Concrete Slab Calculator
Cubic yards and bag counts for a floor, patio, shed pad or garage slab at any thickness.
This concrete slab calculator works out the ready-mix volume in cubic yards, or the number of pre-mix bags, for a poured pad — a shed base, patio, garage floor or walkway.
Enter the finished slab dimensions and the thickness in inches. The tool converts to a common unit, divides by 27 for cubic yards, and adds a waste allowance for uneven sub-grade.
How thick should a slab be?
Four inches is standard for patios, shed floors, sidewalks and residential garage floors. Go to five or six inches where vehicles heavier than a car will park, and thicken the edge to 8–12 inches (a turned-down edge) on slabs that carry a wall or frost load.
A slab is only as good as what is under it: 4 inches of compacted gravel base, a vapour barrier under heated or interior slabs, and control joints cut at roughly 24 to 30 times the slab thickness in feet to steer the inevitable cracking.
Quick coverage figures
At 4 inches thick, one cubic yard of concrete covers about 81 square feet. At 5 inches it covers about 65 square feet, and at 6 inches about 54 square feet. A 12 ft × 12 ft patio at 4 inches needs roughly 1.8 cubic yards.
For anything over about half a yard, order ready-mix. Mixing 25-plus bags by hand for a slab invites cold joints and an uneven finish.
Order a little extra
Add 5 to 10%. Sub-grade is never perfectly flat, forms bulge under the weight of wet concrete, and some is always lost to spillage and the chute. On a slab, running short means a cold joint you will see for the life of the pour.
Common slab mistakes
Skipping the gravel base. 4 inches of compacted crushed stone gives uniform support and drainage; without it the slab cracks over soft spots.
No control joints. Concrete will crack — control joints cut at 24–30× the thickness (in feet) decide where.
Rebar or mesh on the ground. Reinforcement does nothing lying in the dirt; it belongs in the middle third of the slab on chairs.
Pouring to the exact number. Order 5 to 10% over — you cannot add concrete to a slab that has started to set.
| Product | Yield | Per cubic yard | Best for | Notes |
|---|---|---|---|---|
| Ready-mix (delivered) | 1 yd³ per yd³ | — | Slabs, driveways, footings > ½ yd³ | Short-load fee under ~1 yd³ |
| 80 lb bag | ~0.60 ft³ | 45 bags | Small pads, posts, repairs | ~4,000 psi mixed |
| 60 lb bag | ~0.45 ft³ | 60 bags | Post setting, small footings | Common fence-post size |
| 50 lb bag | ~0.375 ft³ | 72 bags | Small repairs | |
| 40 lb bag | ~0.30 ft³ | 90 bags | Patching, setting small posts | |
| Cured concrete weight | — | ~4,000 lb (2 tons) | — | 145–150 lb/ft³ |
Every material calculator
Each of these uses the same measured-drawing tool with the right density and bag sizes built in: gravel calculator , mulch calculator , concrete calculator , sand calculator , topsoil calculator , paver sand calculator , crushed stone calculator , sonotube concrete calculator , pea gravel calculator , river rock calculator , decomposed granite calculator , road base calculator . Or start from the material calculator homepage and pick a material there.
Frequently asked questions
How many cubic yards of concrete do I need for a 20x20 slab?
At 4 inches thick, a 20 ft × 20 ft slab (400 sq ft) needs about 4.9 cubic yards. At 6 inches it needs about 7.4 cubic yards. Add 5 to 10% for waste.
How much concrete do I need for a 10x10 slab?
About 1.23 cubic yards at 4 inches thick, or roughly 1.85 cubic yards at 6 inches.
How thick should a concrete slab be?
Four inches for patios, sidewalks and shed or garage floors. Five to six inches for heavy vehicles, with a thickened edge where the slab carries a load.
How many 80 lb bags for a slab?
About 45 bags per cubic yard. A small 6 ft × 6 ft pad at 4 inches (0.44 cu yd) needs about 20 bags — near the limit of what is sensible to mix by hand.
How much does a concrete slab cost?
Ready-mix material is roughly $125 to $175 per cubic yard. A finished slab including forms, base, rebar and labour typically runs $5 to $10 per square foot.
Do I need gravel under a concrete slab?
Yes — about 4 inches of compacted gravel base gives uniform support and drainage, which reduces cracking and settlement.
How far apart should control joints be?
In feet, roughly 24 to 30 times the slab thickness in inches. A 4-inch slab gets joints every 8 to 10 feet, cut within the first day.
Can I pour a slab in sections?
You can, but each cold joint is a plane of weakness and a visible line. For a driveway or patio, pour monolithically or use planned, doweled construction joints.
How many bags of concrete for a 12x12 slab?
A 12 ft × 12 ft slab at 4 inches is about 1.78 cubic yards — roughly 80 × 80 lb bags. That is firmly ready-mix territory; hand-mixing that many invites cold joints.
How thick should a garage slab be?
Five to six inches for a residential garage, with a thickened 8–12 inch edge under any wall, and 4 inches of compacted gravel base beneath.
Do I need a vapour barrier under a slab?
Yes for any interior or heated slab, and for a garage in a damp climate — 6–10 mil poly under the slab, over the gravel, with taped seams.
How long before I can drive on a new slab?
Foot traffic after 24–48 hours, vehicles after 7 days, full design strength at 28 days. Keep it damp or covered for the first week to cure properly.
How much does a poured concrete slab cost?
Ready-mix material is roughly $125–$175 per cubic yard. A finished slab with forms, base, mesh and labour usually runs $5 to $10 per square foot.