How to Choose the Right Augur Size for Pre-Drilling Fence Post Holes? 

Recently, we had a client who wanted to DIY their Horserail fencing project and came to us for advice on selecting the right augur. They had hundreds of posts to install, and even though their property had softer soil, some pre-drilling was necessary. They wanted to ram the posts in, and we provided them with 120mm round posts for this purpose. But, the key challenge was determining what size augur to use for pre-drilling. 

Over the years we’ve seen many property owners, especially those opting for a DIY fencing project, often face confusion about post-installation. One common question is, “What size augur should we purchase to pre-drill the post holes?” This is a critical question because proper pre-drilling not only helps in installing the fence post but also ensures long-term stability, even in challenging soil conditions. 

If you too are planning a fencing project for your property and want to know the right method to pre-drill the holes, then this blog is for you.  

When it comes to fence post installation, we at Stock & Noble have learned through years of experience that the augur size can make a significant difference in the overall stability of the fence. Choosing the correct augur size ensures that the post fits snugly into the ground, offering the much needed stability to face environmental factors like wind, soil erosion, or even the weight of animals pushing against it. 

This blog will walk you through the process of choosing the correct augur size and understanding the nuances of installing posts, especially if you’re working with plastic or Hogan posts. 

Whether you’re a DIY enthusiast or working with a fencing contractor, knowing the right method to pre-drill the holes is essential to ensure your fence stands strong and straight for years to come.  

How Deep Should Fence Posts Be Installed? 

The answer, as we’ll see, isn’t one-size-fits-all—it depends on several factors, including the type of posts, the soil conditions, and how deep the posts need to be installed. 

The Rule of Thumb for Augur Size 

• Use a smaller augur than the post size: 

• Augur should be slightly smaller than the post for a secure fit. 

• Example: For a 120mm OD (outside diameter) post, use a 100mm augur. 

• This leaves 10mm on either side, allowing the soil to naturally compress and create a snug fit. 

Benefits of a Snug Fit 

• Natural soil compression: 

• Soil crumbles into the hole as the post is inserted, eliminating the need for concrete in softer soils. 

• Ensures a secure and stable fit for the post. 

Hogan and Plastic Posts 

• Why pre-drilling is essential: 

• Hogan and plastic posts require a tight fit for better anchoring. 

• Pre-drilling allows these posts to grip the soil firmly, preventing movement. 

When Pre-Drilling Isn’t Necessary 

• Steel posts don’t always need pre-drilling: 

• Steel posts have thinner walls and are strong enough to cut through the soil with less resistance. 

• Direct installation: Steel posts can often be driven directly into the ground without the use of an augur, especially in softer soils. 

For most posts (timber/plastic): Augur should be slightly smaller than the post for a snug fit.  

For Hogan/plastic posts: A snug fit is essential for anchoring securely.  

For steel posts: Pre-drilling may not be necessary due to their ability to cut through soil directly. If you want, you can learn more about how to install steel fence posts. 

How Deep Should You Drill? 

The depth of the post hole is another factor that plays a key role in the stability of your fence. Typically, fencing experts recommend that for a 2.4m post, around 1.5m should remain above ground in typical equine applications. That means 900mm should be buried in the soil. This depth provides enough support to keep the post upright and secure. 

Soften vs. Hard Soils: Adjusting Augur Depth (based on the post size mentioned above) 

Soft Ground 

• Augur half the post’s buried length: 
• For softer soil types, approximately 450mm deep is sufficient. 
• The remaining depth can be rammed without much resistance, ensuring stability. 

Hard or Rocky Soil 

• Pre-drill deeper into the soil: 
• In hard or rocky conditions, augur up to 90% of the post’s depth.
• This translates to 800-850mm, leaving the last 100mm to be rammed directly into the ground. 
• Why this helps: Leaving a small portion un-augured acts like an anchor, improving soil grip and post stability. 

Extremely Hard Soil 

• Augur the full post depth: 
• For extremely hard soil, augur the full depth to 900mm. 
• This ensures the post can be rammed effectively for maximum support and resistance to shifting. 

Benefits of Properly Drilled Post Holes 

Why is it important to go through the effort of pre-drilling? A well-drilled post hole does more than just make installation easier. It ensures that your fence remains straight and beautiful for years to come. When the posts are properly secured, the entire structure becomes much more resistant to leaning or shifting, even in extreme weather conditions. 

Our clients who inquired about the augur size, hired a local fencing contractor to install the fence posts for them, while they took on the task of installing the rails and end assemblies themselves. By following the correct augur guidelines, they were able to complete the project with a fence that not only looked impressive but was also highly functional. 

Other Considerations: Post Sizes and Soil Types 

When it comes to fencing, post sizes can vary significantly depending on the material being used. For most timber and plastic posts, the standard size is around 120mm OD. For these types of posts, as mentioned earlier, a 100mm augur is ideal. This size ensures a snug fit and makes the installation process straightforward. 

However, it’s essential to adjust your approach based on the soil type. Softer soils allow for more leeway, meaning you can get away with less pre-drilling, while harder, more compact soils will require more intensive auguring. 

When using Hogan or plastic posts, ensure that the augur is neither too big nor too small. If the augur is too large, the post will fit too loosely, requiring extra materials like gravel or concrete to fill in the gap. On the other hand, an augur that’s too small will make it difficult to fit the post, potentially leading to damage and a much slower installation. 

Are You Confident to Pre-Drill Your Fence Post Holes? 

By following these guidelines for selecting the right augur size and understanding the importance of pre-drilling, you’ll be well on your way to creating a fence that stands the test of time. Whether you’re using timber, plastic, or Hogan posts, and whether your property is situated on soft or hard soil, these tips will ensure your fence posts are installed securely and efficiently. 

The right augur size is a small but crucial detail that can save you time, effort, and money in the long run. Your fence will not only look great but will also provide lasting durability. If you’re ever in doubt or need more information, our Learning Hub is just a click away, where we provide answers to all your fencing-related questions and more. 

So, if you’re ready to take the next step, start by choosing the right augur, and let the natural tight fit of your posts do the rest. For any further advice or to discuss your fencing project in detail, reach out to our fencing experts. We’re always here to help you make the best decisions for your property.