How the math works
Sections = fence length ÷ spacing, rounded up (100 ÷ 8 = 12.5 → 13)Posts = sections + 1 (14)Concrete depth = hole depth − gravel base (36" − 4" = 32")Hole volume = π × (diameter ÷ 2)² × concrete depth, minus the post (3-1/2" × 3-1/2" for a 4x4)Per hole: 1.228 cu ft × 1.05 ÷ 0.375 = 3.44 → 4 bags
Add one post for each corner, gate side and end of every separate run.
Trade pro tips
- Dig to 1/3 of the post’s above-ground height, and below your local frost line in cold climates.
- Holes should be about 3× the post width: 10"–12" for 4x4s, 12" for 6x6s.
- Put 3"–6" of gravel in the bottom for drainage so the post end isn’t sitting in water.
- Crown the concrete at the top so water sheds away from the post.
- Call 811 before you dig — it’s free and required in every state.
Frequently asked questions
How many bags of concrete per fence post?
For a 4x4 in a 10" × 36" hole with a 4" gravel base: 4 bags of 50 lb fast-setting concrete. An 8" × 24" hole needs 2.
How many fence posts do I need for 100 feet?
14 posts at 8 ft spacing or 18 at 6 ft spacing, for one straight run with posts at both ends.
How deep should fence post holes be?
About one-third of the post’s height above ground — 24"–36" for a 6 ft fence — and below the frost line where the ground freezes.
Do I mix fast-setting concrete with water first?
No. Quikrete Fast-Setting is poured dry into the hole around the braced post, then about a gallon of water per 50 lb bag is added. It sets in 20–40 minutes.