Gravel Under Concrete Slab Calculator
Find the stone for the layer below a concrete slab. Measure the slab, pick the depth, and see cubic yards and tonnes.
Measure your project
Your answer
That is 12.46 m³ and weighs about 21.19 tonnes. We added 10% extra.
If you buy bags
This is a guide. A slab carries weight, so check your local building rules or ask a builder or engineer.
Which costs less?
This price is for picking it up yourself. Add a delivery fee in "Change prices" if they bring it.
¾ in clear stone: $60 a cubic yard, the middle of 14 Canadian yards (Oct 2026). Pickup price, before delivery and tax. Bags: crushed stone at Ontario stores, about $9.48 for 30 kg (only 3 stores).
Quick answers
- Under a house floor
- At least 100 mm (4 in) of clean stone, in Ontario's 2024 building code.
- Which stone
- ¾ in clear stone. Quebec's home warranty sheet names it as clean stone.
- Packing
- Clean stone needs none. Other fill is packed in layers of 300 mm or less.
- Under a concrete driveway
- 75 mm (3 in) of Granular A, in Niagara Region's rule for driveways at the road.
- 40 × 30 ft floor, 4 in deep
- About 14.81 cubic yards (11.33 m³) of clear stone. It weighs about 19.26 tonnes.
How to work it out
- Find the slab size in feet. Length × width is the square feet.
- Multiply by the stone depth in inches, then ÷ 324. You now have cubic yards.
- Multiply by 1.3 for tonnes of clear stone.
- Using Granular A? Turn on packing. It orders 35% more.
40 × 30 = 1,200 sq ft
1,200 × 4 ÷ 324 = 14.81 cubic yards
14.81 × 1.3 = 19.26 tonnes
With 10% extra: 21.19 tonnes
What the building code asks for
Under a house floor on the ground, Ontario's building code (Part 9) wants clean stone. It must be at least 100 mm (4 in) deep. We checked the 2024 code, now in force.
BC's 2012 code said the same. BC's 2018 code calls this a drainage layer and moves the depth to a note.
Clean means no more than 10% passes a 4 mm sieve. ¾ in clear stone fits that.
Granular A does not fit. At least 15% of it passes an even finer 1.18 mm sieve.
The Ontario rule leaves out slabs in garages, carports and sheds.
A BC builders' drainage guide, still a draft from 2017, says 6 in is better. It names clear stone from ¾ to 1 in.
A floor slab carries weight. Check your local building rules or ask a builder or engineer.
Pouring a footing, not a slab? See our concrete footing calculator.
Packing the stone layer
Ontario's 2024 code and BC's 2012 code say clean stone under a floor does not need packing. That saves a step with clear stone.
Quebec's home warranty sheet says other fill must be packed. That covers MG-20 stone and MG-112 sand. Pack it in layers of 300 mm or less.
The sheet also wants each layer tested, to prove it is packed well.
The BC guide adds that sand alone is not fit to go under a slab. It needs gravel on top.
In the calculator, pick Granular A and turn on packing. It adds 35%, as two Quebec yards do.
Driveways, garages, walks and patios
Outdoor slabs and garage floors follow other advice. Here is what we found in Canada.
| Slab | Stone layer | Who says |
|---|---|---|
| House floor on the ground | 100 mm clean stone | Ontario code (2024), BC code (2012) |
| Home concrete driveway | 75 mm Granular A | Niagara Region |
| Garage floor or driveway | Often none on sandy soil, else 75 mm packed | Concrete Manitoba |
| Walk or patio | At least 50 mm packed crushed stone | Sakrete Canada |
| Flatwork in Alberta | 20 or 25 mm stone, no depth given | Concrete Alberta |
Niagara's figure is for driveways it rebuilds at the road. One region only.
Under a 20 × 20 ft concrete driveway, 75 mm of packed Granular A means ordering about 6.7 tonnes. That includes 35% for packing.
Stone for common slab sizes
| Slab | Cubic yards | Tonnes | Cost at a yard |
|---|---|---|---|
| 10 × 10 ft | 1.23 | 1.60 | $74 |
| 12 × 16 ft | 2.37 | 3.08 | $142 |
| 20 × 20 ft | 4.94 | 6.42 | $296 |
| 24 × 24 ft | 7.11 | 9.24 | $427 |
| 30 × 40 ft | 14.81 | 19.26 | $889 |
No extra added. Cost at $60 a cubic yard, the middle of 14 yards, Oct 2026, before tax and delivery.
Bags only suit a small pad. A 10 × 10 ft pad takes 54 bags of 30 kg. That is about $512, against $74 loose.
Questions people ask
How much gravel goes under a concrete slab?
Ontario's 2024 building code asks for clean stone under a house floor. It must be at least 100 mm (4 in) deep. A 40 × 30 ft floor takes about 14.81 cubic yards.
How much gravel goes under a concrete patio or walk?
Sakrete Canada asks for at least 50 mm (2 in) of packed crushed stone. For driveways and garage floors on sandy soil, Concrete Manitoba says often none is needed.
How much gravel under a garage slab?
The Ontario code's clean-stone rule leaves out garages. Concrete Manitoba says a 75 mm packed base may be used. Ask your town before you pour.
Does gravel need to be packed before pouring concrete?
Clean stone does not, say the Ontario and BC codes. Quebec's warranty sheet says other fill must be packed, in layers of 300 mm or less.
How thick should gravel be under a concrete driveway?
Niagara Region puts 75 mm (3 in) of Granular A under a home concrete driveway. Other towns may differ.
Can I put sand under a concrete slab?
Not on its own, says a draft BC builders' guide. Sand needs a layer of gravel between it and the floor slab.
How much gravel for a 4 inch concrete slab?
The slab's thickness does not set the stone depth. At 4 in of stone, a 20 × 20 ft slab needs about 4.94 cubic yards.
Can Granular A go under a basement floor?
Not as the code's clean stone. At least 15% of Granular A is fine sand and dust. Clean stone allows only 10% through a 4 mm sieve.
Where our numbers come from
- Ontario Building Code 2024, Part 9 (ministry compendium)
- 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
- Floor slabs: BC Building Code 2018, Section 9.16 (at least 75 mm)
- Under-slab stone: Garantie GCR sheet FT-9.16.2.2-01 (Quebec)
- Under-slab stone: BC builder's drainage guide (draft, 2017)
- Driveway layers: Niagara Region specifications
- Driveways and garage slabs: Concrete Manitoba (thickness, base, steel)
- Driveways, joints and slope: Concrete Alberta flatwork guide
- Bags per tube and post, walk thickness: Sakrete Concrete Mix (Canada) data sheet
- Granular A and B: City of Toronto TS 1010 (OPSS 1010 definitions)
- Weights per cubic yard: Canadian yards and producers (few sources each)
- Loose prices: the middle of up to 23 Canadian yards (Oct 2026)
- Store prices: Ontario Home Depot, RONA, Home Hardware and Chamberlain Timber Mart (Oct 2026)
- Quebec yards: bulk calculators (tonne, packing)
- #57 stone extra: Palmetto Sand & Gravel (add 10%)
Made by BuildersCount. Last checked 4 Oct 2026.
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