Fence calculator

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.

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 4 worked examples whose answers come from independent sources; for example, “100 ft run, 8 ft spacing, 3½ in pickets with 1½ in gaps” is checked against Python decimal: ⌈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.

Quikrete — setting posts in concrete (hole 3× post width, depth ⅓–½ of the above-ground height plus 6 in of gravel); Quikrete Concrete Mix (No. 1101) data sheet — bag yields 40 lb 0.30, 60 lb 0.45, 80 lb 0.60 ft³.