Fence Post Depth Calculator
How deep a fence post should go and how much concrete the hole needs — using the one-third rule and the hole size.
How deep to dig the post hole.
How the fence post depth calculator works
The rule of thumb is that a third of the total post length goes in the ground, or 600 mm, whichever is greater. A 2.4 m post above ground therefore wants about 900 mm buried, giving a 3.3 m post.
Hole diameter should be about three times the post width, so a 100 mm post wants a 300 mm hole. Concrete volume is the hole less the buried post.
Formula: depth = max(height/2, 600 mm); concrete = hole − post
Worked examples
| Inputs | Recommended depth | Note |
|---|---|---|
| 1.8 m fence, 100 mm posts | 0.9 | 900 mm deep, 2.7 m posts |
| Soft soil | 1.08 | deeper again |
| A low fence | 0.6 | the 600 mm minimum applies |
FAQFrequently asked questions
How deep should a fence post go?
A third of the total post length, or a minimum of 600 mm. For a 1.8 m fence that means about 900 mm buried and a 2.7 m post.
How wide should the hole be?
About three times the post width — a 300 mm hole for a 100 mm post. Narrower holes do not give the concrete enough leverage against the soil.
Should I concrete every post?
Corner, end and gate posts always. Intermediate posts can often be well-compacted gravel, which drains better and rots the post more slowly.
How much concrete per hole?
About 55 liters for a 300 mm hole 900 mm deep, once the post is deducted — roughly six 20 kg bags.
Do solid fences need deeper posts?
Yes. A solid panel catches far more wind, and wind load is what pushes posts over. Go deeper, wider, or both.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Gypsum Association — gypsum board sizes — 4 × 8 and 4 × 12 ft sheets
- ADA Standards for Accessible Design — ramps (§405) — maximum running slope 1:12
- International Code Council — the International Residential Code
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.