How Much Concrete Do You Need for a Fence Post? Bags per Hole for 4×4 and 6×6 Posts

A 4×4 post (3.5 in actual) in a 3 ft hole, the depth for a 6 ft fence, takes about 1.55 cubic feet of concrete: buy 3 bags of 80 lb mix. A 6×6 post (5.5 in actual) takes about 3.82 cubic feet: buy 7.

That is 2.6 bags for the 4×4 and 6.4 for the 6×6 before rounding up. The numbers follow the QUIKRETE rule of a hole three times the post width and as deep as half the post height above ground. Frost depth and local code can call for deeper holes.

How big is the hole for a fence post?

QUIKRETE’s Fast-Setting Concrete Mix data sheet gives two sizing rules for a post set in concrete: the hole diameter should be 3 times the post width, and the hole depth should be one-half of the post height above ground. A 6 ft fence therefore gets a 3 ft deep hole, and a 4×4 post, which is really 3.5 in square, gets a hole 10.5 in across.

Because the post sticks out 6 ft and goes 3 ft down, it is 9 ft long, and one-third of it is underground. That is the “one-third in the ground” rule of thumb, and it is the same rule seen from the other side. It is a manufacturer’s setting guide, not a code requirement.

The formula

hole volume (cu ft) = π × (hole diameter in ÷ 24)² × depth (ft)
post volume (cu ft) = (post width in ÷ 12)² × depth (ft)      square post
concrete per post   = hole volume − post volume
bags = ceil( posts × concrete per post × (1 + allowance) ÷ yield per bag )

1. Use the actual post size, not the nominal one. A 4×4 is 3.5 in and a 6×6 is 5.5 in.
2. Hole diameter is 3 × the post width. Depth is half the above-ground height, unless frost depth or local code asks for more.
3. Subtract the post. It takes up room in the hole, so the concrete is the hole minus the post.
4. Divide by the bag yield: 0.60 cubic feet for an 80 lb bag of standard mix, 0.45 for a 60 lb bag and 0.375 for a 50 lb bag, as listed on the QUIKRETE data sheets. If your bag prints a different yield, use that.
5. Round up once, on the total for all posts, not post by post.

For a round post, replace the post volume with π × (post diameter ÷ 24)² × depth. The yields and the conversion from cubic feet are the same as in how many bags of concrete do I need.

Bags per post: 4×4 and 6×6

This table applies the hole rule above to four common fence heights. Bag counts are for one post and have no allowance. The 80 lb column uses the 0.60 cubic foot yield of standard QUIKRETE Concrete Mix No. 1101; the 60 lb column uses the 0.45 cubic foot yield printed on the Fast-Setting sheet.

A 6×6 post uses about two and a half times the concrete of a 4×4 at the same height, because the hole is wider and the area grows with the square of the diameter. For a 6 ft fence, the 4×4 hole is 1.804 cubic feet and the post is 0.2552, which leaves the 1.5488 in the table.

Worked example: 17 posts for a 120 ft fence

A 6 ft tall fence, 120 ft long, with posts every 8 ft and one gate has 120 ÷ 8 = 15 sections and 15 + 1 + 1 = 17 posts, the layout rule in the Outdoor Essentials fence guide. Each 4×4 post sits in a 3 ft deep hole.

• Concrete per post: 1.8040 − 0.2552 = 1.5488 cubic feet.
• All 17 posts: 17 × 1.5488 = 26.329 cubic feet, which is 0.975 cubic yards.
• 80 lb bags (0.60): 26.329 ÷ 0.60 = 43.88, so 44 bags.
• With a 10% allowance: 26.329 × 1.10 = 28.962 cubic feet, 48.27 bags, so 49 bags.
• 60 lb bags (0.45) with the allowance: 28.962 ÷ 0.45 = 64.36, so 65 bags.

Rounding each post up to 3 bags would have given 17 × 3 = 51 bags. Rounding the total once gives 44, or 49 with the allowance. The 10% is a convention for spillage and uneven holes, not a standard, and it is multiplied in, not added to the bag count.

What changes the count

• Depth. Each extra foot of depth on a 4×4 adds about 0.52 cubic feet, roughly 0.9 of an 80 lb bag. Gate posts, corner posts and holes dug deeper for frost cost more.
• Hole width. The table assumes a hole exactly three times the post width. A hole dug wider than that holds more concrete, and the area grows with the square of the diameter.
• Round posts. A round post of the same width takes up less of the hole than a square one, so it needs slightly more concrete.
• Mix and bag size. Yield comes from the bag. Standard mix, fast-setting mix and specialty mixes can differ, and the 80 lb yield for fast-setting mix was not found on the data sheets used here.
• Gravel base. Anything you put in the bottom of the hole takes the place of concrete. The table does not subtract it.

Water, mixing and setting times vary by product. They are on the bag. For a whole fence the bags come on pallets: see how many bags of concrete are on a pallet.

Frost depth, code and limits

• The half-height rule is a manufacturer’s setting guide. Frost depth and local code can require holes deeper than that, and gate posts are often set deeper. Ask your building department or check your permit before you dig.
• Call 811 before you dig, and check property lines, height limits and HOA rules first.
• This guide counts concrete for fence posts. Posts that carry structural loads, such as deck posts, need footings sized for the load, soil and local code, which is a design task. Ask your building department or a licensed engineer.
• Wet concrete is caustic. Wear gloves and eye protection.

Common questions