How Many Bags of Concrete per Fence Post?
Concrete per hole
Volumes below are the hole minus the post, plus 10% because the sides of a hand-dug hole always bulge outward. Remember a 4x4 post is actually 3.5 in square and a 6x6 is 5.5 in.
| Post | Hole | Depth | Concrete | 80 lb bags | 50 lb bags |
|---|---|---|---|---|---|
| 4x4 | 8 in | 24 in | 0.6 cu ft | 1 | 2 |
| 4x4 | 10 in | 24 in | 1.0 cu ft | 2 | 3 |
| 4x4 | 10 in | 36 in | 1.5 cu ft | 3 | 4 |
| 4x4 | 12 in | 36 in | 2.4 cu ft | 4 | 7 |
| 6x6 | 12 in | 36 in | 2.1 cu ft | 4 | 6 |
| 6x6 | 14 in | 48 in | 3.9 cu ft | 7 | 11 |
| Gate or corner 6x6 | 16 in | 48 in | 5.3 cu ft | 9 | 15 |
Multiply by the number of posts and buy two bags over — the last hole is always the one that swallows more than the others.
How wide and how deep
- Width: about three times the post dimension. A 4x4 wants a 10–12 in hole, a 6x6 wants 12–16 in. The concrete collar needs enough thickness around the post to resist bending; a narrow collar cracks.
- Depth: one third of the post's above-ground height, or 2 ft, whichever is greater. A 6 ft fence on an 8 ft post goes 24–30 in down.
- Below the frost line in freezing climates — 36 to 48 in in northern states. Above it, frost heave lifts posts over winter and the fence never comes back level.
- Gate and corner posts carry the load. Go wider and deeper than the line posts, and use 6x6 if the design allows.
Wet mix, dry-pour, or foam
| Method | How it works | Where it fits |
|---|---|---|
| Standard mix, mixed wet | Mix in a tub or barrow, pour in | Strongest and cheapest per hole; use it for gate and corner posts |
| Fast-setting mix, dry-poured | Pour dry into the hole, add the water the bag specifies | Line posts where you want to brace and move on; sets in 20–40 minutes |
| Expanding foam | Two-part foam poured in, expands to fill | Fast and light to carry, costs more per post, less mass to resist racking |
Dry-pouring works for fence posts specifically because the hole is narrow, the surrounding soil supplies moisture, and the load is light. It is not a technique to carry over to a slab or a footing.
Details that decide whether the post lasts
- Gravel at the bottom. Four to six inches of compacted gravel under the post lets water drain away from the end grain instead of sitting in a concrete cup.
- Crown the top. Slope the concrete away from the post at grade so water runs off. A flat or dished top holds a puddle against the wood.
- Brace before it sets and check plumb on two adjacent faces. Fast-setting mix gives you minutes, not hours.
- Avoid backfilling soil over the concrete against a wood post — it traps moisture at exactly the height where posts rot.
Use the round column or post hole mode in the concrete calculator and set the quantity to your number of posts: it totals the bags for the whole fence in one step.
Work out how much concrete a pour needs in cubic yards — then see the same volume as 80, 60, 50 and 40 lb bags so you can compare bagged mix against a ready-mix delivery before you order.
Frequently asked questions
How many bags of concrete for a 4x4 fence post?
For a 10 inch hole 2 feet deep, about 1 cubic foot of concrete: two 80 lb bags or three 50 lb bags. The same width at 3 feet deep needs three 80 lb bags.
How deep should a fence post hole be?
One third of the post height above ground or 2 feet, whichever is greater, and always below the local frost line in freezing climates — which can mean 36 to 48 inches. Gate and corner posts should go deeper than line posts.
Can I pour dry concrete into a post hole?
For fence posts, yes, using a fast-setting mix poured dry and watered per the bag instructions. The narrow hole and moisture in the surrounding soil make it work. Do not use the technique on slabs or footings.
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