Fence Post Concrete Calculator

US & Canada

Pick your post, hole size and depth to see how many bags of concrete each post takes.

Your posts

1. Post and hole

in
ft
posts

Quikrete and Sakrete both say: a hole about 3 times the post width, and about 1/3 of the post in the ground. Deck and structural posts go below the frost line.

2. Allowance

%

Result

0

Bags to buy

The post's own volume is taken out of each hole. Fast-setting yields are from the Quikrete data sheet (Nov 2025); regular 80 lb mix yields 0.6 cu ft.

How the post concrete maths works

  • Hole volume = π × (hole diameter ÷ 2)² × depth
  • Post volume = width × width × depth for a square post, or π × (diameter ÷ 2)² × depth for a round one
  • Concrete = hole volume minus post volume, times the number of posts, plus the allowance
  • Bags = concrete ÷ bag yield, rounded up

Fast-setting yields come from the Quikrete Fast-Setting Concrete Mix data sheet (revised November 2025): 0.375 cu ft for a 50 lb bag, and 0.41 cu ft and 0.5 cu ft for the 25 kg and 30 kg bags sold in Canada. Sakrete lists 0.38 cu ft for its 50 lb bag, close enough that the bag counts match.

Bags of fast-setting concrete per post

One post, 10% extra, before rounding. Hole sizes start at about twice the post width.

Bags of fast-setting concrete per post by post size, hole diameter and depth
Post Hole Depth Concrete 50 lb bags 30 kg bags
4 × 4 8 in 2 ft 0.58 cu ft 1.5 1.2
4 × 4 8 in 2.5 ft 0.73 cu ft 1.9 1.5
4 × 4 8 in 3 ft 0.87 cu ft 2.3 1.7
4 × 4 8 in 4 ft 1.16 cu ft 3.1 2.3
4 × 4 10 in 2 ft 1.01 cu ft 2.7 2
4 × 4 10 in 2.5 ft 1.27 cu ft 3.4 2.5
4 × 4 10 in 3 ft 1.52 cu ft 4.1 3
4 × 4 10 in 4 ft 2.03 cu ft 5.4 4.1
4 × 4 12 in 2 ft 1.54 cu ft 4.1 3.1
4 × 4 12 in 2.5 ft 1.93 cu ft 5.1 3.9
4 × 4 12 in 3 ft 2.31 cu ft 6.2 4.6
4 × 4 12 in 4 ft 3.08 cu ft 8.2 6.2
4 × 4 16 in 2 ft 2.88 cu ft 7.7 5.8
4 × 4 16 in 2.5 ft 3.61 cu ft 9.6 7.2
4 × 4 16 in 3 ft 4.33 cu ft 11.5 8.7
4 × 4 16 in 4 ft 5.77 cu ft 15.4 11.5
4 × 4 18 in 2 ft 3.7 cu ft 9.9 7.4
4 × 4 18 in 2.5 ft 4.63 cu ft 12.3 9.3
4 × 4 18 in 3 ft 5.55 cu ft 14.8 11.1
4 × 4 18 in 4 ft 7.4 cu ft 19.7 14.8
6 × 6 12 in 2 ft 1.27 cu ft 3.4 2.5
6 × 6 12 in 2.5 ft 1.58 cu ft 4.2 3.2
6 × 6 12 in 3 ft 1.9 cu ft 5.1 3.8
6 × 6 12 in 4 ft 2.53 cu ft 6.8 5.1
6 × 6 16 in 2 ft 2.61 cu ft 7 5.2
6 × 6 16 in 2.5 ft 3.26 cu ft 8.7 6.5
6 × 6 16 in 3 ft 3.91 cu ft 10.4 7.8
6 × 6 16 in 4 ft 5.22 cu ft 13.9 10.4
6 × 6 18 in 2 ft 3.43 cu ft 9.1 6.9
6 × 6 18 in 2.5 ft 4.28 cu ft 11.4 8.6
6 × 6 18 in 3 ft 5.14 cu ft 13.7 10.3
6 × 6 18 in 4 ft 6.85 cu ft 18.3 13.7

Setting the post

  • Dig about 3 times the post width, with a third of the post in the ground. Add 6 in for a compacted gravel base.
  • Fill with dry fast-setting mix to 3 to 4 in below the top, then add about 1 gal (3.8 L) of water per 50 lb bag, as Quikrete directs.
  • Wait 4 hours before putting strain on the post, per the Quikrete data sheet.
  • Deck posts sit on footings below the frost line. See frost depth in Canada and the sonotube calculator.

Common questions

How many bags of concrete per fence post?
For a 4 × 4 post in a 10 in hole 3 ft deep, the hole holds 1.38 cu ft once the post is in. That is 3.7 bags of 50 lb fast-setting mix, or 4.1 with a 10% allowance.
How many bags of concrete for 10 fence posts?
Ten 4 × 4 posts in 10 in holes 3 ft deep take 41 bags of 50 lb fast-setting concrete, with 10% extra.
How many bags for a 4 × 4 post 2 ft deep?
In a 12 in hole (3 times the post width, as Quikrete and Sakrete advise), 2 ft deep: 1.4 cu ft, or 3.7 bags of 50 lb fast-setting mix.
How much concrete for a 6 × 6 post?
A 6 × 6 in a 16 in hole 3 ft deep needs 3.56 cu ft: 9.5 bags of 50 lb fast-setting mix, or 7.1 bags of 30 kg.
How many 30 kg bags per post in Canada?
A 4 × 4 in an 8 in hole 1.2 m deep holds 29 litres once the post is in: 2.1 bags of 30 kg fast-set, which yields about 14 L (0.5 cu ft) a bag. Sakrete Canada's own chart gives 2 bags for that hole.
How big should a fence post hole be?
Quikrete's data sheet says to dig the hole "about 3 times the diameter of the post", with "1/3 the overall post height" in the ground. Its project sheet adds 6 in for a gravel base. For a 4 × 4 that is a 10 to 12 in hole.
How deep should a fence post be in Canada?
No building code sets fence post depth, but going below frost stops heaving. Deck and other structural posts must reach the depth your municipality sets, often 1.2 m (4 ft). A 4 × 4 in a 10 in hole 4 ft deep takes 4.9 bags of 50 lb fast-setting mix.