Concrete per fence post calculator

Find how many bags of concrete set each fence post, and the total for the job, from the hole and post size — for 40, 50, 60 and 80 lb bags.

Confirm panel/picket dimensions and bag yield against your product and order a little extra (~10%) for waste, corners, terrain and uneven runs. Panel sizes, picket widths, gaps and bag yields vary by product and brand.

Calculator

posts
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Hole diameter is usually ~3× the post width.
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A nominal 4×4 measures 3.5"; a 6×6 measures 5.5".
Total concrete (60 lb, 26 posts)78 bags
Per post — 60 lb3 bags
Per post — 80 lb2 bags
Per post — 50 lb4 bags
Per post — 40 lb4 bags
Net hole volume1.15 cu ft/post

A 10" × 30" hole around a 3.5" post is about 1.15 cu ft — 3 × 60 lb (or 2 × 80 lb) bags per post, so 78 bags for 26 posts. Yields are labeled typicals — confirm on your bag.

Setting a post in concrete fills the ring of the hole around the post. The concrete you need per hole is the hole volume minus the volume the post itself takes up — then you divide by the yield of one bag and round up, because you cannot buy a fraction of a bag.

Bag yields are the key labeled typical: an 80 lb bag makes about 0.60 cu ft, a 60 lb bag about 0.45, a 50 lb bag about 0.375 and a 40 lb bag about 0.30. Bigger bags mean fewer bags but more weight to carry. This tool shows the per-post count for all four sizes side by side, plus the total for your whole post count so you can price the pour in one trip.

Hole size drives everything. A common rule of thumb is a hole about three times the post width in diameter and set roughly one-third of the above-ground height deep — check depth against your climate and code with the post-hole depth reference.

Formula

hole_vol = π × (hole_dia_ft ÷ 2)² × depth_ft − post_width_ft² × depth_ft\nbags/post = ceil(hole_vol ÷ bag_yield_cuft)\ntotal     = bags/post × posts

Yields: 40 lb ≈ 0.30, 50 lb ≈ 0.375, 60 lb ≈ 0.45, 80 lb ≈ 0.60 cu ft/bag (labeled typicals — confirm on the bag).

Worked example

10" hole, 30" deep, 4×4 post (3.5"), 26 posts, 60 lb bags:

  • hole = π × (0.417)² × 2.5 − (0.292)² × 2.5 ≈ 1.15 cu ft
  • per post = ceil(1.15 ÷ 0.45) = 3 (60 lb); 2 × 80 lb; 4 × 50 lb; 4 × 40 lb
  • total = 3 × 26 = 78 bags of 60 lb

Reference table

Bags per post for your 10"×30" hole around a 3.5" post (net 1.15 cu ft):

Bag sizeYield/bagBags per post
40 lb0.300 cu ft4
50 lb0.375 cu ft4
60 lb0.450 cu ft3
80 lb0.600 cu ft2

Yields are labeled typicals — confirm the figure printed on your bag.

Frequently asked questions

How many bags of concrete per fence post?
For a typical 10"×30" hole around a 4×4 post (about 1.15 cu ft), you need 3 bags of 60 lb or 2 bags of 80 lb concrete per post. Smaller 40 or 50 lb bags take 4 per post.
Does the post volume really matter?
Yes, a little. The concrete only fills the ring around the post, so the tool subtracts the post cross-section from the hole volume. For a slim 4×4 the difference is small; for a 6×6 gate post it is worth counting.
What hole size should I dig?
A common planning rule is a hole about three times the post width in diameter, set roughly one-third of the above-ground post height deep. Frost depth, soil and code may require deeper — check the post-hole depth reference and local rules.
Fast-setting versus standard concrete — does the yield change?
Yields differ a little by product, and this tool uses labeled typicals (0.30 to 0.60 cu ft by bag weight). Always confirm the cubic-foot yield printed on your specific bag before buying in bulk.

Related calculators

Post-hole depth referenceHow deep should fence posts be? A labeled planning reference: buried depth is about one-third of the above-ground height plus a 6-inch gravel base.Fence post calculatorFence post calculator: how many posts for your fence? Enter length and spacing to get line posts, sections and total posts including gates, ends and corners.Fence calculatorAll-in-one fence calculator: enter length, post spacing and style to get posts, sections, rails, pickets and concrete bags for the post holes, with the formula.