Easy Garden Calc

Fence post concrete calculator

Enter the number of posts, the hole size and depth, and your bag size to get how many bags of concrete to buy.

Enter your measurements

in
Often about 3× the post width
in
Often ⅓ of the post’s total length, below the frost line where required
in
For drainage · 0 if none
Yield is printed on the bag
More options: Extra · Price
%
Hand-dug holes are rarely neat cylinders
$
Blank to skip
Bags of concrete28 bags50 lb, incl. fast-setting (≈ 0.375 cu ft)
Bags per post
2.7 bags
Concrete volume
10.1 cu ft (0.38 cu yd)
Gravel for the hole bottoms
2.73 cu ft
How we got this

Hole: 3.1416 × (10 in ÷ 2)² × 24 in of concrete, minus the post

+ 10% = 1.01 cu ft per post ÷ 0.375 cu ft per bag = 2.7 bags per post

× 10 posts = 28 bags

  • If you mix each hole separately, round up per post: 3 bags a post, 30 in all. Wet-mixing in a wheelbarrow lets you share a batch between holes and buy closer to the total.
  • Call 811 (in the US) or your local utility locate service a few days before you dig.
Check your codeCode limits vary by jurisdiction — confirm with your building department before you cut or build. Fence height, setbacks and how deep posts go (often below the local frost line) are set by local rules, not a national code.

For this job · affiliate link
Post-hole digger10 holes this deep by hand go much faster with a proper clamshell post-hole digger than with a spade. For dozens of holes, rent a powered auger.
View on Amazon (opens in a new tab)

Last reviewed

Quick answers for common sizes

Worked examples for the Fence Post Concrete Calculator
ExampleAnswer
How many bags of concrete per 4×4 fence post3 bags
How many bags of concrete for a 6×6 post3 bags

Bags per post by hole diameter (depth and post as above)

Bags per post by hole diameter (depth and post as above)
Hole diameterBags per postBags of concreteConcrete volume
8 in1.5 bags16 bags5.81 cu ft
10 in2.7 bags28 bags10.1 cu ft
12 in4.1 bags42 bags15.4 cu ft
14 in5.8 bags58 bags21.6 cu ft
16 in7.7 bags77 bags28.8 cu ft

The concrete fills the hole around the post, above a few inches of gravel. For a 4×4 in a 10 in hole with 2 ft of concrete, that comes to roughly 2.7 of the 50 lb bags, or 1.7 of the 80 lb bags, per post — the calculator works out the real number for your holes.

How much concrete per fence post

The concrete is the volume of the hole, above any gravel, minus the part of the hole the post itself takes up. A 4×4 post (really 3½ × 3½ in) takes up about 12 sq in of a 10 in hole’s 79 — not much, but it adds up over a fence line.

hole volume = 3.1416 × (hole diameter ÷ 2)² × concrete depth
concrete per post = hole volume − post cross-section × concrete depth
bags = concrete ÷ bag yield (50 lb ≈ 0.375 cu ft, 80 lb ≈ 0.60 cu ft)
Yields from Quikrete’s data sheets for its Concrete Mix and Fast-Setting Concrete Mix. Other brands are similar but check your bag.
BagTypical yieldBags per cubic foot
40 lb≈ 0.30 cu ft≈ 3.3
50 lb (incl. fast-setting)≈ 0.375 cu ft≈ 2.7
60 lb≈ 0.45 cu ft≈ 2.2
80 lb≈ 0.60 cu ft≈ 1.7

Worked examples

How deep and how wide?

Quikrete’s post-setting instructions say to dig the hole about three times the post width and make it one-third of the post’s total length deep — so an 8 ft post goes about 32 in into the ground, leaving about 5 ft 4 in above it. Where the ground freezes, local codes and good practice often call for the bottom to sit below the frost line. Your local building department or fence permit spells out what applies.

Gravel or concrete in the bottom? Many fence builders put a few inches of gravel under wood posts so water drains away from the end grain. Quikrete’s own Concrete Mix instructions instead start with 6 in of dry mix in the bottom, and its fast-setting instructions pour dry mix around the post to 3–4 in below the surface and backfill the rest with soil. Set the gravel field to 0 if you follow those.

Slabs, pads and footings

Pouring something other than a post hole — a pad for a shed or AC unit, or a round deck footing? Use the concrete bag calculator, or see how many bags of concrete for a 10×10 slab and how many bags of concrete in a cubic yard.

Tips

  • Slope the top of the concrete away from the post so water sheds instead of pooling against it.
  • Brace posts plumb before the concrete sets; check two faces with a level.
  • Wood posts should be rated for ground contact.
  • Wait before hanging rails or gates: Quikrete says 24 hours for its standard mix, 4 hours for fast-setting.
CautionAlways have underground utilities marked before digging post holes — in the US, contact 811 a few business days ahead. It is free.

What you’ll need

As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.

Frequently asked questions

How many bags of concrete per 4×4 fence post?

Usually two to three 50 lb bags in a 10 in hole about 30 in deep — the worked example how many bags of concrete per 4×4 fence post shows 2.7. Charts that say one bag per post assume a narrow or shallow hole.

How many bags of concrete per 6×6 post?

In a 12 in hole 36 in deep over 6 in of gravel, about 2.6 of the 80 lb bags. Deeper holes and wider holes, worked through: how many bags of concrete for a 6×6 post.

Should I put gravel under a fence post?

A few inches of gravel in the bottom of the hole is common practice for wood posts: it lets water drain away from the end grain instead of sitting in a concrete cup. Some concrete makers’ instructions put concrete mix in the bottom instead; either way, slope the top so water runs off.

How deep should fence post holes be?

Quikrete’s rule is one-third of the post’s total length (half its height above ground), and below the frost line in cold climates. Local codes and your fence manufacturer take priority.

Sources

Related calculators