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Horizontal Fence Calculator
Work out posts, horizontal boards and concrete for a horizontal slat fence.
Your result
Enter your numbers and press the button. Your result appears here.
Calculation Formula
Posts
\[\text{Number of Posts} = \left\lceil \frac{\text{Fence Length (ft)}}{\text{Post Spacing (ft)}} \right\rceil + 1\]
Boards
\[\text{Boards per Section} = \left\lceil \frac{\text{Fence Height (ft)} \times 12 - \text{Top Gap} - \text{Bottom Gap} + \text{Board Gap}}{\text{Board Height (in)} + \text{Board Gap (in)}} \right\rceil\]
Total
\[\text{Total Boards} = \text{Boards per Section} \times (\text{Number of Posts} - 1)\]
Concrete
\[\text{Bags per Post} = \left\lceil \frac{\left(\pi \left(\frac{D}{2}\right)^2 - w^2\right) \times d \div 1728}{0.375} \right\rceil\]
A stack of n boards has n − 1 gaps, which is why one board gap is added on top. Boards span one section, so buy boards at least as long as your post spacing. D is the hole diameter (3 × the post width), w is the actual post width (3½ in for a 4×4), d is the depth of concrete in inches and 0.375 ft³ is the yield of a 50 lb bag of fast-setting concrete. Posts go in the ground half as deep as the fence is tall, with 6 in of gravel underneath.
Example: 100 ft of fence needs 14 posts, 156 boards (12 per section) and 70 bags of fast-setting concrete.
Rules of thumb
| Item | Typical |
|---|---|
| Depth in the ground | ⅓ of the post length, or below the frost line |
| Hole width | About 3 × the post width |
| Wood post spacing | 6 to 8 ft |
| Chain link line posts | No more than 10 ft |
| T-posts for barbed wire | 8 to 12 ft |
| Field fence | Wood posts 8–10 ft, T-posts 6–8 ft |
Common questions
How many rails does a fence need?
Two rails for fences up to about 4 ft, three for 5 to 6 ft and four for 8 ft fences.
Should posts be 4×4 or 6×6?
4×4 posts are standard for fences up to 6 ft. Use 6×6 for 8 ft fences, gate posts and windy sites.
How long should fence posts be?
Height above ground plus the depth in the ground. For a 6 ft fence that is usually a 9 ft post, so buy 10 ft stock and cut it, or set 8 ft posts deeper only where frost and soil allow.
How many bags of concrete per fence post?
For a 4×4 post in a hole about 10 to 12 in wide and 2 to 3 ft deep, plan on 2 to 4 bags of 50 lb fast-setting mix. Bigger holes and 6×6 posts take more.
Sources
- Quikrete, Concrete Mix data sheet (bag yields): https://www.quikrete.com/PDFs/DATA_SHEET-Concrete%20Mix%201101.pdf
- Quikrete, project calculator (post depth example): https://www.quikrete.com/calculator/