Fence calculator: posts, rails, pickets and bags of concrete for a straight fence run, from its length, post spacing, picket width and gap.
By AdminDrafted with AI writing toolsUpdated Checked examples: 4
A 100 ft run with posts every 8 ft needs 14 posts, 26 rails and 252 pickets. Setting the posts takes 16.1 ft³ of concrete, 27 bags of 80 lb.
Rounded up; a straight run has one more post than sections. The last section is 4 ft.
Each picket takes its own width plus one gap. Rounded up.
The hole volume minus the part of the post inside it.
Rounded up to whole bags; yield per 80 lb bag from Quikrete/Sakrete data sheets.
A straight fence has one more post than it has sections, and sections are the length divided by the post spacing, rounded up. Rails are sections × rails per section. Pickets are the length divided by one picket width plus one gap, with a margin for spares. Concrete per post is the hole's cylinder minus the square post inside it, times the concrete depth.
The default 100 ft run with posts 8 ft apart has 13 sections, 14 posts, 26 rails and 252 pickets 3½ in wide with 1½ in gaps. Setting the posts in 10 in holes with 30 in of concrete takes 16.1 ft³, or 27 bags of 80 lb mix at Quikrete's 0.60 ft³ per bag.
For an L-shaped or enclosed fence, run the calculator once per straight side and subtract one post for each shared corner.
Round the run length ÷ post spacing up to whole sections; posts are one more than sections, rails are sections × rails per section, and pickets are length ÷ (picket width + gap). An 80 ft run at 8 ft spacing has 10 sections, 11 posts and 20 rails at two a section.
Rounding up leaves twelve 8 ft sections and one of 4 ft. Spacing the same 14 posts evenly instead puts them 100 ÷ 13 = 7.69 ft apart, about 7 ft 8.3 in.
Count each straight side on its own, then subtract one post for every corner the sides share. An L-shaped fence with two 50 ft sides at 8 ft spacing has 8 posts per side, 15 in all.
A gate splits a side into two runs, each ending at a gate post. Count the two runs separately, leaving out the opening, and add the gate and its hardware on top.
Concrete per post is the hole's cylinder minus the square post: (π × D² ÷ 4 − a²) × h, with D the hole diameter, a the post width and h the concrete depth. Divide cubic inches by 1,728 for cubic feet.
Quikrete's post-setting guide sizes the hole at three times the post width. Its depth is ⅓ to ½ of the post's height above ground, plus 6 in for a gravel base.
The concrete stops 3–4 in below ground, so h is the depth that gets concrete, not the whole hole.
The post's diagonal, a × √2, must be smaller than the hole. A 3½ in post has a 4.95 in diagonal; an 8 in post has an 11.31 in diagonal and cannot fit a 10 in hole.
The Quikrete Concrete Mix data sheet gives yields of 0.30, 0.375, 0.45 and 0.60 ft³ for 40, 50, 60 and 80 lb bags. The 16.11 ft³ above is 27 bags of 80 lb, 36 of 60 lb or 54 of 40 lb.
Kilogram bags are estimated at the same yield per pound: about 0.33 ft³ for 20 kg, 0.41 ft³ for 25 kg and 0.50 ft³ for 30 kg. Check the yield printed on the product you buy.
"Default" is the 100 ft run worked above. "Exact sections" is the 80 ft run, and "One section" is a single 8 ft section with no spare pickets.
"Closed privacy" is 50 ft of 5½ in boards with no gap or spares, three rails, and 5½ in posts in 12 in holes with 36 in of concrete.
"Metric run" is 30 m at 2.4 m spacing with three rails, 90 mm pickets and 30 mm gaps, and 90 mm posts in 250 mm holes with 60 cm of concrete, set with 50 lb bags.
| Scenario | Posts | Pickets | Concrete bags |
|---|---|---|---|
| Default | 14.000000 | 252.000000 | 27.000000 |
| Exact sections | 11.000000 | 202.000000 | 22.000000 |
| One section | 2.000000 | 20.000000 | 4.000000 |
| Closed privacy | 8.000000 | 110.000000 | 24.000000 |
| Metric run | 14.000000 | 263.000000 | 33.000000 |
Total the lumber for posts, rails and pickets with the board foot calculator. For footings of other shapes, use the concrete calculator.
Checked against: ⌈100/8⌉ = 13 sections, 14 posts; 1200 in ÷ 5 in = 240 × 1.05 = 252; (π·5² − 3.5²) × 30 in³ × 14 = 16.11 ft³ ÷ 0.6 = 26.9 → 27
Checked against: Hand calculation: 80 ÷ 8 = 10 sections exactly, one more post than sections
Checked against: 12.5 → 13 sections; 30000 ÷ 120 = 250 × 1.05 = 262.5 → 263; (π·125² − 90²) × 600 mm³ × 14 = 344.3 L ÷ 0.375 ft³
Checked against: 600 in ÷ 5.5 = 109.1 → 110; (π·6² − 5.5²) × 36 × 8 posts = 13.81 ft³ ÷ 0.6 → 24
One more than the number of sections. Divide the fence length by the post spacing and round up to get sections, then add 1: a 100 ft run at 8 ft spacing is 12.5, rounded up to 13 sections and 14 posts, with a last section of 4 ft. For a fence with corners, count each side separately and subtract one post for every shared corner.
Quikrete's guide says to dig one third to one half of the post's above-ground height, plus 6 in for a gravel base, and to make the hole three times the post's width. A post standing 6 ft out of the ground needs a 30–42 in hole, 10½ in wide for a 3½ in 4 × 4. For load-bearing posts, Quikrete refers you to local building codes.
Divide the concrete in each hole by the bag's yield. A 10 in hole with 30 in of concrete around a 4 × 4 post holds 1.15 ft³. Quikrete's data sheet gives 0.60 ft³ per 80 lb bag of Concrete Mix, 0.45 ft³ per 60 lb and 0.30 ft³ per 40 lb, so that post takes 2 bags of 80 lb, 3 of 60 lb or 4 of 40 lb.
Divide the fence length by one picket's width plus the gap. For 100 ft (1,200 in) of 3½ in pickets with 1½ in gaps that is 1,200 ÷ 5 = 240, or 252 with 5% extra for splits and cuts. A closed privacy fence of 5½ in boards with no gap needs 1,200 ÷ 5.5 = 218.2, so 219 boards before any extra.
With Quikrete Concrete Mix, which is mixed with water before it goes in the hole, the data sheet says to wait 24 hours before the post is subjected to any strain. Quikrete's fast-setting concrete is poured dry into the hole, then water is added, and it sets in 20 to 40 minutes.
This calculator includes 4 worked examples with independently sourced answers. They run in the test suite, and you can run them here too.