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How to Calculate Concrete Footing Size for Posts and Walls
Undersized footings are one of the most common causes of fence, pergola, and wall failure. Here is the formula for footing depth, diameter, and volume for any post or wall load.
A footing transfers load from a structure to the ground. Undersizing it causes settling, heaving, or collapse. Oversizing it wastes concrete and money. The calculation depends on the load, the soil type, and the frost depth.
Minimum Footing Depth
UK frost depth: typically 450mm in most of England and Wales.
Footings must extend below the frost line to prevent frost heave.
Minimum practical depth for posts:
Fence post (1.8m high): 600mm depth
Garden pergola post: 600-900mm depth
Structural column: 900mm-1,200mm
Rule of thumb for fence posts:
Post depth in ground = 1/3 of total post length
For a 2.4m post (1.8m above ground + 0.6m below):
Depth = 2.4 / 3 = 0.8m → use 600-800mm ✓
Footing Diameter for Round Posts
Standard practice:
Footing diameter = 3x the post diameter
Minimum: 150mm diameter regardless of post size
75mm fence post:
Footing diameter: 75 x 3 = 225mm → use 250mm hole
(Standard post hole borer: 150mm, 200mm, 250mm)
For structural posts (pergola, carport):
100mm x 100mm timber post
Minimum footing: 300mm diameter
Recommended for 2.4m height: 350-400mm
Concrete tube formers (Sonotubes):
Available in 200mm, 250mm, 300mm, 350mm diameter
Use one size up from minimum for safety margin
Volume of Concrete per Footing
Volume = pi x r^2 x depth
(r = radius of hole = diameter/2)
250mm diameter, 600mm deep:
r = 0.125m
Volume = 3.14159 x 0.125^2 x 0.6 = 3.14159 x 0.015625 x 0.6
= 0.0294 m3 per footing = 29.4 litres
For 10 fence posts:
Total volume = 10 x 0.0294 = 0.294 m3
Concrete bag equivalents:
25kg bag yields approximately 0.012 m3
0.294 m3 / 0.012 = 24.5 bags → order 26 bags
Or: one 20kg rapid-set post-fix bag covers one 150-250mm hole to 600mm.
Dry-pour method: pour dry mix, add water, no mixing required.
Mix Specification for Footings
Standard structural concrete mix (C20/25):
1 part cement : 2 parts sharp sand : 3 parts aggregate (coarse)
Water: approximately 0.55 litre per kg of cement
For post footings in non-aggressive soil:
C20P mix (prescriptive mix for foundations) is sufficient
= 1:2.5:3.5 by volume
For aggressive soil (sulfate-bearing clay, near sewers):
Use sulfate-resistant cement
Or: use Postcrete / rapid-set product (already formulated correctly)
Compressive strength guide:
Fence post: plain Class C20 adequate
Structural column: Class C25-C30 recommended
Retaining wall: C25 minimum, structural engineer advice needed