A nominal 4x4 measures 3.5 in; a 6x6 measures 5.5 in. Enter 0 to ignore the post.
Over-excavation, spillage and uneven subgrade. 5-10% for formed work, 10-15% on rough ground.
Yields are the figures printed on the bag. Speciality mixes differ — check yours.
Optional. Leave at zero to skip the cost estimate.
Material quantity estimate for planning and purchasing. Not structural engineering advice, not a bid, and not a code determination.
How deep and how wide?
Two rules cover almost every fence and mailbox post:
- Depth: one third of the above-ground post height, or below the frost line, whichever is deeper. A 6 ft fence on an 8 ft post gives 2 ft of burial.
- Diameter: three times the post width. A 4x4 wants a 10-12 inch hole, a 6x6 wants 16-18 inches.
Structural posts carrying a deck or roof are a different calculation entirely — those are footings sized to a load, not fence posts, and they follow the frost-depth and bearing rules.
| Post | Hole | Depth | Concrete per hole | 50 lb fast-set bags |
|---|---|---|---|---|
| 4x4 fence post | 10 in | 24 in | 0.92 ft³ | 3 bags |
| 4x4 fence post | 12 in | 30 in | 1.75 ft³ | 5 bags |
| 4x4 mailbox post | 10 in | 18 in | 0.69 ft³ | 2 bags |
| 6x6 corner post | 18 in | 36 in | 4.67 ft³ | 13 bags |
| 6x6 gate post | 18 in | 42 in | 5.45 ft³ | 15 bags |
Fast-setting mix and the gravel base
Fast-setting post mix is poured in dry, then water is added to the hole — no mixing, set in 20-40 minutes. A 50 lb bag yields about 0.375 ft³, less than a 50 lb bag of standard mix, so the bag count runs higher.
Put 4-6 inches of compacted gravel in the bottom of the hole before the concrete. It drains water away from the end grain, which is where posts rot first. Crown the concrete above grade and slope it away from the post for the same reason.
That gravel is also why you will see two different bag counts quoted for what sounds like the same hole. The table above is full-depth concrete: a 4x4 in a 10 inch hole 24 inches deep holds 0.92 ft³, which is three bags. The familiar two-bags-per-hole figure assumes 6 inches of gravel in the bottom, leaving 18 inches of concrete — 0.69 ft³, or two bags. Both are correct. They are different holes, and it is worth knowing which one you are digging.
Common questions
How many bags of concrete per fence post?
For a 4x4 in a 10 inch hole filled with concrete to the full 24 inches, three 50 lb bags of fast-setting mix — the hole holds 0.92 cubic feet once you deduct the post. If you put 6 inches of gravel in the bottom first, which you should, the concrete depth drops to 18 inches and two bags does it. That is where the two-bags rule of thumb comes from.
Should I subtract the post from the hole volume?
Yes. A 3.5 inch square post 24 inches deep displaces 0.17 cubic feet — about 11% of a 12 inch hole. Across 30 fence posts that is five cubic feet of concrete you would otherwise buy and not use.
Do fence posts need concrete at all?
Not always. Compacted gravel alone drains better and lets you reset a post later, and in well-drained soil it holds a fence line fine. Concrete is the right answer for gate posts, corner posts and anything carrying lateral load.
How deep should a 6 foot fence post be?
Two feet minimum, using the one-third rule on an 8 ft post — deeper if your frost line is below 24 inches. Corner and gate posts go a further 6-12 inches deeper because they take the racking load.