Fence Calculator

How the fence calculator works

A post-and-rail fence breaks down into repeating sections (bays). Each section spans the gap between two posts, carries a set number of rails, and is covered by pickets or boards (or one panel). The calculator divides each run by your post spacing, rounding up, to count sections. It then adds the posts at the ends, the rails, the pickets, and the concrete to set every post.

How the runs connect changes the post count. Runs joined end to end share their corner posts, so a chain needs one more post than it has sections. A fully enclosed loop needs exactly as many posts as sections. Separate runs each need their own end posts. Each gate adds one post, a simple allowance for the extra post a gate opening needs. Check this against your own layout, because gate hardware and opening widths vary.

The formulas

Sections per run = run length ÷ post spacing   (round up)
Posts            = sections + 1 (joined) | sections (closed loop) | sections + runs (separate), + gates
Rails            = sections × rails per section
Pickets          = (total length − gate widths) ÷ (picket width + gap)
Concrete / post  = π × (hole Ø ÷ 2)² × fill depth − post width² × fill depth
Fill depth       = hole depth − gravel layer − unfilled top

Post-hole guidance from the manufacturer

The default hole numbers follow QUIKRETE’s published post-setting instructions. Make the hole about three times the post’s width. Make it 1/3 to 1/2 of the post’s above-ground height deep, plus 6 in. Put 6 in of gravel in the bottom, and fill the concrete to 3–4 in below ground so soil or sod can finish the top. QUIKRETE’s Fast-Setting data sheet also gives a check figure: two 50 lb bags set a 3 in square post in a 9 in hole that is 30 in deep. Filled all the way, that hole works out to about 2.5 bags by geometry. So expect real-world use to land near the calculator’s figure, and keep a spare bag.

In the UK, products such as Blue Circle Postcrete come in 20 kg bags. Their bag yield isn’t listed on the product page, so check the bag or the technical data sheet before using it here.

Worked examples

100 ft of picket fence with one gate

100 ÷ 8 = 12.5, so 13 sections and 14 posts, plus 1 for the gate: 15 posts. Two rails per section is 26 rails. With 5.5 in boards butted together over 96 ft (100 ft less a 4 ft gate): 1,152 ÷ 5.5 = 209.5, so 210 pickets. With 10.5 in holes 30 in deep, 6 in of gravel and 3 in left at the top, each post takes 0.90 ft³ of concrete. That is 2.4 bags of 50 lb fast-setting mix, or 37 bags for all 15 posts.

Three sides of a garden in 1.83 m panels (metric)

Runs of 10, 15 and 10 m joined at the corners: 6 + 9 + 6 = 21 panels and 22 posts. With 100 mm posts in 300 mm holes 750 mm deep (150 mm gravel, 75 mm top), each hole takes about 32 litres of concrete, or 2.75 bags of 25 kg. That comes to about 61 bags in total.

Before you dig

• Find buried services. In the US, contact 811 a few days before digging; utilities will mark their lines for free. Elsewhere, check with your utilities and use a cable detector.
• Check the rules. Height limits, permits, boundary lines, frost depth and HOA rules vary by place. Confirm them with your local authority before buying materials.
• Brace posts while the concrete sets. QUIKRETE says to wait 4 hours before putting strain on posts set with fast-setting concrete.
• Buy a few spare boards. Lumber includes the odd warped or split piece.