Fence post concrete calculator

How this calculator works

Concrete for a fence post fills a cylinder with a post standing in the middle of it. The calculator works out that ring of concrete for each hole, adds them up, adds a margin, and converts to bags.

Volume per hole = [π × (D ÷ 2)² − post area] × concrete height

• D is the hole diameter, usually about 3 times the post width.
• Post area is the cross-section of the post, using actual sizes. A 4x4 is 3½ × 3½ in, or 12.25 in². A 6x6 is 5½ × 5½ in, or 30.25 in². Round chain link posts use their outside diameter.
• Concrete height is the embed depth minus the 6 in gravel base at the bottom of the hole.

Divide cubic inches by 1,728 to get cubic feet.

The calculator groups holes by class (line, corner and end, gate), because each class can have a different diameter and depth. It sums every hole, adds 10% for irregular holes and spills, then divides by the yield of the bag you choose and rounds up.

Bags = ceil(total volume × 1.10 ÷ bag yield)

Worked example

Sixteen 4x4 line posts in 10 in holes with 24 in of concrete, and six corner, end and gate posts in 12 in holes with 30 in of concrete.

Class	Per hole	Holes	Subtotal
Line	(78.54 − 12.25) × 24 ÷ 1,728 = 0.92 ft³	16	14.7 ft³
Corner, end, gate	(113.10 − 12.25) × 30 ÷ 1,728 = 1.75 ft³	6	10.5 ft³
Total, plus 10%			27.8 ft³
80 lb bags at 0.60 ft³	27.8 ÷ 0.60		47 bags

Buying by the post, one bag per hole, would have left this job about 25 bags short.

Bag yields

These are the published yields for standard concrete mix. Check the bag you buy, since high-strength and specialty mixes can differ.

Bag size	Yield	Bags per cubic yard
40 lb	0.30 ft³	90
50 lb	0.375 ft³	72
60 lb	0.45 ft³	60
80 lb	0.60 ft³	45

The 80 lb yield of 0.60 ft³ comes from the Quikrete Concrete Mix data sheet. The 50 lb figure of 0.375 ft³ matches the Quikrete Fast-Setting Concrete data sheet.

Bags per post

Per-post figures before the 10% margin. Multiply by your hole count, add 10%, then round up.

Post	Hole diameter	Concrete height	Volume	80 lb bags	50 lb bags
4x4 wood	8 in	24 in	0.53 ft³	0.9	1.4
4x4 wood	10 in	24 in	0.92 ft³	1.5	2.5
4x4 wood	10 in	30 in	1.15 ft³	1.9	3.1
4x4 wood	12 in	30 in	1.75 ft³	2.9	4.7
4x4 wood	12 in	36 in	2.10 ft³	3.5	5.6
6x6 wood	12 in	30 in	1.44 ft³	2.4	3.8
6x6 wood	12 in	36 in	1.73 ft³	2.9	4.6
6x6 wood	16 in	36 in	3.56 ft³	5.9	9.5
5x5 vinyl	10 in	30 in	0.93 ft³	1.6	2.5
4x4 vinyl	8 in	24 in	0.48 ft³	0.8	1.3
Chain link line, 1⅝ in	6 in	24 in	0.36 ft³	0.6	1.0
Chain link terminal, 2⅜ in	8 in	30 in	0.80 ft³	1.3	2.1
Chain link gate, 2⅜ in	12 in	36 in	2.26 ft³	3.8	6.0

Hole diameter matters more than depth. Going from a 10 in to a 12 in hole adds about 44% to the area of the hole, and the concrete goes up with it.

Fast-setting or standard mix

Fast-setting concrete is made for posts. You set the post, pour the dry mix into the hole and add water. Quikrete's data sheet says two 50 lb bags set a 4 in post in a 10 in hole 2 ft deep, which lines up with the formula: a 2 ft hole with a 6 in gravel base leaves 18 in of concrete, or about 1.8 bags of 50 lb mix. It sets quickly, so you can hang rails sooner, but you have less time to plumb the post.

Standard concrete mix is mixed in a wheelbarrow or mixer before you pour. It costs less per cubic foot, and 80 lb bags mean fewer bags to carry per hole. It also gives you time to adjust the post.

For a few posts, fast-setting is simpler. For a whole fence, standard mix usually costs less.

Gravel base

Every hole gets a 6 in layer of gravel at the bottom before the concrete goes in. It lets water that reaches the bottom of the post drain away instead of sitting in a concrete cup and rotting the post.

Gravel per hole = π × (D ÷ 2)² × 6 in ÷ 1,728

The calculator counts gravel in 0.5 ft³ bags. A 10 in hole takes about 0.27 ft³ and a 12 in hole about 0.39 ft³, so two holes per bag is a fair rule for small holes.

What depth to use

The concrete height depends on how deep the post goes, and that depends on the fence height and your frost line. Use the fence post depth calculator to get the embed depth first, then bring it here. Remember to subtract the 6 in gravel base, and to use the deeper figure for corner and gate posts.

GuideHow many bags of concrete per fence postBags of concrete per fence post for 4x4, 6x6, chain link and vinyl posts in common hole sizes, in 50 lb and 80 lb bags, with the formula and gravel.