This fence calculator estimates the posts, rails, pickets and bags of concrete you need for a straight run of wood fence, based on its length, post spacing, height and picket size. Use it to build a shopping list for a privacy or picket fence, or to sanity check a quote before work starts.
How to Use the Fence Calculator
- Measure the total length of the fence line in feet. For a fence with several sides, add the sides together.
- Choose the post spacing. Eight feet is common for wood fences built with 8 foot rails. Six feet makes a stiffer fence and suits windy sites or heavy boards.
- Choose the fence height.
- Enter the picket width in inches. A standard “1 x 6” fence board is actually about 5.5 inches wide.
- Enter the gap between pickets. Use 0 for a solid privacy fence, or a gap such as 2 to 3 inches for a spaced picket look.
- Read the posts, sections, rails, pickets, post hole depth and concrete bags in the results.
How the Calculation Works
The fence length is divided by the post spacing and rounded up to get the number of sections. Posts are sections plus one, because a straight run needs a post at each end. Fences taller than 5 feet get 3 rails per section; fences 5 feet and under get 2. Rails are sections times rails per section.
Pickets are the fence length in inches divided by the picket width plus the gap, rounded up. Post hole depth is one third of the fence height, with a minimum of 2 feet. Concrete is worked out for a 10 inch diameter hole at that depth, minus the space taken by a 4 x 4 post (3.5 x 3.5 inches actual). That volume is divided by 0.375 cubic feet, the rough yield of a 50 lb bag of concrete mix, and rounded up to whole bags per post.
Worked example with the default values: a 100 foot fence, 8 foot post spacing, 6 feet tall, 5.5 inch pickets with no gap.
- Sections: 100 / 8 = 12.5, rounded up to 13 sections
- Posts: 13 + 1 = 14 posts
- Rails: 6 feet is over 5 feet, so 3 per section, and 13 x 3 = 39 rails
- Pickets: 100 x 12 = 1,200 inches, and 1,200 / (5.5 + 0) = 218.2, rounded up to 219 pickets
- Post hole depth: 6 / 3 = 2 feet (which is also the minimum)
- Hole volume: 3.1416 x (10 / 24) x (10 / 24) x 2 = 1.09 cubic feet
- Post volume: (3.5 / 12) x (3.5 / 12) x 2 = 0.17 cubic feet
- Concrete per post: (1.09 – 0.17) / 0.375 = 2.46, rounded up to 3 bags
- Total concrete: 3 x 14 = 42 bags of 50 lb mix
Fence Materials by Height
These figures come from the same rules the calculator uses, shown per 8 foot section with 8 foot post spacing and a 10 inch hole. They are a quick way to compare heights before you commit.
| Fence height | Rails per section | Post hole depth | 50 lb bags per post | Typical post length |
|---|---|---|---|---|
| 3 ft | 2 | 2 ft | 3 | 6 ft |
| 4 ft | 2 | 2 ft | 3 | 6 to 8 ft |
| 5 ft | 2 | 2 ft | 3 | 8 ft |
| 6 ft | 3 | 2 ft | 3 | 8 ft |
| 8 ft | 3 | 2.7 ft | 4 | 10 to 12 ft |
Typical post length is the fence height plus the hole depth, rounded to a common lumber length. In cold climates the hole often needs to go below the local frost line, which can be deeper than these figures.
Tips and Common Mistakes
- Check property lines and HOA rules first. Confirm where your boundary is, using a survey if needed, and read any HOA or local rules on height, style and setback. Many towns require a permit for fences over a certain height.
- Call 811 before digging. In the US, call 811 or use your state’s online request a few days before you dig post holes so buried utility lines can be marked. It is free.
- Corners and gates add posts. The count assumes one straight run. Each extra corner and each gate opening usually needs at least one more post, and gate posts are often set deeper with more concrete.
- Gates are not included. Subtract gate widths from the fence length for pickets, then plan the gate hardware and framing separately.
- Buy a few extra pickets. The picket count has no waste allowance. Add about 5% for split boards, crooked pieces and cuts at the ends.
- Larger holes take more concrete. The concrete estimate assumes a 10 inch hole. A 12 inch hole or a 6 x 6 post changes the volume, so adjust the bag count if you dig wider.
Frequently Asked Questions
How many posts do I need for a 100 foot fence?
With 8 foot spacing, a straight 100 foot run needs 13 sections and 14 posts. With 6 foot spacing it needs 17 sections and 18 posts. Add posts for corners and gates.
How deep should fence posts be?
A common rule of thumb is one third of the post’s above ground height, with at least 2 feet in the ground. Local frost depth and soil can require deeper holes, so check local practice.
How many bags of concrete per fence post?
For a 4 x 4 post in a 10 inch hole about 2 feet deep, plan on about 3 bags of 50 lb concrete mix. Taller fences with deeper holes need about 4 bags per post.
Should I leave a gap between privacy fence boards?
Freshly milled boards can shrink as they dry, so many builders butt them tight or leave a very small gap. Dry or pressure treated boards that have already dried out can swell, so a small gap helps there.