Fence Post Concrete Calculator

Concrete for fence and deck post holes: hole size, depth, post size and number of posts give the bags you need, with the post's own volume taken off.

Each post hole needs the volume of the hole minus the volume of the post. For a 4×4 post (actually 3.5 inches square) in a 10-inch hole 2 feet deep, that's (π × 5² − 3.5²) × 24 = 1,591 cubic inches, about 0.92 cubic feet: two 80 lb bags or three 50 lb bags per post. In metric, a 100 mm post in a 300 mm hole 600 mm deep takes about 0.036 m³, or four 20 kg bags. Enter your hole size, depth, post size and number of posts, and the calculator gives the total concrete and bags with the post taken off. If you put gravel in the bottom of each hole for drainage, enter only the depth of concrete.

Units
Bag sizes
Pour
in
in
in
Common posts:
%
Bags
17

80 lb bag (0.60 ft³)

Concrete
0.38 yd³

10.13 ft³ · 0.287 m³

Weight when mixed
1,517 lb

0.76 tons

Premixed bags needed by size
80 lb bag (0.60 ft³)17
60 lb bag (0.45 ft³)23
50 lb bag (0.375 ft³)28
40 lb bag (0.30 ft³)34
20 kg bag31
25 kg bag25
30 kg bag21
How it was worked out
  1. (π × (10 in ÷ 2)² − 3.5² for the post) × 24 in deep × 10 holes = 9.21 ft³ (0.34 yd³)
  2. Plus 10% extra = 10.13 ft³ (0.38 yd³)
  3. 80 lb bag (0.60 ft³): 10.13 ft³ ÷ 0.6 ft³ made per bag = 16.88, rounded up to 17 bags

Bag yields are typical figures: 0.30, 0.375, 0.45 and 0.60 ft³ for 40, 50, 60 and 80 lb bags, and about 108 × 20 kg bags per m³. Check the yield printed on the bag you buy. Mixed concrete is taken as 2,400 kg/m³ (about 4,050 lb per cubic yard).

Common use cases

  • Buy the right number of bags for a new fence
  • Set deck, pergola or mailbox posts
  • Concrete for gate posts and steel posts
  • Compare post-hole concrete with fast-setting post mix

How to use

  1. Choose your units. Post holes is already selected.
  2. Enter the hole diameter and the depth of concrete in each hole.
  3. Pick a common post size or enter your own, square or round. Choose No post to fill a hole solid.
  4. Enter the number of holes, then read the bags you need.

FAQ

How big should a fence post hole be?

A common rule is a hole about three times the post's width: 10 to 12 inches for a 4×4. Depth is often given as about a third of the post's height above ground (many UK guides say a quarter of the post's full length), plus a few inches of gravel for drainage. Wind, soil and local rules matter, so check for your fence.

How many bags of concrete per fence post?

For a 4×4 in a 10-inch hole with 2 feet of concrete, about 0.92 ft³: 2 × 80 lb or 3 × 50 lb bags. For a 100 mm post in a 300 mm hole with 600 mm of concrete, about 4 × 20 kg bags. Bigger holes take a lot more, because the volume grows with the square of the diameter.

Should the gravel at the bottom count?

No. Enter only the depth of concrete. If the hole is 30 inches deep with 6 inches of gravel, enter 24 inches. Work out the gravel with the gravel calculator if you need to buy it.

Is fast-setting post mix different?

It's poured into the hole dry or with water added in the hole, depending on the product, but it still fills the same volume. Use the bag yield printed on the product: 50 lb fast-setting bags in the US typically make about 0.375 ft³, the same as the 50 lb option here.

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