Fence calculator panels, posts and Postcrete

How the calculator sets out a fence

A panel fence is a row of bays: a post, a panel, a post, a panel, and a post to finish. So a run of panels always needs one more post than it has panels. Each bay takes up one panel width plus one post width, and the calculator divides each side by that, rounding up.

The method comes from Grange Fencing's How To Put Up A Fence guide. It sets out 1.83m panels between 75mm posts, and its diagram measures the run at each post: 1.980m after one panel, 3.885m after two, and 11.505m after six. The calculator's formula gives the same figures.

Where a side does not divide exactly, the last panel has to be cut down. Grange makes the same point: a reduced size panel is needed when a run falls short.

Worked example: a 12m back garden boundary, one straight side, 1.83m panels, 75mm timber posts, gravel boards, and 2 bags of Postcrete per post.

• Bay: 1.83 + 0.075 = 1.905m.
• Panels: (12 − 0.075) ÷ 1.905 = 6.26, so 7 panels.
• Posts: 7 + 1 = 8 posts: 2 end posts and 6 in between.
• Gravel boards: one per panel, 7.
• Post caps: one per timber post, 8.
• Postcrete: 8 × 2 = 16 bags.
• The cut panel: six full bays reach 11.505m, leaving 0.495m. Take off the last post and the seventh panel is cut to about 0.42m wide.

Timber or concrete posts, corners and ends

Timber posts take the panel on their face, fixed with clips. Grange's guide uses 75mm posts with 1.83m panels, and the calculator defaults to 75mm. Pick 100mm timber posts in the list and each bay grows by 25mm.

Slotted concrete posts hold the panel inside a slot on each side, so the panel's ends sit within the post's width. That makes each bay a little shorter than panel plus post. The calculator allows 50mm per concrete post by default, which is our estimate from a typical slot of 55mm by 23mm deep, listed by the merchant Bendrey Bros. Check it against your posts, or set the fence out using the panel itself.

Concrete posts come in three types, and the calculator lists them separately. Intermediate posts are slotted on both sides. End posts are slotted on one side, for the two ends of the fence. Corner posts have slots at right angles, one for each side that meets there.

Worked example with concrete posts: an L-shaped fence of 8m and 5m, with one corner, 1.83m panels, concrete gravel boards and 2¾ bags of Postcrete per post.

• Bay: 1.83 + 0.05 = 1.88m.
• First side: (8 − 0.05) ÷ 1.88 = 4.23, so 5 panels.
• Second side: (5 − 0.05) ÷ 1.88 = 2.63, so 3 panels.
• Posts: 8 panels + 1 = 9: 2 end posts, 1 corner post and 6 intermediate posts.
• Gravel boards: 8. No caps are needed on concrete posts.
• Postcrete: 9 × 2.75 = 24.75, so 25 bags.

How many bags of Postcrete per post

It depends on the post and the hole, so the calculator lets you set it. The defaults are 2 bags for a 75mm timber post, and 2¾ bags for a 100mm timber post or a slotted concrete post, all in a 600mm deep hole. Those come from the table in Tarmac's Postcrete product data sheet, which gives bags of 20kg by post size and hole size.

The hole sizes follow Jacksons Fencing's advice: a standard depth of 600mm and a hole about three times as wide as the post. A 75mm post needs a hole about 225mm across, which rounds up to the 25cm row in Tarmac's table. A 100mm post needs about 300mm. Jacksons' own Postcrete guide says to expect at least one bag per post.

Tarmac's method is to fill the hole a third deep with water, then pour the Postcrete in evenly around the post until no water shows, and level the post. It sets in 5 to 10 minutes. Do not mix it first.

How long the posts need to be

Add at least 600mm to the height of the fence, including any gravel board. That is Jacksons' rule, from its standard 600mm hole. Grange's guide puts it another way: a quarter of the post's total length should be below ground. For a 2.4m post, both give 600mm.

Jacksons publishes the post length it pairs with each panel height, all set 600mm deep. Its table is below. Above 2.14m it switches to heavy duty posts.

Jacksons also says a timber post should sit on at least 2 inches (about 50mm) of ballast or gravel at the bottom of the hole, so water can drain away from the end grain. Allow for that in the depth you dig.

Gravel boards and post caps

A gravel board runs along the bottom of each bay, under the panel, so the panel never touches the soil. Grange says all timber panels should be isolated from ground level to avoid rot, and the boards fill the gaps on level and sloping ground. It uses metal gravel board clips to fix them to timber posts. Concrete gravel boards slot between concrete posts.

They are sold to match the panel: Grange's timber gravel board is 183cm wide, and concrete ones are commonly listed at 1,829mm. So the calculator gives one gravel board per panel, including the cut one, which you trim to suit.

On timber posts, the calculator adds one cap per post. Grange recommends a cap or finial on each post to protect the top from the weather, since the end grain soaks up water.

Before you dig the first hole

Check the height. In England, the permitted development rules mean a new fence over 2m, or over 1m next to a road used by vehicles, needs planning permission, as does a fence at or around a listed building. The Planning Portal adds that an Article 4 direction or a planning condition can remove those rights. Agree the boundary line with the neighbour before the posts go in.

Check what is underground. Jacksons advises looking for mains electrics, or asking the utility company to mark cables, before you dig.

Measure each side from where the first post will stand to where the last one will. If the fence ends against a wall or an existing post, the calculator's end post may not be needed, so take one off your order.

• What the calculator does not cover: gates and gate posts, featheredge or closeboard built on site, trellis toppers, clips and screws, and stepped fences on a slope.
• Slopes: Grange's guide steps panels down the slope and uses gravel boards to fill the gap at the base, so posts on a slope need to be longer.
• Panel sizes vary. Most panels are 1.83m wide, but some contemporary slatted panels are 1.8m. Measure yours and change the width.
• Quoting a customer? List the panels, posts and your time, with a valid-until date, on the quote template.

Common questions