Fence Calculator

How the Fence Calculator works

Fence calculator: bays first, then everything hangs off them

A fence is a series of bays between posts. The number of bays is the fence length divided by the post spacing, rounded up so the last bay is shorter rather than longer than the rest. Posts are bays plus one, because a run needs a post at both ends. Rails are the bays multiplied by the rails per bay — two for a fence up to about 1.2 m, three for a 1.8 m paling fence — and the page also gives the lineal length of rail so a timber yard can quote it. Pickets, or palings, are the fence length divided by the picket width plus the gap. For a lapped paling fence with no gap, enter the gap as 0 and the page counts one paling per width; then add the lapping course separately, which is usually about one extra paling per three.

A fence with a corner or a gate is two or more runs. Enter each run on its own and add the results; corners share a post, so subtract one post per corner, and a gate needs its own heavier posts and usually a bag or two more concrete each.

The assumed waste allowance is 5%

Fencing generates less waste than flooring or decking because most pieces are used whole. Posts are not wasted at all, so the allowance is applied to rails and pickets only, where it covers the odd split or bowed length and the cuts at the end of each run. Five per cent is enough for straight runs of treated pine; raise it to 10% for hardwood, which is more often rejected for defects, or for a fence with steps, angles or many short runs.

Post spacing and concrete

Post spacing is normally the rail length divided into equal bays. Rails come in 2.4, 3.0, 3.6 and 4.8 m lengths, so 2.4 m centres let rails span one bay each and 3.6 m rails span a bay and a half, joining on a post. Timber posts are usually set 600mm into the ground for a 1.8 m fence, in a hole about 300mm across, which takes roughly two 20 kg bags of rapid-set concrete. Sandy or soft ground, taller fences and gate posts want a wider or deeper hole and three or four bags. Steel posts and post-hole foam have their own requirements.

What is not included

Fixings, caps, plinth boards, gates and hinges are separate. Paling fences on a boundary are often built with a plinth along the bottom, one per bay, and a capping along the top, which is the same lineal length as one rail. Add those from the rails row if the design calls for them.

Fence materials by length

Posts at 2.4 m, two rails a bay, 100 mm pickets with a 10 mm gap and 5% waste.

Posts, rails and pickets by fence length

Fence formula and worked examples

The formula: bays = fence length ÷ post spacing, rounded up; posts = bays + 1; rails = bays × rails per bay × (1 + waste %); pickets = fence length ÷ (picket width + gap) × (1 + waste %); concrete = 2 bags per post.

First example. A 20 m fence with posts at 2.4 m needs 9 bays and 10 posts. With 2 rails per bay and 5% waste that is 19 rails (45.4 m), 234 pickets at 70 mm with a 20 mm gap, and 20 bags of concrete.

Second example. A 100 ft fence with posts every 8 ft needs 13 bays and 14 posts, 41 rails (327.6 ft) at 3 rails per bay, 281 pickets at 3.5 in with a 1 in gap, and 28 bags of concrete.

How many fence posts do I need? One more than the number of bays, because each run starts and ends on a post. Corners and gates add a post each.

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