UtilibyFence Post Calculator
Find how many posts, rails and concrete bags you need for your fence. Free, private and no sign-up.
How to use this free fence post calculator online
- Enter your total fence length — measure every straight run and add them together.
- Choose your fence type. The calculator will suggest a standard post spacing for that material.
- Adjust the spacing if your project requires something different.
- Enter the number of gates. Each gate replaces a section of fence but still needs two gate posts.
- Choose straight run or enclosed loop.
- Set post size, hole diameter and hole depth for the concrete estimate.
- Read the post count, rail count, concrete per hole and total bags below.
Post count formula
The calculator uses the classic "fence-post problem" formula from carpentry and mathematics:
- Straight run: Posts = (Length ÷ Spacing) + 1, rounded up. You need one post at the start, one at the end, and one between every pair of sections.
- Enclosed loop: Posts = Length ÷ Spacing, rounded up. The start post and end post are the same one.
- Gates: Each gate replaces one section, but the two posts flanking the gate still count. The calculator subtracts one section per gate and does not reduce the post count.
Source: this is a standard formula used by fence professionals and building suppliers.[reference:8][reference:9]
Standard post spacing by fence type
| Fence type | Typical spacing |
|---|---|
| Wood / privacy | 6–8 ft (1.8–2.4 m) |
| Chain-link | 4–10 ft (1.2–3.0 m), usually 8 ft |
| Vinyl | 6–8 ft (1.8–2.4 m) |
| Split rail | 8–10 ft (2.4–3.0 m) |
| Wire / livestock | 8–12 ft (2.4–3.6 m) |
Spacing is measured on-centre — from the middle of one post to the middle of the next.[reference:10]
Concrete per post
The concrete needed per hole is the hole volume minus the volume of the post sitting inside it. Hole diameter is typically about three times the post width, and depth is about one-third of the post length.[reference:11]
- 4 × 4 post, 8-inch hole, 30 inches deep: about 1.5 bags of 50 lb concrete, or 1 bag of 80 lb.
- 6 × 6 post, 12-inch hole, 36 inches deep: about 4 bags of 50 lb, or 3 bags of 80 lb.
Always add 10 percent extra for spillage and settling.[reference:12]
Worked example
150 ft straight fence, 8 ft spacing, 4 × 4 posts, 10-inch holes 24 inches deep, 80 lb bags.
- Posts: (150 ÷ 8) rounded up = 19, plus 1 end post = 20 posts
- Sections: 19
- Rails (2 per section): 38 rails
- Hole volume: π × (10/2/12)² × (24/12) = 1.09 cu ft
- Buried post volume: (3.5/12)² × (24/12) = 0.17 cu ft
- Concrete per hole: 1.09 − 0.17 = 0.92 cu ft
- Total concrete: 0.92 × 20 = 18.4 cu ft, plus 10% waste = 20.2 cu ft
- Bags: 20.2 ÷ 0.60 = 34 bags of 80 lb
Frequently asked questions
How many fence posts do I need for 100 feet?
At 8-foot spacing on a straight run, 100 feet needs 14 posts: (100 ÷ 8) rounded up is 13, plus 1 end post equals 14. At 6-foot spacing it needs 18 posts.
How far apart should fence posts be?
Wood privacy fences: 6 to 8 feet. Chain-link: 4 to 10 feet, usually 8. Vinyl: 6 to 8 feet. Split rail: 8 to 10 feet. Wire or livestock: 8 to 12 feet.
How many bags of concrete per fence post?
A typical 4×4 post in an 8-inch hole 36 inches deep needs 1 to 2 bags of 50-pound concrete. A 6×6 post in a 12-inch hole 48 inches deep needs 3 to 4 bags of 80-pound concrete.
How deep should fence posts be buried?
Bury at least one-third of the post length underground. For a 6-foot fence, use 8-foot posts buried 2 feet. In cold climates, bury below the local frost line, typically 36 inches in northern areas.
What is the difference between a straight run and an enclosed loop?
A straight run has two end posts, so it needs one more post than the number of sections. An enclosed loop shares its start and end post, so the post count equals the number of sections.
Is this fence post calculator free?
Yes. It is free, runs entirely in your browser and requires no sign-up. Nothing you enter is sent to our servers.