"How many bags of concrete per fence post?" depends on three numbers: the hole diameter, how deep the concrete goes, and how much room the post itself takes up. A standard 4×4 wood post set in a 10 inch hole, 36 inches deep with 6 inches of gravel at the bottom, takes about 1.15 ft³ of concrete: roughly 3 bags of 50 lb fast-setting mix or 2 bags of 80 lb concrete per post. This calculator works it out for your posts, subtracts the post's own volume, adds gravel for drainage and totals the bags for the whole fence.
Planning the whole fence? The fence calculator counts posts, rails and pickets. For deck piers and sonotubes use the concrete column & post hole calculator.
How to use the fence post concrete calculator
- Enter the number of posts, including corner, end and gate posts.
- Choose the post size. Wood posts are listed by their actual size (a 4×4 is 3.5 × 3.5 inches).
- Enter the hole diameter. A common guideline is a hole about three times the post width: 10 to 12 inches for a 4×4 and 16 to 18 inches for a 6×6.
- Enter the hole depth and the gravel layer. Many fence builders bury about a third of the post's above-ground height and put 4 to 6 inches of gravel in the bottom so water can drain away from the end grain.
- Pick fast-setting or standard concrete mix and keep a little extra, because dug holes are rarely perfectly round.
Formula
All dimensions are in inches; dividing cubic inches by 1,728 gives cubic feet. Yields: 50 lb fast-setting = 0.375 ft³, 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³.
Worked example
Ten 4×4 posts in 10 inch holes, 36 inches deep, with 6 inches of gravel, set with 50 lb fast-setting concrete and 10% extra.
- Hole area: π × 5² = 78.54 in². Post area: 3.5 × 3.5 = 12.25 in². Net: 66.29 in².
- Concrete depth: 36 − 6 = 30 in. Per post: 66.29 × 30 = 1,988.7 in³ ÷ 1,728 = 1.151 ft³, or 3.07 bags of fast-setting mix.
- All posts with 10%: 1.151 × 10 × 1.10 = 12.66 ft³ ÷ 0.375 = 33.8, so 34 bags of 50 lb fast-setting (or 22 × 80 lb).
- Gravel: 78.54 × 6 ÷ 1,728 = 0.273 ft³ per hole × 10 × 1.10 = 3.0 ft³, or 6 bags of 0.5 ft³ gravel.
Bags of concrete per post chart
Bags per post for common posts and holes, with a 6 inch gravel base and no waste. "Fast" is 50 lb fast-setting mix; the second figure is 80 lb concrete mix.
| Post and hole | 24 in deep | 30 in deep | 36 in deep | 42 in deep |
|---|---|---|---|---|
| 4×4 wood in a 10 in hole | 1.8 fast · 1.2 × 80 lb | 2.5 fast · 1.5 × 80 lb | 3.1 fast · 1.9 × 80 lb | 3.7 fast · 2.3 × 80 lb |
| 4×6 wood in a 12 in hole | 2.6 fast · 1.6 × 80 lb | 3.5 fast · 2.2 × 80 lb | 4.3 fast · 2.7 × 80 lb | 5.2 fast · 3.3 × 80 lb |
| 6×6 wood in a 16 in hole | 4.7 fast · 3.0 × 80 lb | 6.3 fast · 4.0 × 80 lb | 7.9 fast · 4.9 × 80 lb | 9.5 fast · 5.9 × 80 lb |
| 2-3/8 in steel in an 8 in hole | 1.3 fast · 0.8 × 80 lb | 1.7 fast · 1.1 × 80 lb | 2.1 fast · 1.3 × 80 lb | 2.5 fast · 1.6 × 80 lb |
Doubling the hole diameter roughly quadruples the concrete, so a 12 inch hole for a 4×4 uses about 52% more than a 10 inch hole at the same depth.
Fast-setting vs standard concrete for posts
Fast-setting post mix is poured dry into the hole around a braced post and then soaked with water; it sets in minutes, so you can keep building the same day. It yields less per bag (about 0.375 ft³ per 50 lb) and costs more per cubic foot. Standard concrete mix is mixed in a wheelbarrow first, gives more concrete per dollar and per pound, and needs the post braced for longer while it cures. For a long fence, standard mix is usually cheaper; for a few posts or a quick repair, fast-setting saves time.
Depth, frost and drainage
In cold climates the bottom of the hole should be below the local frost depth so frost heave doesn't lift the post. Your building department can tell you the local figure. Crown the concrete at the top so water runs away from the post, and leave the gravel uncovered at the bottom so water can drain from the end grain. Once the posts are in, the deck stain calculator estimates stain for the fence boards, and the pea gravel calculator sizes gravel for a path along it.
Caveats and assumptions
- Post volume is subtracted only over the concrete depth; the post also sits in the gravel, which we ignore because gravel is cheap and bags are sold whole.
- Auger holes are close to round; holes dug by hand are often bell-shaped or wider at the top, so raise the waste allowance.
- Gate posts and corner posts often need bigger, deeper holes. Enter them separately if they differ.
- Some fence builders set wood posts in compacted gravel only, with no concrete. Follow local practice and the fence manufacturer's instructions.
Frequently asked questions
How many bags of concrete do I need per fence post?
For a 4×4 post in a 10 inch hole with 30 inches of concrete above a 6 inch gravel base, about 3 bags of 50 lb fast-setting concrete or 2 bags of 80 lb concrete mix.
How many bags of concrete for a 6x6 post?
In a 16 inch hole with 30 inches of concrete, about 3.0 ft³: roughly 8 bags of 50 lb fast-setting or 5 bags of 80 lb mix per post.
How big should a fence post hole be?
A common guideline is a diameter about three times the post width and a depth of about one third of the post height above ground, plus a few inches for gravel. Go below the frost line where required.
How many 50 lb bags of Quikrete per 4x4 post?
About two to three for a typical 24 to 36 inch deep, 10 inch hole. At 24 inches deep with 6 inches of gravel it is about 1.8 bags; at 36 inches it is about 3.1 bags.
Should I put gravel under a fence post?
Many builders put 4 to 6 inches of gravel at the bottom of the hole so water can drain away from the end of the post, which helps wood posts last longer.
Sources
- Quikrete — Setting posts — hole size, gravel base and fast-setting method
- Quikrete — Fast-Setting Concrete Mix — product use and yield
- Quikrete — Concrete Calculator and product yields — yield per 80, 60 and 40 lb bag
- NIST — PS 20 American Softwood Lumber Standard — actual dimensions of 4×4 and 6×6 lumber
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.