Fence Calculator UK · metric

Panels, posts, postcrete and cost for panel fencing by the metre. Metric units and £ costs, updated for 2026.

Last reviewed July 2026 · Free · Nothing you enter is stored

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UK panel fencing uses 1.83 m (6 ft) panels, so panels = length ÷ 1.83 rounded up, and you need one more post than panels (panels + 1). Each post takes about two bags of postcrete. Costs cover materials and a supply-and-install range per metre.

The waste allowance

Panel fencing does not waste material the way picket fencing does, because the panel arrives finished and the only cut is the last bay in the run. That last bay is where the planning goes wrong. A fence line is almost never an exact multiple of a panel plus a post, so the final bay comes up short and has to be filled with a cut-down panel or closed with a closeboard bay built on site. A featheredge panel cuts down reasonably neatly; a waney lap panel usually does not, because cutting through it leaves nothing holding the slats. Decide which end of the run takes the odd bay before you set any posts. Postcrete is the other place to be generous: bags are cheap next to the labour of digging a hole, and a hole that ends up 50 mm wider than intended will swallow the difference without warning.

The bay, not the picket: how British fencing is counted

British garden fencing is built in bays. The standard panel is 1.83 metres wide, which is the old six feet, and it drops between two posts rather than being assembled board by board on site. Panel heights run from 0.9 m up to 1.83 m in 300 mm steps, and the width does not change with the height.

That makes the count straightforward. Posts are one more than the number of panels, so ten panels take eleven posts. What catches people out is that a bay measures the panel plus a post, not the panel alone. With 100 mm posts each bay eats 1.93 metres of the run, so a 20 metre boundary is a little over ten bays rather than the eleven you get by dividing by 1.83. Set the run out on the ground with a line and a tape before ordering.

The relevant British standards are BS 1722-11 for prefabricated wood panel fences and BS 1722-5 for close-boarded and palisade fences. Neither is a legal requirement for a garden fence, but a supplier quoting them is telling you something about how the panels are built.

Posts: 75 or 100 mm, timber or concrete

Timber posts come 75 mm and 100 mm square. Seventy-five millimetre posts suit lower fences and sheltered runs; 100 mm is the sensible choice for a full 1.83 m panel, for gate posts and for exposed boundaries. Concrete posts are the durable alternative, and the slotted type takes the panel and the gravel board without a single fixing.

Post length follows the fence height plus the buried depth plus the gravel board. A 1.2 m fence takes a 1.8 m post, a 1.5 m fence takes a 2.1 m post, and a 1.83 m fence takes a 2.4 m post. Ordering 1.8 m posts for a 1.8 m fence is a genuine and expensive mistake.

Depth decides whether the fence is still upright in ten years. The working rule is to bury roughly a third of the above-ground height, which puts 600 mm in the ground for a 1.83 m fence and makes 600 mm the practical minimum for garden fencing. On exposed sites, in soft or wet ground, and for gate posts, go to 750 mm or more. The hole wants to be about three times the post width, so 225 to 300 mm across, with 50 mm of hardcore or gravel in the bottom so water running down the post drains away instead of standing against the end grain.

Postcrete, and how many bags a post really takes

Postcrete is the British quick-set post mix and it is sold in 20 kg bags. It is used dry: fill the hole to about a third of its depth with clean water, pour the dry mix straight in, and it firms up within minutes. No mixing, no barrow and no gauging, which is why it has almost entirely replaced site-mixed concrete on domestic fencing.

For a 1.83 m panel fence on 100 mm posts, with a hole 600 mm deep and 200 to 250 mm across, the trade figure is one and a half to two bags per post. Seventy-five millimetre posts in the same hole take one to one and a half. Widen the hole to 300 mm and it goes to two bags, and a 300 mm hole at 750 mm deep is three.

It is worth understanding why the numbers move so fast, because hole diameter is squared in the volume. A 300 mm hole holds 0.042 cubic metres over 600 mm, and taking off the post leaves about 0.036 cubic metres of concrete; the same hole at 250 mm across takes barely two thirds of that. The figure we could not verify at source is the yield of a 20 kg bag. The value in circulation, quoted from a Blue Circle product data sheet, is about 0.018 cubic metres, and we were unable to open the data sheet itself to confirm it. If the true yield is nearer 0.012, every hole in the table below wants an extra bag, so buy two or three spare and keep them dry.

Gravel boards, and why British fences have them

A gravel board is a plain board fitted horizontally at the bottom of each bay, between the posts and under the panel, so the panel itself never touches the ground. It is a sacrificial part. Soil, leaf litter and splash sit against the gravel board rather than the panel, and when it eventually rots you unscrew one board rather than replacing a fence.

Timber boards come 1.83 m long by 140 mm by 28 mm for slotted posts, and 2.4 m by 150 mm by 25 mm for traditional runs. Concrete gravel boards are commonly 1.83 m by 150 mm by 50 mm, with a 300 mm deep version for ground that falls away. The board also raises the fence, which changes the post length you need and counts towards the overall height: a 1.83 m panel on a 140 mm timber board stands about 1.97 metres, just under the two metre limit and no accident.

What actually governs a British fence, and it is not frost

American fencing guidance spends a great deal of time on frost depth and footings below the frost line. That is not the British constraint. Fences are not subject to the Building Regulations here, and winter ground freezing in most of Britain is shallow compared with the 600 mm a post is buried at anyway.

The constraint that does bite is planning. Under permitted development you can put up a fence, wall or gate up to two metres high without applying for permission, but only one metre where it is next to a highway used by vehicles or the footpath of such a highway. Height is measured from ground level and includes the gravel board and any trellis on top. The allowance does not apply to a listed building or its curtilage, and it can be removed by an article four direction or a planning condition, which is common on newer estates.

The other real constraint is wind. A close-boarded 1.83 m panel is a solid sail, and on an exposed boundary the load on the post is far larger than anything the fence weighs. That is what the deeper hole, the wider hole and the 100 mm post are for. Where wind is a persistent problem, a slatted or hit-and-miss panel lets air through and loads the posts far less than a solid one.

Post length, hole depth and postcrete by fence height

Post lengths assume a gravel board under the panel. Bag counts are for 20 kg bags of quick-set post mix and follow the hole volume less the post, so they move quickly with hole diameter. Round up and keep a spare bag or two.

Post lengths and burial depths from Fencing and Landscape Supplies and Wern Wood, both working to the one-third rule with 600 mm as the domestic minimum. Bag counts for 100 mm and 75 mm posts from CE Clarke Fencing, and the 300 mm hole figure from TradeCalculator's volume calculation. The yield of a 20 kg bag is the weak link: the circulating figure of roughly 0.018 cubic metres is attributed to a Blue Circle Postcrete data sheet that we were unable to open, so treat the bag counts as a guide and buy spare.

What goes wrong on panel fencing

Dividing the run by 1.83 and ordering. The bay is the panel plus a post. With 100 mm posts that is 1.93 metres, so a 20 metre boundary is a little over ten bays, not eleven, and the odd bay has to be cut.

Buying a 1.8 m post for a 1.8 m fence. A 1.83 m panel needs a 2.4 m post: fence height, plus 600 mm in the ground, plus the gravel board. The same logic gives 2.1 m posts for a 1.5 m fence and 1.8 m posts for a 1.2 m fence.

Fitting the panel straight to the ground. Without a gravel board the panel wicks up ground water and rots along its bottom edge, and replacing a panel costs several times what replacing a board does.

Forgetting that the gravel board counts towards the height. Permitted development allows two metres, or one metre next to a highway used by vehicles. A 1.83 m panel plus a gravel board is about 1.97 m, and adding trellis on top of that puts you over.

A narrow hole with a lot of concrete in it. Depth without width does not hold a post. Aim for a hole about three times the post width, with 50 mm of hardcore in the bottom so water drains away from the end grain.

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