Fence Post Hole Concrete Calculator
Find how many 30 kg bags you need for each fence post, with holes in feet or metres.
Tell us about your fence
Your answer
That is about 3.4 bags per post, for 10 posts.
This answer is a guide. Bomix says ideally go below the frost line. For gates and posts that carry weight, check your local building rules or ask a builder or engineer.
What it costs
Middle prices at Ontario stores, Oct 2026, before tax. Post mix: 4 stores. Fast-set: only 2. Concrete mix: 5.
Quick answers
- A 4×4 post, 6 ft fence
- About 3.4 bags of 30 kg post mix. Dig 10½ in (267 mm) wide and 42 in (1,067 mm) deep.
- Hole width
- 3 times the post, says Bomix. Sakrete uses an 8 in (20 cm) hole for a 4×4.
- Hole depth
- ⅓ of the whole post, says Bomix, ideally below the frost line. Bomix puts that at about 1.2 m (4 ft), but frost depth changes by city.
- What a 30 kg bag makes
- Post mix: about 13 L (0.45 cubic foot). Fast-set and concrete mix: 14 L.
How to work it out
- Dig the hole 3 times as wide as the post really is. By the US lumber standard, a 4×4 is 3½ in.
- For a light fence, put ⅓ of the whole post in the ground. That is 0.5 × the height above ground.
- Add 6 in (150 mm) of crushed stone. We put it under the ⅓, so the hole is a little deeper.
- Take the post's area from the hole's area. Multiply by the concrete height, then divide by 1,728.
- Divide by 0.45, what one 30 kg bag of post mix makes.
Hole: 3 × 3½ = 10½ in wide
Depth: 72 × 0.5 + 6 in stone = 42 in
3.142 × 5.25 × 5.25 − 3½ × 3½ = 74.3 sq in
74.3 × 36 in ÷ 1,728 = 1.55 cubic feet (44 L)
1.55 ÷ 0.45 = 3.4 bags per post. 10 posts: 35 bags
Hole size: three Canadian guides
We found no fence-post depth in the building code or any city rule. So these are makers' guides, not rules.
- Bomix concrete mix: a hole 3 times the post. Put ⅓ of the whole post in the ground, ideally below the frost line. Bomix puts that at about 1.2 m (4 ft). Add 150 mm of crushed stone at the bottom.
- Bomix post mix: dig 15 cm below the frost line, at least 122 cm (48 in). Set it on 15 cm of crushed stone.
- Sakrete: leave 5 cm on each side of the post. A 4×4 gets an 8 in (20 cm) hole. Go below the frost line.
Frost depth changes by city. See the depths we found on the tube form (sonotube) calculator. It also counts the concrete for deck posts set in tubes.
For gates and posts that carry weight, the deeper hole is the safer choice. Check your local building rules or ask a builder or engineer.
| Post and hole rule | Hole (wide × deep) | Concrete | Post mix bags | Concrete mix bags |
|---|---|---|---|---|
| 4×4, light fence (⅓ rule) | 10½ in × 42 in | 44 L | 3.4 | 3.1 |
| 4×4, below frost (Bomix) | 10½ in × 48 in | 51 L | 4 | 3.6 |
| 4×4, Sakrete hole | 8 in × 120 cm | 29 L | 2.3 | 2.1 |
| 6×6, light fence (⅓ rule) | 16½ in × 42 in | 108 L | 8.5 | 7.6 |
| 6×6, below frost (Bomix) | 16½ in × 48 in | 126 L | 9.9 | 8.9 |
Light fence: 6 ft high. Bomix holes stand on 6 in of stone. Filled to the top, no extra.
So a 4×4 in Bomix's 48 in post-mix hole takes about 4 bags of post mix.
Sakrete's bags-per-post table
Sakrete Canada prints a table for a 4×4 in an 8 in (20 cm) hole. It fills the hole to the top.
| Hole depth | Sakrete says | Our maths |
|---|---|---|
| 45 cm (17½ in) | 1 | 0.8 |
| 60 cm (23½ in) | 1.25 | 1 |
| 90 cm (35½ in) | 1.75 | 1.6 |
| 120 cm (47 in) | 2 | 2.1 |
Our maths takes away a 3½ in post and uses 0.5 cubic foot per bag.
The two differ by 0.2 of a bag at most. Sakrete's hole is narrower than Bomix's, so it takes less concrete.
Which bag, and what it costs
All three bags weigh 30 kg. Post mix makes a little less, because Quikrete and Bomix both print 0.45 cubic foot. Sakrete's post mix says 14 L.
| Bag | Makes | Price (stores) | Per 4×4 post |
|---|---|---|---|
| Post mix 30 kg | 13 L | $10.99 (4) | $37.82 |
| Fast-set 30 kg | 14 L | $14.27 (2) | $44.20 |
| Concrete mix 30 kg | 14 L | $6.99 (5) | $21.65 |
Middle prices at Ontario stores. Fast-set comes from only 2 stores. Per post: the 6 ft fence example above.
Post hole cement or concrete?
Many people call the mix in a post hole "cement". Cement is only one part of concrete. Sakrete's cement sheet says to mix 1 part cement with 2.5 parts sand and 2.5 parts stone.
Bomix calls its post mix a concrete mix, and its data sheet says it has Portland cement in it. So for post hole cement, use this calculator: it counts the same bags.
Frost lift
Frost can push posts up. Bomix says a cardboard tube form around the concrete greatly cuts the risk of frost lifting the post.
Questions people ask
How much concrete per fence post?
For a 4×4 on a 6 ft fence, about 44 L. That is 3.4 bags of 30 kg post mix in Bomix's hole. Sakrete's narrower 8 in hole, 120 cm deep, takes about 2.
How much concrete for a post hole?
Take the post's area from the hole's area, then multiply by the depth. A 4×4 in an 8 in hole, 120 cm deep, takes about 29 L.
Can you mix concrete in the post hole?
With a post mix, yes. Quikrete's Canadian sheet says fill the hole half full of dry mix. Add about 3 L of water per bag, then fill the rest.
How deep should fence posts be in Canada?
We found no code depth for fences. Bomix says ⅓ of the post, ideally below the frost line, which it puts at about 1.2 m. Frost depth changes by city, so ask yours.
How do I stop frost lifting my fence posts?
Bomix says go below the frost line. Its sheet adds that a cardboard tube around the concrete greatly cuts the risk of frost lift.
How much does fence post concrete cost in Canada?
Post mix is about $10.99 a 30 kg bag, and concrete mix $6.99. The middle of 4 and 5 Ontario stores, before tax.
Post mix or regular concrete mix?
A 30 kg bag of post mix makes about 13 L, and concrete mix makes 14 L. Post mix costs more per bag.
Where our numbers come from
- Bag size and fence posts: Bomix Concrete Mix data sheet (Quebec)
- Post mix, litres per bag and depth: Bomix Postmix-30 data sheet (Quebec)
- Post mix, litres per bag and hole size: Sakrete Rapid Post (Canada)
- Fast-set concrete, litres per bag and bags per post: Sakrete Fast Set (Canada)
- Bags per tube and post, walk thickness: Sakrete Concrete Mix (Canada) data sheet
- Post mix, litres per bag: Quikrete Fence N' Post (Canada) data sheet
- Bag size: Sakrete Canada Concrete Mix (30 kg makes 0.014 m³, or ½ cubic foot)
- Real size of a 4×4 and 6×6: NIST lumber standard PS 20
- Store prices: Ontario Home Depot, RONA, Home Hardware and Chamberlain Timber Mart (Oct 2026)
- Cement bags and mix: Sakrete Portland Cement (Canada) data sheet
Made by BuildersCount. Last checked 4 Oct 2026.
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