WoodCalcs.uk
Post Foundation Calculator — Hole Depth & Concrete
Determine post hole depth, concrete quantities, and post sizes for secure foundations.
How We Calculate This
This calculator determines the required hole depth and concrete quantity based on post height, soil type, and hole diameter.
Depth calculation
Hole depth = Above-ground height x Burial ratio (min 600mm)
Burial ratios are taken from the visible fence height (UK rule of thirds): firm and normal soil = 33%, soft/sandy soil = 40%. A 600mm practical minimum always applies, so a 1.8m (6ft) fence uses a 2.4m post set 600mm into the ground.
Concrete volume
Volume = pi x (Hole radius)² x Depth - Post cross-section x Depth
Postcrete bags are estimated at about 0.016m³ effective in-hole fill per 20kg bag (one bag sets a 100mm post in a 200mm x 750mm hole, per Blue Circle guidance). Round up to the nearest whole bag.
Frequently Asked Questions
The general rule is one-third of the total post length should be buried. For a 1.8m (6ft) fence, the post hole should be at least 600mm deep (total post length 2.4m). In soft or sandy soil, increase the depth to 40% of total length.
Postcrete (quick-set post mix) is faster and easier - just pour it dry into the hole and add water. It sets in 5-10 minutes. Mixed concrete is cheaper for large numbers of posts. For fence posts, postcrete is typically 1-2 bags per post; for pergola posts, 2-3 bags.
The hole diameter should be at least three times the post width. For a 100mm post, dig a 300mm diameter hole. For a 150mm post, aim for 400-450mm diameter. This provides enough concrete around the post for stability.
Posts must be UC4 pressure-treated for ground contact. Even with treatment, you can extend life by coating the buried section with bitumen paint. Never use untreated timber in the ground - it will rot within 2-3 years.
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Last updated: February 2026
All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.