Concrete for Fence Posts: How Many Bags Per Post Hole?
For a standard fence post hole — 12 inches wide, 36 inches deep, for a 4×4 post — the answer most people are looking for is below. The table covers the other common hole sizes.
Those counts are the exact fill for an empty hole. Post holes come with a built-in cushion: the post itself displaces about 0.25 ft³ of concrete in a standard hole, which roughly offsets the usual 10% waste allowance. If you want the cushion on paper anyway, buy 5 × 80 lb bags per hole — the table below shows both counts for every size.
Bags Per Post Hole, by Hole Size
Counts are per hole, rounded up. The "+10%" column multiplies the volume by 1.10 before dividing — the number to buy when you want a guaranteed cushion. Yields: 80 lb bag = 0.60 ft³, 60 lb bag = 0.45 ft³.
| Hole (diameter × depth) | Volume | 80 lb exact | 80 lb +10% | 60 lb exact | 60 lb +10% |
|---|---|---|---|---|---|
| 8 in × 24 in | 0.70 ft³ | 2 | 2 | 2 | 2 |
| 10 in × 30 in | 1.36 ft³ | 3 | 3 | 4 | 4 |
| 12 in × 36 in | 2.36 ft³ | 4 | 5 | 6 | 6 |
| 12 in × 48 in | 3.14 ft³ | 6 | 6 | 7 | 8 |
Buying for a whole fence run? Multiply the per-hole count by the number of posts, then add 10% once on the total instead of per hole. Rounding up on every hole quietly over-buys on a long run; one cushion at the end is enough.
How the Math Works
A post hole is a cylinder, so the volume is π × radius² × depth. Work in feet: the radius in feet is the diameter in inches ÷ 24, and the depth in feet is inches ÷ 12.
Volume (ft³) = π × (D ÷ 24)² × (depth ÷ 12) · Bags = ft³ ÷ yield, rounded up
Worked example, 12 in × 36 in: π × 0.5² × 3 = 2.36 ft³. Bags of 80 lb mix: 2.36 ÷ 0.60 = 3.93, so 4 bags. Bags of 60 lb mix: 2.36 ÷ 0.45 = 5.24, so 6 bags. With a 10% cushion the 80 lb count goes to 5; the 60 lb count stays at 6.
Odd sizes — a 9 in auger, a 42 in frost depth — will not be in any table. The calculator on the main page has a post hole mode that takes any diameter, depth, and post count.
How Deep and Wide Should a Post Hole Be?
Width: about 3× the post. A 4×4 post (actually 3.5 in square) wants a 10–12 in hole; a 6×6 post wants 16–18 in. That ring of concrete around the post is what holds it against lateral loads like wind and gate swing.
Depth: below the frost line, and about a third of the post buried. Two rules, and the deeper one wins:
- Frost line first. The bottom of the hole must sit below your local frost depth, or freeze-thaw cycles will heave the post out of line. Frost depth runs from roughly 12 in in the deep South to 48 in or more in the upper Midwest and New England. Check your local building code.
- The third rule. Bury about ⅓ of the post's total length. A 6 ft fence typically uses an 8 ft post with about 2 ft in the ground — in frost country that is not deep enough on its own, so rule 1 governs.
- Add a gravel pad. Dig about 6 in deeper than the post bottom and fill that with gravel for drainage. The gravel displaces concrete, which is another reason the exact bag counts above work out in practice.
That is why the table's depths are what they are: 24 in holes only make sense where frost is shallow, 36 in is the workhorse for most of the country, and 48 in is for deep frost, tall posts, and heavy gate or corner posts. Size corner and gate posts up a class — they carry the fence's tension and take the most abuse.
Bags-per-Post Rules of Thumb
- One 80 lb bag fills 0.60 ft³ of hole — about a quarter of a standard hole. If you can estimate a hole's volume, divide by 0.6 and round up.
- Small holes (8 in): 2 bags. Standard fence holes (10–12 in × 30–36 in): 3–4 bags of 80 lb mix per post. Deep or wide holes (12 in × 48 in): 6 bags.
- Fast-setting mix counts differ. It usually comes in 50 lb bags (0.375 ft³ each), so a standard 12 in × 36 in hole takes about 7 bags.
- Bag weight adds up: 20 posts at 4 bags each is 80 bags — 6,400 lb. Have them delivered, or stage them along the fence line before you start digging.
Run Your Exact Holes
Enter your hole diameter, depth, and number of posts in the calculator's post hole mode and get exact bag counts in 40, 50, 60, and 80 lb bags, with the waste allowance you choose.
Post Hole Concrete FAQ
How many bags of concrete do I need for a fence post?
For a typical 4×4 fence post in a 12 in × 36 in hole: 4 bags of 80 lb mix (6 bags of 60 lb). Smaller holes take less — a 10 in × 30 in hole takes 3 × 80 lb bags, and a shallow 8 in × 24 in hole takes 2. Deep 12 in × 48 in holes take 6 × 80 lb bags. Multiply by your post count and add 10% once for the whole job.
How deep should a fence post hole be?
Deep enough that the bottom of the hole is below your local frost line — that is the rule that matters most, and it ranges from about 12 in in warm climates to 48 in or more in cold ones. As a starting point, bury roughly ⅓ of the post: an 8 ft post for a 6 ft fence puts about 2 ft underground. Dig about 6 in past the post bottom and backfill that space with gravel for drainage.
Can I use fast-setting concrete for fence posts?
Yes — it is made for exactly this. You set the post, pour the dry mix into the hole around it, and add water on top; no premixing in a wheelbarrow, and the post is firm in under an hour. It costs a bit more per bag and usually comes in 50 lb bags, so a standard 12 in × 36 in hole takes about 7 bags instead of 4 × 80 lb. Regular mix is cheaper and works fine if you brace the post plumb while it cures.
Estimates Only
Results are planning estimates. Actual concrete needs vary with ground conditions, hole accuracy, over-digging, and product yield. Bag yields follow manufacturer figures for standard mixes; your product may differ slightly. Frost depth requirements vary by jurisdiction — follow your local building code for post depth. For structural work (foundations, load-bearing footings, retaining walls), consult a licensed professional.