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 Size

Soil Type

Post Height Above Ground

Number of Posts

Hole Diameter

Postcrete Price per Bag (optional)

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 = Total post length x Burial ratio

Burial ratios: Firm soil = 25%, Normal soil = 33%, Soft soil = 40% of total length.

Concrete volume

Volume = pi x (Hole radius)² x Depth - Post cross-section x Depth

Postcrete bags are estimated at 20kg per 0.01m³. 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.