The Question Behind Every Fence and Deck Project
Ask a store associate how much concrete a post hole needs and you will get a shrug and a guess, usually two bags, whatever the hole. The real answer is arithmetic: a hole is a cylinder, a bag yields a known volume, and the post itself displaces some of both. Get it right and a fence line goes up in a day with no mid job store run and no pallet of leftover bags fossilizing in the garage. Get it wrong in the other direction and you pay to haul the overage back. This guide gives the yields, the table, and a full fence example, and it pairs with our Fence Calculator, which already includes per post concrete in its material list, and the main Concrete Calculator for converting bigger jobs to cubic yards. Prices referenced are October 2026 national home center pricing: standard mix about $5 to $7 per 80 lb bag, fast setting about $6 to $9 per 50 lb bag.
Start With Yields: What a Bag Actually Makes
| Bag size | Yield, mixed and placed | Typical price |
|---|---|---|
| 40 lb | About 0.30 cubic feet | $4 to $6 |
| 50 lb fast setting | About 0.375 cubic feet | $6 to $9 |
| 60 lb | About 0.45 cubic feet | $5 to $7 |
| 80 lb | About 0.60 cubic feet | $5 to $7 |
The formula is the same one the manufacturers print in their data sheets. Hole volume is pi times the radius squared times the depth, in feet. Subtract the post volume, which for a nominal 4x4 (a true 3.5 by 3.5 inches) is about 0.085 square feet times the buried depth. Divide what remains by the yield per bag and round up, then add a small allowance for holes that bell out at the bottom and soil that caves in. A 12 inch hole is the standard for a 4x4 post: three times the post width gives room for concrete to surround and lock it, and it is the size most power augers are rented to dig.
Bags Per Hole: The Table
| Hole (diameter x depth), 4x4 post | Net concrete | 80 lb bags | 60 lb bags | 50 lb fast set bags |
|---|---|---|---|---|
| 8 in x 24 in | About 0.53 cu ft | 1 | 2 | 2 |
| 10 in x 30 in | About 1.15 cu ft | 2 | 3 | 4 |
| 12 in x 36 in | About 2.10 cu ft | 4 | 5 | 6 |
| 12 in x 48 in (deep frost country) | About 2.80 cu ft | 5 | 7 | 8 |
Two things in this table surprise people. First, depth is expensive: going from 36 to 48 inches adds nearly 40 percent more concrete per hole, which is why frost depth research matters before the auger is rented. Second, hole diameter is the lever you control. Dropping from 12 to 10 inches on a shallow, sheltered fence cuts the bill by almost half, and in firm soil with short fence runs it is a legitimate choice. In loose or sandy soil the wider hole is doing structural work, and the savings are not worth a fence that leans by spring.
Worked Example: A 100 Foot Fence, 14 Posts
Take a 100 foot privacy fence with posts every 8 feet: 13 spaces, 14 posts including both ends. Holes are 12 inches across and 36 inches deep in a moderate climate, with 4x4 posts. Each hole needs about 2.10 cubic feet of concrete, so the line needs 14 times 2.10, about 29.4 cubic feet. In 80 lb bags at 0.60 cubic feet each, that is 49 bags, call it 51 or 52 with the spillage allowance. In 50 lb fast setting bags it is about 79 bags, which is why fast set is a convenience product for a few posts, not a material for a fence line: the same concrete costs roughly twice as much in small bags. At $5 to $7 per 80 lb bag, the concrete bill lands around $250 to $360. Cross the 60 bag line and it is worth a phone call about a ready mix short load or a mixer rental for a day, both of which price against your Saturday.
Technique Notes That Decide How Long Posts Live
- Gravel bed first: 4 to 6 inches of gravel in the bottom, post set on the gravel, not encased at the base, so end grain can drain.
- Brace before you pour: every post plumb and braced in both directions before any concrete, mixed or dry, goes in.
- Crown the top: slope the concrete surface away from the post so water sheds instead of pooling at the wood line.
- Mind the frost line: hole bottom below local frost depth, plus the gravel bed below that. Heaved posts are dug twice.
- Gate posts are special: gates concentrate torque. Go a size deeper and wider on hinge posts, or set them in a continuous concrete grade beam with the adjacent post.
- Deck and structural posts are footings, not fence posts: they follow local code for size and depth and get inspected in most jurisdictions. This guide covers landscape and fence work.
Dry Pour Versus Mixed, Settled Honestly
Dry pouring fast setting mix around a braced fence post, then watering the hole, is manufacturer approved and field proven for fences, mailboxes and signs. It trades a small amount of ultimate strength for a large amount of speed, and for a fence that trade is rational. Mixed concrete wins where loads are real: gate posts, deck footings, pergolas in wind country, and any post a structure depends on. The cost difference between the methods is mostly labor and bag size, not the concrete itself. Choose per post, not per project: many smart fence builds dry pour the line posts and properly mix the corners and gates, where the forces actually live.
Digging Day: Augers, Rocks and the Plan
The concrete math is the easy half; the holes are the work. A two person power auger rents for about $75 to $125 a day in 2026 and turns a 14 hole fence line into a morning, provided the soil cooperates. Rent it after the line is laid out, batter boards set and strings run, and call the utility locating service several working days before any digging, which is free and legally required in most states before mechanized digging. Dig a test hole first: if you hit rock, roots thicker than your wrist, or soil that caves like sugar, the day plan changes, and it is better to know at hole one than hole nine. Keep the auger holes plumb by starting slow and checking against the string, and finish each hole with a hand post hole digger to clean the bottom and bell it slightly if your soil holds a shape. In clay, a few inches of water in a finished hole softens the walls and lets them slump, so dig on the day you set posts or the day before a dry forecast. Set posts the same day the holes are open whenever possible: an open hole is a rain gauge, a trip hazard and a cave in waiting for a schedule.
Cold Weather and Hot Weather Setting
Concrete and weather have opinions about each other. In cold conditions, standard mixes gain strength slowly, and a hard freeze in the first days can damage young concrete in a hole just as on a slab: use fast setting mixes rated for cold, warm the mixing water for hand mixed work, and cover set posts overnight when frost threatens. Post holes forgive more than flatwork because the earth itself insulates, but a hole full of water before a freeze is a post heaved by spring. In hot weather the problem flips: dry soil drinks the water out of mixed concrete before it cures, so pre wet the hole walls, work in the morning, and keep the top of the set concrete damp under a cover for the first day on structural posts. Neither adjustment appears on the bag, which is written for a mild day that rarely attends a fence project. Plan around the forecast the way you planned around the frost line, and the bags per hole table above holds in any season.