Fence Calculator
Five counts come out of two measurements: posts, rails, pickets, hole depth and bags of concrete, from a length and a height. Most of the errors come from lumber being narrower than its name, which is why this calculator works in actual sizes.
How it works
Sections come first: the run divided by the post spacing, rounded up. Spacing defaults to 8 ft because that is the rail length: Master Halco's specification puts line posts at the length of the rail plus 2 in, on center, and notes that the exact figure moves with the rails, the height and the slope. This calculator counts sections off the 8 ft rail itself, since that is what decides how many rails you buy — set your own spacing if you are working to a different one. Posts are sections plus one, so a 100 ft run is 13 sections and 14 posts.
Rail rows follow the height at one row per 2 ft of fence height, with a floor of two. A 6 ft fence takes 3 rows, so those 13 sections need 39 rails. The height that counts is the fence you see, not the post: Mansfield's fence standard states the rule and its own worked case — six feet of fence, three stringers — and keeps how deep the post is buried in a separate clause.
Pickets are where nominal sizes bite. A 1×6 picket is 5-1/2 in wide, not 6 in, because dressed softwood is milled under its nominal size. 1,200 in of run divided by 5.5 is 219 pickets — not the 200 the name implies. A 4×4 post is likewise 3-1/2 in, which is what sets the hole diameter.
Post holes use the manufacturer's rule: diameter is 3 times the post width, depth is 1/3 of the overall post length. A 6 ft fence therefore wants a 9 ft post buried 36 in in a 10-1/2 in hole. The bottom 6 in of that hole takes dry mix, so wet concrete fills only the top 30 in, less the post itself: 1.29 ft³. The dry layer is the same bagged product and you buy it too, so the bag count covers the whole hole — 1.59 ft³, or about 2.7 of the 80 lb bags per post, which is 38 bags for a 100 ft run.
Formula: Sections = length ft ÷ 8 ft spacing, rounded up · Posts = sections + 1 · Rails = sections × rail rows · Pickets = length ft × 12 ÷ picket width in · Concrete per post = (hole area − post cross-section) × depth above the dry base, plus hole area × the 6 in dry base
Measuring on site
- Measure each straight leg separately, corner to corner along the ground. A 40 ft leg and a 60 ft leg are not a 100 ft fence: each rounds up to its own sections and each needs its own end post.
- Decide gates before you count. A gate opening is not a fenced section, and its two posts carry more load than a line post.
- Set the height you want above grade, not the post you plan to buy. The calculator adds the buried third: ask for 6 ft of fence and it tells you to buy 9 ft posts.
Waste and what it covers
There is no waste percentage here, because fence parts are counted, not poured. What varies is the picket gap: a solid fence at 0 in takes 219 pickets per 100 ft, the same run at 2 in takes 160. Concrete is the one item worth a margin — a hole that hits a root takes more than the geometry says.
| Run | Sections | Posts | Rails | Pickets | 80 lb bags |
|---|---|---|---|---|---|
| 50 ft | 7 | 8 | 21 | 110 | 22 |
| 100 ft | 13 | 14 | 39 | 219 | 38 |
| 150 ft | 19 | 20 | 57 | 328 | 54 |
| 200 ft | 25 | 26 | 75 | 437 | 69 |
Questions
- How many fence posts for 100 feet?
- 14, at the standard 8 ft spacing. 100 ÷ 8 is 12.5, which rounds up to 13 sections, and 13 sections need 14 posts. The last section is short — 4 ft rather than 8 — so decide where that shortfall goes before you dig.
- How many pickets do I need for a 100 ft fence?
- 219 for a solid fence built from 1×6 pickets. The picket is 5-1/2 in wide, not 6, so 1,200 in divided by 5.5 gives 218.2, rounded up. Assuming 6 in pickets would have you order 200 and come up 19 short.
- How deep should fence post holes be?
- QUIKRETE's rule is 1/3 of the overall post height, which puts a 6 ft fence on a 9 ft post set 36 in deep. The 'at least 2 ft' half of the popular rule has no primary source behind it, so we do not apply it. Your frost line does override the rule: in cold regions the hole has to reach below it, and that depth comes from your building department.
- How many bags of concrete per fence post?
- About 2.7 of the 80 lb bags for a 6 ft fence on 4×4 posts, which is 38 bags for a 100 ft run. That is a 10-1/2 in hole 36 in deep: 1.29 ft³ of wet concrete in the top 30 in, less the post, plus 0.30 ft³ of dry mix in the bottom 6 in. The dry layer is the same bagged product and you have to buy it, so it is in the bag count even though it is kept out of the wet volume.
Sources
- Master Halco — PostMaster+® Steel Post for Wood Fence — CSI Specification, Section 32 31 29“Line Posts spaced length of rail +2” (50.8mm) on center. Exact spacing may be modified depending on fencing rails used, fence heights and ground slope.”
- City of Mansfield, Texas — New or Replacement Wood Privacy Fence Construction Standards (11 Feb 2020)“One stringer is required for every two feet (2’) of height. Fences that are six feet (6’) in height must have three (3) horizontal stringers. Fences that are eight feet (8’) in height must have four (4) horizontal stringers.”
- City of Mansfield, Texas — New or Replacement Wood Privacy Fence Construction Standards (11 Feb 2020)“The maximum height for a residential privacy fence is eight (8) feet.”
- NIST — Voluntary Product Standard PS 20, American Softwood Lumber Standard, Table 3“Nominal and minimum-dressed sizes of boards, dimension, and timbers: nominal thickness 1 in → minimum dressed dry 19 mm (3/4 in); nominal thickness 2 in → 38 mm (1-1/2 in); nominal width 4 in → 89 mm (3-1/2 in); nominal width 6 in → 140 mm (5-1/2 in).”
- QUIKRETE — Concrete Mix data sheet (Product No. 1101)“Start by digging the post hole about 3 times the diameter of the post. Hole depth should be 1/3 the overall post height. Place 6 in (150 mm) of dry concrete mix in the bottom of the hole.”
- QUIKRETE — Concrete Mix data sheet (Product No. 1101)“An 80 lb (36.2 kg) bag yields approximately 0.60 ft3 (17 L).”
Figures and sources last checked by The BuilderTally Team in August 2026.