Fence Post Calculator

Fence Installation Guide

What's the key thing to understand about Post Depth Rule?

For fences up to 1.5m: minimum post depth of 450–600mm. For 1.8m fences: 600mm minimum, 750mm on exposed or windy sites. Rule of thumb: post depth = 1/3 of total post length. A 2.4m post (giving 1.8m above ground) needs 600mm buried.

What do I need to know about Concrete Quantities?

Each post hole typically needs a mix 300mm in diameter, 600mm deep = approximately 0.042 m³ = about 1.5 bags of 25kg postcrete. Postcrete is a rapid-set product that sets around the post in minutes when water is added.

What's the difference between Metal and Timber Posts?

Galvanised steel post spikes avoid concrete and can be quicker to install on firm ground. Metal posts last significantly longer than timber. For fence panels, steel H-frame posts designed for your panel size are the most secure and best-looking option.

What's the key thing to understand about Safety and Regulations?

Most structural and electrical DIY work in England and Wales must comply with Building Regulations and is notifiable to local Building Control. Work that is not notifiable (like-for-like replacements, cosmetic changes) can be done without notification. Notifiable work done without approval is technically illegal and can cause problems when selling the property. Electrical work in most rooms requires a Part P competent person or Building Control inspection. Gas work must always be performed by a Gas Safe registered engineer — it is illegal for anyone else to carry out gas work in the UK, and using an unregistered installer invalidates both your home insurance and any warranty on the work. For structural changes (removing load-bearing walls, altering the roof structure), always get a structural engineer's assessment before starting, even for jobs that look straightforward.

What do I need to know about Post Spacing Accuracy?

This calculator estimates post count based on total fence length and panel width, but the actual spacing between posts on site needs to be set out and measured carefully, since even a small cumulative error across a long run of panels can leave the final panel not fitting the remaining gap. The standard approach is to set the first and last post positions first, based on the actual start and end points of the fence line, then divide the remaining distance evenly by the number of panels, rather than simply measuring a fixed panel width repeatedly from one end and hoping it lines up with the far end, since ground isn't always perfectly straight and small measuring errors compound over a long run. Posts should be set to the outside measurement of the panel's actual width, not a rounded standard figure, since even panels sold under the same nominal size occasionally vary by a centimetre or two between batches or suppliers, and a post spacing based on the exact panels in hand, measured before digging any holes, avoids a frustrating situation where a panel is a close but imperfect fit between two already-concreted posts.

What should I know about Setting Posts in Ground vs Post Mix Speed-Set?

Traditional post concreting uses a wet-mixed concrete, poured around the post in a dug hole and left to cure over several days to weeks before the post can safely take the weight and wind-loading of attached fence panels. Fast-setting post mix, sold as a dry granular product, is a popular DIY alternative that's simply poured dry into the hole around the post, with water poured on top afterwards to trigger a chemical reaction that sets the mix firm within as little as 10-15 minutes, meaning a post can often be supported well enough to continue work, or even attach a lightweight panel, the same day rather than waiting the full curing period of traditional wet-mixed concrete. The trade-off is typically cost per bag compared with ordinary bagged concrete, and the fact that fast-setting post mix doesn't fully cure to its ultimate strength as quickly as the quoted working time suggests, so while it allows quick progress on a job, it's worth treating a freshly speed-set post gently for the first day or two rather than immediately hanging a full-weight panel and expecting it to withstand strong wind loading right away.

What's the difference between Concrete-In and Bolt-Down Post Bases?

Concreting posts directly into the ground is the traditional method and remains the strongest for freestanding fences, but bolt-down (also called post-fix or Metpost-style) bases offer real advantages in certain situations. A bolt-down base — a galvanised steel shoe fixed to a concrete pad, patio, or existing slab with resin or mechanical anchors — avoids digging a deep hole, which is useful where there's a patio, driveway, or shallow services nearby that can't be dug through. It also keeps the timber post slightly raised above ground level, away from standing water, which significantly reduces base rot compared with timber buried directly in soil or concrete. The trade-off is that bolt-down bases generally aren't rated for the same wind loads as a post set 600mm deep in concrete, so they're best suited to garden fencing and lighter screening rather than tall, exposed boundary fences. They also rely entirely on the strength of the base they're fixed to, so a thin or cracked existing slab will undermine the fixing regardless of how good the post base itself is.

Concrete Mix Quantities by Post Size (Reference Table)

Once you know how many posts a fence run needs, the next practical question is how much concrete or postcrete to buy. Underestimating means a wasted trip to the merchant partway through the job; overestimating wastes money on bags you'll never use. The figures below are typical for a dry-mix fast-setting postcrete product in a hand-dug hole.

Worked Example: Concrete for a 24m Fence Run on 2.4m Panel Centres A 24m boundary using 2.4m fence panels needs 24 ÷ 2.4 = 10 panel spacings, which means 11 posts (one more than the number of panels, since each panel shares a post with its neighbour). Using 100x100mm timber posts at 3 bags of postcrete per hole, that's 11 x 3 = 33 bags of 20kg postcrete. At an approximate cost of £5.50 per bag, the concrete alone comes to 33 x £5.50 = £181.50, before adding the cost of the posts, panels, and gravel boards themselves. Buying 2-3 spare bags is sensible, since hole depth and width always vary slightly with real ground conditions.

Checking Ground Conditions Before You Dig

The post depth and concrete quantities above assume reasonably firm, well-drained soil. Ground conditions can change those numbers significantly, and it's worth checking before you commit to a material order. Heavy clay soil holds water and expands and contracts with moisture changes (a process called heave), which can gradually push shallow posts out of plumb over a few winters — in clay-heavy areas, going a little deeper than the standard one-third-of-post-length rule, or using a wider-bottomed hole (sometimes called a bell-shape) to resist upward heave forces, is common practice. Very sandy or loose soil, conversely, drains well but holds a post less firmly, so some installers widen the hole diameter rather than increasing depth to get adequate concrete bearing area. If you hit standing water, rock, or old foundations/drains while digging — all common in gardens with a building history — it's worth repositioning the post slightly rather than forcing a hole through an obstruction, since a few centimetres of repositioning is far cheaper than cracking a drain or footing. Before digging anywhere near a boundary or driveway, it's also worth checking for buried services (gas, water, electricity, drainage) using a cable/pipe locator or your utility provider's plans, since fence post holes are a common cause of accidental service strikes in older gardens.

How Fence Height Changes Post Spacing Recommendations

Taller fences catch more wind load, and that extra sideways force is exactly why post spacing recommendations tighten as fence height increases rather than staying fixed at a standard panel width. A 0.9m-1.2m low boundary fence can often tolerate standard panel-width spacing of 1.8-2.4m without issue, but a 1.8m-1.98m privacy fence — especially a solid, closed-board style that offers no wind-through gaps — puts considerably more leverage on each post in a storm. In exposed, elevated, or coastal locations, reducing spacing below the standard panel width, upsizing the post itself, or increasing concrete depth are all common adjustments, and manufacturers of taller fence panels often publish their own recommended maximum spacing for exactly this reason rather than leaving it to a generic rule of thumb.