How to use the fence post calculator
Enter the total fence length, the spacing between posts and the fence height. Count the corners and gates on your plan. Pick the post size and how deep to set it: one-third of the height is standard, half suits tall fences, windy sites and loose soil. For posts, rails and pickets together, use the fence calculator.
The formulas
- Posts = fence length ÷ spacing (rounded up) + 1 + corners + gates
- Hole width = 3 × post width; concrete depth = fence height × 1/3 (or 1/2); hole depth = concrete depth + 6 in of gravel
- Post length = fence height + concrete depth, then the next stock length (8, 10, 12, 14 or 16 ft)
- Concrete per post = (hole area − post area) × concrete depth ÷ 0.6 cu ft per 80 lb bag
Worked example: 100 ft of 6 ft fence, 4×4 posts 8 ft apart
- 100 ÷ 8 = 12.5 → 13 sections + 1 = 14 posts (2 ends, 12 line posts)
- Concrete depth: 6 ÷ 3 = 2 ft; hole 3 × 3 1/2 = 10 1/2 in wide × 2 ft 6 in deep
- Post length: 6 + 2 = 8 ft
- Concrete: (π × 0.4375² − 0.2917²) × 2 = 1.03 cu ft ÷ 0.6 = 1.72 bags per post → 25 bags of 80 lb
- Gravel: 6 in in each hole = 4.2 cu ft
Posts and holes by fence height
4×4 posts set one-third deep.
| Fence height | Post length (buy) | Hole, 10 1/2 in wide | Concrete per post |
|---|---|---|---|
| 3 ft | 4 ft (8 ft) | 1 ft 6 in deep | 0.86 bags |
| 4 ft | 5 ft 4 in (8 ft) | 1 ft 10 in deep | 1.15 bags |
| 5 ft | 6 ft 8 in (8 ft) | 2 ft 2 in deep | 1.43 bags |
| 6 ft | 8 ft (8 ft) | 2 ft 6 in deep | 1.72 bags |
| 8 ft | 10 ft 8 in (12 ft) | 3 ft 2 in deep | 2.29 bags |
Common mistakes
- Forgetting corner and gate posts. A straight count from length and spacing comes up short on any fence that turns or has a gate.
- Holes too narrow. A post in a tight hole has a thin shell of concrete that cracks; dig three times the post width.
- Skipping the gravel. Six inches of gravel under the post lets water drain away from the end grain instead of pooling in the concrete.
- Stopping above the frost line. Where the ground freezes, frost heave lifts shallow posts. Check your local frost depth and dig below it if it’s deeper than the rule gives.
- Buying posts to the exact length. Lumber comes in 2 ft steps; buy the next length up and cut the top level after the concrete sets.
Frequently asked questions
How many fence posts do I need?
Divide the fence length by the post spacing, round up and add one for the end. Then add one post for each corner and each gate. 100 ft at 8 ft spacing is 14 posts; with 2 corners and a gate it's 17.
How deep should a fence post be?
Set it in concrete 1/3 to 1/2 as deep as the fence is tall, on 6 in of gravel (QUIKRETE). For a 6 ft fence that's 2 ft of concrete in a hole 2 ft 6 in deep, or 3 ft of concrete in a 3 ft 6 in hole for windy sites. Go below the frost line where the ground freezes.
How long should fence posts be?
The fence height plus the depth in concrete. A 6 ft fence needs 8 ft posts at one-third depth or 9 ft posts at half depth (buy 10 ft and cut). A 4 ft fence needs 5 ft 4 in posts.
How wide should a fence post hole be?
Three times the width of the post. A 4×4 (3 1/2 in actual) needs a 10 1/2 in hole and a 6×6 (5 1/2 in actual) a 16 1/2 in hole.
How much concrete per fence post?
For a 6 ft fence with a 4×4 in a 10 1/2 in hole and 2 ft of concrete, about 1.72 bags of 80 lb per post. A 6×6 in a 16 1/2 in hole takes about 4.25 bags.
Why does the fence calculator give a different concrete figure?
It uses the hole size you type in (10 in by 2 ft by default, about 1.53 bags for a 4×4). This page sizes the hole from the post and the fence height with QUIKRETE's rule, so the hole and the concrete come out a little bigger.
Do I need a post at every corner and gate?
Yes. A corner post takes the pull from two directions, and a gate needs a post on each side. One of the gate posts usually replaces a line post, so each gate adds one post to the count.
Sources
Formulas and defaults last reviewed October 30, 2026. How we check our math.