Concrete Slab Calculator
Concrete for patios, garage & shed floor slabs
Planning a patio, garage, shed or driveway slab? Enter the slab dimensions and thickness to get the concrete you need in cubic yards and in bags. Below you'll find a slab size chart, the bags-vs-ready-mix decision, a worked patio example, real costs, and the mistakes that ruin a pour.
How thick should a slab be?
Thickness is driven by what will sit on the slab, not by its size. Going thicker is cheap insurance compared with breaking out a cracked slab later.
| Use | Thickness | Reinforcement | Gravel base |
|---|---|---|---|
| Walkway or path | 4 in | Fibre mesh or none | 4 in |
| Patio | 4 in | Wire mesh optional | 4 in |
| Shed floor | 4 in | Wire mesh | 4 in |
| Driveway (cars) | 5 to 6 in | Rebar or mesh | 4 to 6 in |
| Garage floor | 4 to 6 in | Rebar | 4 to 6 in |
| RV pad or heavy truck | 6 to 8 in | #4 rebar grid | 6 to 8 in |
The printable material coverage chart has the same slab volumes plus the gravel-base and bags-per-yard tables, which is what you actually need when the base and the pour are ordered together.
Concrete slab size chart
Cubic yards of concrete for common slab sizes. These are bare volumes, so add about 10% on top for spillage, uneven subgrade and the bit you always need at the far corner.
| Slab size | Area | 4 in thick | 5 in thick | 6 in thick |
|---|---|---|---|---|
| 10 × 10 ft | 100 ft² | 1.23 yd³ | 1.54 yd³ | 1.85 yd³ |
| 12 × 12 ft | 144 ft² | 1.78 yd³ | 2.22 yd³ | 2.67 yd³ |
| 10 × 20 ft | 200 ft² | 2.47 yd³ | 3.09 yd³ | 3.70 yd³ |
| 12 × 20 ft | 240 ft² | 2.96 yd³ | 3.70 yd³ | 4.44 yd³ |
| 20 × 20 ft | 400 ft² | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ |
| 24 × 24 ft (2-car) | 576 ft² | 7.11 yd³ | 8.89 yd³ | 10.67 yd³ |
Concrete bags vs. ready-mix for a slab
Bagged mix and ready-mix are priced completely differently, and the crossover comes sooner than most people expect. The table below gives concrete bags per cubic yard for every common bag size.
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 bags |
| 50 lb | 0.375 ft³ | 72 bags |
| 60 lb | 0.45 ft³ | 60 bags |
| 80 lb | 0.60 ft³ | 45 bags |
Under about half a cubic yard, bags win. Between half and one yard it's a judgement call. Past one cubic yard (that's 45 eighty-pound bags to mix by hand) ready-mix is cheaper, stronger and far more consistent.
There is also a quality argument. A slab poured in batches from a mixer has cold joints between loads and varying water content; a ready-mix truck delivers the whole slab as one continuous, uniformly mixed pour.
Worked example: concrete for a 12 × 20 ft patio slab
A 12 by 20 ft patio is 240 ft², poured 4 inches thick over a 4 inch gravel base.
| Item | Working | Order |
|---|---|---|
| Concrete volume | 240 ft² × 0.333 ft | 80 ft³ = 2.96 yd³ |
| Plus 10% | 2.96 × 1.10 | 3.26 yd³, order 3.5 |
| Gravel base (4 in) | 240 ft² × 0.333 ft +10% | ~4.6 tons |
| If bagged instead | 88 ft³ ÷ 0.6 | 147 × 80 lb bags |
147 bags makes the decision for you: this is a ready-mix job. Ready-mix is usually sold in quarter-yard increments, so round up rather than risk stopping mid-pour. Run the base through the gravel calculator to price the stone separately.
Concrete slab cost: what it costs to pour a slab
Use the price field above as a concrete slab cost calculator: enter your local rate per cubic yard and it prices the pour. Ready-mix commonly runs $120 to $220 per cubic yard delivered, so the concrete delivery cost is usually the largest single line. Small loads attract a short-load fee of roughly $50 to $150 when you order under about 3 to 4 yards, which is why a tiny slab can look surprisingly expensive per yard.
Bagged mix is more expensive per yard, not less: at roughly $5 to $7 for an 80 lb bag, one cubic yard of bagged concrete costs around $225 to $315 before you count your own labour. Bags only win on very small pours where the short-load fee would dominate.
For a patio specifically, concrete patio cost lands around $8 to $16 per square foot installed, or roughly a third of that if you pour it yourself. Budget separately for form lumber and stakes, the gravel base, wire mesh or rebar, a vapour barrier for interior slabs, and tool hire. Prices vary by region, so treat the calculator's cost line as a planning figure and get a local quote.
Base, gravel and rebar
A slab is only as good as what's under it. Pour over 4 to 6 inches of compacted gravel for drainage and a stable, frost-resistant base, and add wire mesh or rebar for anything carrying vehicles. Set reinforcement on chairs so it sits in the middle of the slab depth; mesh lying on the subgrade does nothing at all.
Curing a slab so it doesn't crack
Concrete does not dry, it reacts, and that reaction needs water. Pour on a dry day above 50°F, and keep the surface damp (or covered with plastic or curing compound) for at least 3 to 7 days. Concrete reaches roughly 70% of its strength in the first week and continues gaining for a month.
Cut or tool control joints about every 8 to 10 feet, to a quarter of the slab depth, within the first day. Slabs crack as they shrink; control joints simply decide where. Skipping them is the difference between a neat line and a random crack across your patio.
Slab mistakes that ruin a pour
- Pouring on bare soil. No base means settling, cracking and frost heave.
- Ordering the exact volume. Uneven subgrade eats concrete. Always add 10%.
- No control joints. The slab will crack; joints decide where.
- Reinforcement on the ground. Mesh or rebar must sit mid-depth on chairs to do anything.
- Adding water to make it easier to work. Extra water dramatically weakens the finished slab.
- Finishing while bleed water sits on top. Trowelling that back in causes a dusty, flaking surface.
- Letting it dry fast in sun or wind. Fast drying means a weak, crazed slab. Keep it damp.
How this concrete slab calculator works out its numbers
Volume is area × thickness at 27 ft³ to the cubic yard, with 10% added for spillage and uneven subgrade. Bag counts use the yields printed on the bag by the major manufacturers: an 80 lb bag yields 0.60 ft³, 60 lb yields 0.45 ft³, 40 lb yields 0.30 ft³.
- Bags per cubic yard follows directly: 27 ÷ 0.60 = 45 at 80 lb, 60 at 60 lb, 90 at 40 lb.
- Density is taken as 150 lb/ft³ for normal-weight concrete.
- Thickness guidance reflects common residential practice (4 in for patios and shed floors, 5 to 6 in for vehicle loads) rather than any single code. Check your local building code before pouring anything structural.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Concrete Slab Calculator: frequently asked questions
How much concrete for a 10x10 slab?
A 10×10 ft slab at 4 inches thick needs about 1.23 cubic yards, or roughly 1.36 cubic yards once you add 10% for waste. In bags that is about 61 eighty-pound bags, which is right at the point where ready-mix becomes the better option.
How many bags of concrete in a cubic yard?
45 bags at 80 lb, 60 bags at 60 lb, 72 at 50 lb, or 90 at 40 lb, since a cubic yard is 27 cubic feet and an 80 lb bag yields about 0.6 cubic feet.
How thick should a patio slab be?
4 inches is standard for a residential patio over a compacted gravel base. Go to 5 or 6 inches for driveways, garage floors or anything carrying vehicle weight.
Do I need gravel under a concrete slab?
Yes. 4 to 6 inches of compacted gravel gives drainage and a stable base that resists cracking and frost heave. Pouring directly on bare soil is the most common cause of a failed slab.
Is it cheaper to use bags or ready-mix concrete?
Ready-mix is cheaper per cubic yard once you need more than about one yard. Bagged mix works out around $225 to $315 per cubic yard versus roughly $120 to $220 delivered, though small orders attract a short-load fee of $50 to $150.
How long does a concrete slab take to cure?
It is walkable in 24 to 48 hours and takes vehicle weight after about 7 days, but keep the surface damp for at least 3 to 7 days. Concrete reaches roughly 70% strength in the first week and keeps gaining for a month.
Why do concrete slabs crack?
Concrete shrinks as it cures, so some cracking is inevitable. Control joints cut every 8 to 10 feet to a quarter of the slab depth decide where those cracks go. A poor base, too much water in the mix and fast drying all make cracking worse.
How many bags of concrete do I need for a 10x12 slab?
A 10x12 ft slab at 4 inches thick needs about 44 cubic feet including 10% waste, which is 74 eighty-pound bags or 98 sixty-pound bags. At that quantity ready-mix is cheaper, stronger and far less work than mixing by hand.
How much concrete for a 10x20 slab?
A 10x20 ft slab is 200 ft². At 4 inches thick that is 2.47 cubic yards bare, or about 2.72 cubic yards with 10% waste. In bags it would be 123 eighty-pound bags, so this is firmly ready-mix territory.
How big should a shed slab be?
Match the shed footprint, or extend it 2 to 4 inches past the walls on each side so rain drips clear of the base rather than running down onto the joint. Going much wider just collects standing water against the shed.