Add 10% for waste, so a 6 in pour is an order of about 11.73 cubic yards.
How thick should a concrete slab be for a garage?
Use at least 4 in for a garage that only holds cars, and 5 to 6 in if you will park a truck, store heavy equipment or want extra margin against cracking. The thickness matters less than what sits beneath it: a uniform, compacted 4 in gravel sub-base prevents the soft spots that crack thin slabs. Thickening the edges into a monolithic footing is common for garages in cold climates.
Do I need rebar in a concrete slab?
Rebar does not stop a slab from cracking; it holds the cracks tight so the two sides stay level with each other. A 4 in patio on firm ground often does well with welded wire mesh or fibre, while a garage, driveway or any slab on poor soil benefits from #3 or #4 bar on a 16 to 24 in grid. Keep the steel in the middle third of the thickness, supported on chairs, because rebar lying on the gravel does almost nothing.
How much gravel do I need under a concrete slab?
Most slabs sit on 4 in of compacted gravel. Under a 24 ft × 24 ft slab that is about 7.11 cubic yards compacted, or roughly 10 tons at 1.4 tons per cubic yard. Crushed gravel compacts by around 20%, so order about 8.5 cubic yards loose to end up with a full 4 in after the plate compactor has run.
How do I calculate concrete for an L-shaped slab?
Split the L into two rectangles, find each area as length × width, and add them together. For example, a 20 ft × 10 ft leg plus a 10 ft × 12 ft leg is 200 + 120 = 320 sq ft. At 4 in thick that is 106.7 cubic feet, or 3.95 cubic yards, before waste.
What is the 6 mil vapour barrier under a slab for?
It is a sheet of 6 mil polyethylene laid over the gravel that stops ground moisture rising through the concrete. Without it, a slab that will carry flooring, furniture or stored goods can feel damp and cause adhesives and finishes to fail. Overlap the seams by about 12 in and tape them, which is why the calculator adds 10% to the slab area.
How far apart should control joints be in a slab?
A reliable rule is 24 to 36 times the slab thickness, measured in the same units. A 4 in slab needs joints every 8 to 12 ft, and a 6 in slab every 12 to 18 ft. Cut them to one quarter of the thickness within 6 to 18 hours of finishing, and keep the panels close to square.
How much does it cost to pour a 24x24 concrete slab?
The ready-mix alone for a 6 in slab is about 11.73 cubic yards including waste, which comes to roughly $1,640 at $140 per yard, or $1,525 to $2,110 across the $130 to $180 range. Sub-base gravel, rebar, forms, a pump if needed, finishing labour and delivery fees come on top, and a professional installed price commonly lands at two to three times the concrete cost. Use the estimate for budgeting and get a written quote.
Can I pour a concrete slab myself?
A small slab, such as a 10 ft × 10 ft shed base, is a realistic DIY job with a few helpers and ready-mix or a rented mixer. Anything over about 1 cubic yard should be ordered ready-mix, because you have roughly an hour to place and screed it before it stiffens. Plan for at least three people, hire a bull float and edging tools, and check your local code before pouring anything structural.
How long before I can drive on a new concrete slab?
Wait about 7 days before driving a car onto it, when the concrete has reached roughly 70% of its design strength. Keep heavy trucks, RVs and loaded equipment off for the full 28 days. In cold weather the clock runs slower, so add several days if temperatures stay below 50°F, and keep wheels away from the edges for the first 10 days.
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Related calculators

Results are estimates, not quotes. The cost figure uses a user-editable national-average ready-mix price and excludes labour, forms, reinforcement, pumping and delivery minimums. Sub-base and rebar figures are planning estimates, not engineering specifications. Confirm quantities with your supplier and check local code for structural slabs. For a full line-item price including labour and forms, try the Concrete Slab Cost Calculator. Last updated: October 1, 2026.