How deep should a fence post hole be?

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.

Should I use postcrete or mixed concrete?

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.

How wide should a post hole be?

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.

Do I need to treat the buried part of the post?

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.

WoodCalcs

Post Foundation Calculator — Hole Depth & Concrete

Determine post hole depth, concrete quantities, and post sizes for secure foundations.

Post SizeSelect the post size for your calculation

Soil TypeSoil conditions affect drainage and foundation needs

Post Height Above Ground

Visible post height above ground level, e.g. 1800mm for a 6ft fence panel

Number of Posts

Hole Diameter

Make the hole about 3x the post width, e.g. 300mm dia for a 100mm post

Postcrete Price per Bag (optional)

Enter your supplier price for a cost estimate

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

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Last updated: February 2026

All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.