Fence Post Depth Calculator: Depth Rules, Post Sizing and Concrete vs Driven Installations

Summary

Fence post installation looks simple but has a high failure rate. The most common mistakes are insufficient post depth, posts set in concrete without consideration for frost heave, and wooden posts in direct ground contact with no preservative treatment. A fence post is a cantilevered beam — the load it carries (wind on the fence panels) creates a bending moment at ground level that must be resisted by the buried portion of the post.

In the UK, wind loads on fencing are significant — particularly in exposed locations. A full 6-panel fence run on a windy corner can exert considerable force on each post. The rule of one-third provides a structural margin of safety that has proven reliable in UK conditions for residential fencing over many decades.

Key Facts

• Post depth rule — one-third of above-ground height + 150mm minimum; e.g. 1.8m fence: 1.8 × 1/3 = 600mm + 150mm = 750mm minimum depth
• 75×75mm post — suitable for 1.2m fence and below; minimum post section for domestic fencing
• 100×100mm post — required for 1.8m fencing; the standard for close-board and panel fencing
• 125×125mm post — required for 2.1m fencing; also recommended for gate posts and corner posts at any height
• Post spacing — closeboard — maximum 3.0m centres between posts (including post width); most commonly at 3.0m with 2.7m between post faces
• Post spacing — panel fencing — maximum 1.83m centres (panel width); typically 1.85m to allow for post width; do not stretch panel spans
• Concrete base diameter — minimum 3× the post section; for a 100mm post: minimum 300mm diameter; 350–400mm preferred
• Concrete above ground — form a slight dome 50mm above grade to shed water away from the post base; do not leave concrete flush or below grade (traps water)
• Timber treatment — UC4 — all below-ground post sections must be pressure-treated to BS 8417 Use Class 4 (ground contact); UC3 (green treated) is above-ground use only
• Planning — 2m height limit — fences over 2m high require planning permission anywhere; over 1m high adjacent to a highway (road or public footpath) also requires permission
• Driven posts — steel post spikes (Metpost/Ajax) driven into firm soil; no concrete required; allow natural movement; UC4 or hot-dip galvanised steel post required
• Postmaster steel post system — steel U-section post with concrete collar; no separate timber post needed; very resistant to rot and impact; concrete collar integral to the system
• Concrete mix — site mixed — 1:2:4 (OPC:sharp sand:coarse aggregate) by volume; GEN1 equivalent; do not use cement-only or weak mixes
• Postcrete (rapid-set) — poured dry into hole; add water; sets in 10–20 minutes; 2 bags (20kg each) per post in 300mm diameter × 750mm hole
• Frost heave — concrete fence posts in clay soils can be lifted by frost; keep post base below frost line (375mm minimum); form concrete with a slight taper at the top to reduce frost grip

Quick Reference Table

Detailed Guidance

Calculating Post Hole Dimensions

Step 1: Determine fence height and post depth. Apply the rule: depth = (fence height ÷ 3) + 150mm minimum.

• 1.8m fence: (1.8 ÷ 3) + 0.15 = 0.60 + 0.15 = 0.75m (750mm)
• Total post length = fence height + depth = 1.8m + 0.75m = 2.55m

Step 2: Determine hole diameter. Minimum: 3× the post section. For a 100mm post: 100 × 3 = 300mm. Use 300–350mm diameter for concrete installations. For driven spikes, no concrete hole is needed; the spike diameter determines the driven hole size.

Hole depth = burial depth + 50mm for drainage (loose stone or broken brick at base).

Step 3: Calculate concrete volume. Volume of hole (cylinder) = π × r² × h For a 200mm diameter hole, 600mm deep: = π × (0.1)² × 0.6 = 0.0188m³

But only about 85% of this is concrete (post takes up the rest): ≈ 0.016m³ per hole

A 20kg bag of Postcrete makes approximately 0.012m³. So 2 bags per post (allowing some excess).

Concrete Type and Mixing

Postcrete (dry-mix post concrete): Pour dry into hole around post. Add water as directed. Sets in 10–20 minutes. Ideal for DIY and small-scale fencing. Available in 20kg bags from builders merchants (typically £4–6/bag). Do not pre-mix in a bucket — pour dry into the hole.

ST2 concrete (site-mixed or ready-mix): For large runs, a volumetric concrete lorry delivering ST2 is more economical. Specify the concrete grade and delivery to point of use.

Avoiding waterlogged post bases: Add 50mm of gravel or broken brick to the bottom of the hole before setting the post — this provides drainage and prevents the timber post base from sitting in standing water.

Setting Posts Plumb

1. Dig hole with post hole borer (auger) or manual spade. Clean out loose soil
2. Add gravel drainage layer at base (50mm)
3. Lower post into hole. Check post is at correct height — mark the burial depth on the post before digging
4. Hold post plumb in both directions using a spirit level on two adjacent faces. Use temporary timber props nailed to the post to hold it while concrete sets
5. Pour Postcrete dry into hole to within 50mm of surface
6. Add water as directed (typically 3–4 litres per bag)
7. Form a slight mound above the surface to shed water away from the post
8. Leave undisturbed for at least 30 minutes before releasing temporary props (minimum 2 hours before fitting panels)

Working in sequence: On a long run, set all corner and end posts first, then a string line between them as a height and alignment guide for intermediate posts.

Timber Preservation to BS 8417

UC4 grade (ground contact): All post sections buried in the ground must be UC4 pressure-treated to BS 8417:2011. UC4 treatment penetrates the timber under vacuum pressure with an organic biocide (typically copper-azole). Life expectancy 30+ years. Never substitute UC3 for in-ground use.

UC3 grade (above-ground external): Suitable for fence rails, capping rails, gravel boards — above-ground parts that stay damp (UC3b) or are fully exposed (UC3b). Not for post bases.

End-grain protection: Do not leave cut end-grain timber exposed in the ground. Apply bitumen paint, end-grain preservative, or sealant to all cut ends before burial. End grain absorbs water 10× faster than face grain and will rot from the inside out even in UC4 treated posts if end-grain is left untreated.

Driven Posts: Metpost Spikes and Postmaster

Metpost (Ajax/Fischer post spike): A galvanised steel spike driven into the ground using a Metpost driver (a dead-blow mallet tool). The square socket at the top accepts a matching-size timber post (75mm or 100mm socket sizes). The post is never in contact with the ground — the steel socket supports the base.

Advantages: fast installation (no concrete); post can be removed; no frost-heave of concrete; allows natural drainage around the post.

Limitations: requires firm, stable soil (not waterlogged clay, made ground, or running sand); not suitable for exposed windy locations where the leverage on the spike can rock it over time; not appropriate for posts carrying a gate (gate loads are too high for a driven spike without concrete).

Postmaster steel post system: A hot-dip galvanised or powder-coated steel H-section post that is set in a concrete foundation, with the above-ground portion providing the post. No timber post is needed. The steel section is bolted to fence panels or receives arris rails. Very durable, dimensionally stable, and resistant to impact. Increasingly used on close-board fences as a long-term alternative to timber.

Installation: set the Postmaster post in a 300–350mm diameter hole to the required depth using the same concrete procedure as timber posts.

Planning Permission Height Limits

Under the Town and Country Planning Act 1990 and the GPDO 2015:

• Fences, walls, or gates up to 1m adjacent to a highway (road, public footpath, bridleway) — no planning permission required
• Fences, walls, or gates over 1m adjacent to a highway — planning permission required
• Fences, walls, or gates up to 2m anywhere else — no planning permission required (PD right)
• Fences, walls, or gates over 2m — planning permission required

Highway boundary caution: the highway boundary is often wider than the visible road — it can extend to the back of the footway or the verge. Check the highway boundary with your local council before building a 1m fence near the road.

Conservation areas and listed buildings: Permitted Development rights may be reduced or removed. Consult the LPA before erecting any fence in a conservation area or listed setting.

Frequently Asked Questions

How many Postcrete bags do I need for a run of 6 panels (7 posts)?

7 posts × 2 bags each = 14 bags minimum. Buy 16 bags to allow for spillage and variation in hole size.

My fence posts keep leaning over time. What's wrong?

This indicates insufficient burial depth or soft ground conditions. If the posts were only buried 300–400mm in soft clay, they will eventually rock and lean. The fix requires re-digging to proper depth and resetting in concrete. For the future, in soft clay conditions, increase burial depth to 750–900mm.

Can I use timber closeboard posts in waterlogged ground?

Yes, but with caution. Use UC4 timber, increase burial depth to 900mm, ensure drainage at the post base, and monitor annually. In permanently waterlogged conditions, galvanised steel Metpost spikes or concrete posts may be more durable.

My ground has a lot of flint/rubble just below the surface. What can I do?

If the flint layer is shallow (under 200mm), break through it with a steel bar or SDS drill. Once through, the hole can be dug normally in the soil beneath. If the flint layer is deep and continuous (a dense chalk flint layer), consider bolt-down post bases on a surface pad, or metal spikes driven between flints.

Regulations & Standards

• BS 1722 — specification for fences; covers post specifications, panel types, and construction

• BS 8417:2011 — preservation of wood; use class selection (UC3, UC4) for fence components

• BS EN 350:2016 — durability of wood and wood-based products; natural durability classes

• Town and Country Planning (GPDO) 2015 — Permitted Development rights and height limits for fences

• Approved Document A: Structure (2004) — relevant if the fence supports a load-bearing element or retaining structure

• Tanalith E / Tanalised: UC Class Guidance — UC class guidance and preservative treatment specification

• Postcrete (Heidelberg Materials) — Postcrete product data; mixing and setting times

• Metpost / Fischer: Post Spike Guide — driven post spike selection and installation guidance

• Planning Portal: Fences — official summary of planning rules for fences and walls

• NHBC Standards Chapter 9.2 — fencing specification guidance for new builds

• garden steps — post depths for handrail posts adjacent to garden steps

• decking permits — PD rules for garden structures; fencing in context

• concrete volume — concrete volume calculator for related calculations

• mortar quantities — mortar quantities for masonry posts and piers