Concrete Slab Material Calculator
Find the volume of concrete for a slab, in cubic yards or m³. Pick a thickness, and see what the Canadian rules ask for.
Measure your project
Your answer
That is 88.0 cubic feet, or 2.49 m³. We added 10% extra.
If you buy bags
A 30 kg bag makes 0.014 m³ (0.5 cubic foot), say the makers.
If you order a truck (ready-mix)
Ask for 2.50 m³ (about 3.27 cubic yards). We round up to the next 0.25 m³.
This is a guide. For a driveway or garage floor, check your local building rules or ask a builder or engineer.
Which costs less?
You also skip mixing 176 bags by hand. Have the edges ready before it comes.
Ready-mix (25 MPa): the middle of 4 lists from 3 companies in Ontario and Calgary. Bags: the middle of 8 prices at 5 Ontario stores. October 2026, before GST/HST.
Quick answers
- Thinnest slab allowed
- 75 mm (about 3 in) for a floor on the ground, says the building code (Part 9).
- Patio, walk or car driveway
- 100 mm (4 in). For trucks or RVs, 125 mm (Manitoba) to 150 mm (Alberta).
- Strength for a garage floor
- 32 MPa, with 5–8% air. A heated garage may skip the air, says Concrete Manitoba.
- Control joints
- No more than 2.5 m (8.2 ft) apart, cut ¼ of the way down.
- 12 × 20 ft car pad
- 3.26 cubic yards (2.49 m³) with 10% extra. Ask your plant about its smallest load.
How to work it out
- Find the area: length × width, in feet.
- Multiply the area by the thickness in inches, then divide by 324 to get cubic yards.
- Add 10% extra for spills and dips in the base.
- Plants sell by the m³. To get m³, multiply the cubic yards by 0.765.
12 × 20 = 240 sq ft
240 × 4 ÷ 324 = 2.96 cubic yards
2.96 + 10% = 3.26 cubic yards
3.26 × 0.765 = 2.49 m³ → order 2.50 m³
How thick and how strong?
Each slab has its own rule. Plants sell concrete by strength, in MPa. Air means tiny bubbles mixed in, for slabs that freeze and thaw.
| Slab | Thickness | Strength | Air |
|---|---|---|---|
| Basement or other inside floor | at least 75 mm | 20 MPa | – |
| Garage floor | 100 mm for cars | 32 MPa | 5–8% |
| Driveway, cars | 100 mm | 32 MPa | 5–8% |
| Driveway, trucks or RVs | 125–150 mm | 32 MPa | 5–8% |
| Walk or patio | 100 mm | 32 MPa (Ontario) | 5–8% (Ontario) |
From the building code (Part 9), RMCAO, Concrete Alberta and Concrete Manitoba. Walks and patios at 32 MPa is Ontario's rule.
- Trucks or RVs: Concrete Manitoba says 125 mm or more. Concrete Alberta says at least 150 mm. The two groups differ.
- Garages: 100 mm is enough for cars, says Concrete Alberta. A Quebec contractor pours 120–150 mm. Concrete Manitoba says a heated garage floor may skip the air.
- Old advice: older guides, based on Ontario's 2012 code, give other strengths for floors. Ontario's 2024 code dropped that rule, says RMCAO.
The calculator prices 25 MPa. On only 3 Quebec and Eastern Ontario lists, 32 MPa costs $19 more per m³ in the middle. Ask your plant for a quote, then type its price under "Change prices".
What a yard or a m³ covers
Thin slabs go further. Find your thickness to see how much ground one unit of concrete covers.
| Thickness | Used for | One cubic yard | One m³ |
|---|---|---|---|
| 3 in (75 mm) | house floor, the least allowed | 108 sq ft | 13.3 m² |
| 4 in (100 mm) | walk, patio or car driveway | 81 sq ft | 10 m² |
| 5 in (125 mm) | trucks or RVs (Manitoba) | 65 sq ft | 8 m² |
| 6 in (150 mm) | trucks or RVs (Alberta) | 54 sq ft | 6.7 m² |
No spill extra added. The trucks rows are from two concrete groups that disagree.
What the slab weighs
Concrete weighs about 140 lb per cubic foot, says Quikrete's US sheet. So the 12 × 20 ft pad, 4 inches thick, weighs about 5.1 tonnes.
Control joints and slope
Control joints are cuts that tell the slab where to crack. Concrete Alberta spaces them at most 25 times the thickness. Concrete Manitoba caps them at 2.5 m.
| Thickness | Joints at most | Cut depth (¼) |
|---|---|---|
| 75 mm (3 in) | 1.9 m (6.2 ft) | 19 mm |
| 100 mm (4 in) | 2.5 m (8.2 ft) | 25 mm |
| 125 mm (5 in) | 2.5 m (8.2 ft) | 31 mm |
| 150 mm (6 in) | 2.5 m (8.2 ft) | 38 mm |
We show the smaller of the two rules. Sakrete's sheet says to cut ⅕ deep instead.
The 12 × 20 ft pad
Joints every 8.2 ft or less give 2 × 3 panels. Each is about 6 × 6.7 ft.
That is 44 ft of cuts, each 25 mm deep. Panels should be no more than 1.5 times as long as wide. These are 1.11.
Slope outdoor slabs at least 1%, and 2% is better, says Concrete Alberta. Over 20 ft, that is 2.4–4.8 in of fall.
What goes under the slab
- House floor: Ontario's 2024 building code asks for at least 100 mm of clean stone. BC's 2012 code did too. Check your local code. No more than 10% of it may be finer than 4 mm, so ¾ in clear stone fits. Garage and carport slabs are left out.
- Walk, patio or driveway: on sandy soil, a base is often not needed, says Concrete Manitoba. Otherwise, a 75 mm packed base may be used. Sakrete's own sheet asks for at least 50 mm of packed crushed stone.
- In Alberta: Concrete Alberta asks for a driveway base of 20 or 25 mm stone to level the ground. It gives no depth.
The gravel under concrete calculator works out the stone. For steel in the slab, see the rebar for slab calculator.
Pouring a house foundation? Size its footings with the concrete footing calculator. To mix your own from cement, sand and stone, use the cement calculator.
A garage floor or driveway carries weight. Check your local building rules, or ask a builder or engineer.
Questions people ask
How do I calculate the materials for a concrete slab?
Multiply the length and width in feet by the thickness in inches. Divide by 324 for cubic yards of concrete, then add 10%. The stone base and any steel have their own calculators.
How thick should a concrete slab be in Canada?
At least 75 mm for a floor, says the building code (Part 9). Walks and patios use 100 mm.
How much slope does an outdoor slab need?
At least 10 mm of fall for each metre (1%), says Concrete Alberta. It prefers 20 mm per metre (2%).
What strength of concrete do I need for a garage floor?
32 MPa with 5–8% air. The building code (Part 9) allows 30 MPa only where local stone can't reach 32.
How far apart should control joints be?
No more than 2.5 m both ways, says Concrete Manitoba. Cut them ¼ of the slab deep, within 16 hours of the pour.
How thick should a garage floor be?
100 mm is enough for cars, says Concrete Alberta. A Quebec contractor pours 120–150 mm. For trucks or RVs, Concrete Manitoba says 125 mm and Concrete Alberta 150 mm.
Can I pour a slab in cold weather?
Below 5 °C, the building code (Part 9) says to keep new concrete at 10–25 °C. Then hold it at 10 °C or more for 72 hours.
Where our numbers come from
- Floor slabs: BC Building Code 2018, Section 9.16 (at least 75 mm)
- Floor slabs: Ontario Building Code 9.16.4.3 (at least 75 mm; 2012 text, third-party copy)
- Ontario Building Code 2024, Part 9 (ministry compendium)
- Bags per tube and post, walk thickness: Sakrete Concrete Mix (Canada) data sheet
- Slab thickness in Quebec: Groupe Bellemare guide
- Driveways, joints and slope: Concrete Alberta flatwork guide
- Driveways and garage slabs: Concrete Manitoba (thickness, base, steel)
- Concrete strength: RMCAO residential concrete guide
- Concrete strength in Ontario: RMCAO technical summary (2025, OBC 2024)
- Concrete strength and site mixes: BC Building Code 2018, Section 9.3 (Part 9)
- Under-slab stone: Ontario Building Code 9.16.2.1 (2012 text, third-party copy)
- Under-slab stone: BC Building Code 2012, 9.16.2.1–9.16.2.2
- Ready-mix price, Quebec and Eastern Ontario: the middle of 3 plant lists (2026)
- Ready-mix price: the middle of 4 prices from 3 companies (25 MPa, Oct 2026, before tax)
- Small-load and winter fees: Dufferin Concrete 2026 price list (Ontario)
- Small-load fees: CBM Toronto-area price list
- Bag size: Sakrete Canada Concrete Mix (30 kg makes 0.014 m³, or ½ cubic foot)
- Store prices: Ontario Home Depot, RONA, Home Hardware and Chamberlain Timber Mart (Oct 2026)
- Extra for spills: Sakrete slab calculator (10%)
- Bag sizes and weight: Quikrete Concrete Mix data sheet
Made by BuildersCount. Last checked 4 Oct 2026.
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