A common question that we get asked is "How do I work out the size of hole I need for a fence post?"
Regardless of whether you’re dealing with fence posts, washing line posts or any other type of post, the principals are the same.
The general rule is when setting a fence post into the ground, the height required above the ground should be divided by 3 and this figure is the hole depth required. The width of the post hole should be 3 times the width of diameter of the post.
For example, if you are erecting a 6 foot high fence using 4 inch fence posts, the posts need to be 6 feet above the ground. 6 divided by 3 is 2, therefore the depth of the hole required is 2 feet.
As the post is 4 inches wide, 3 multiplied by 4 is 12, so the hole needs to be 12 inches square.
Please use our table below to work out the width and depth of hole required for your posts. Please note the below requirements are specific for premixed concrete such as Postcrete mentioned here.
|Post size||Height of post above ground||Hole size (width x depth)||No of 20kg bags of premixed Concrete required|
|50mm (2”)||90cm (3′)||15cm (6”) x 30cm (12”)||1/3 of a bag|
|1.2m (4′)||15cm (6”) x 40cm (18”)||1/2 bag|
|1.8m (6′)||15cm (6”) x 60cm (24”)||3/4 of a bag|
|75mm (3”)||90cm (3′)||23cm (9”) x 30cm (12”)||1 bag|
|1.2m (4′)||23cm (9”) x 40cm (18”)||1 and 1/3 bags|
|1.8m (6′)||23cm (9”) x 60cm (24”)||1 and 1/2 bags|
|100mm (4”)||90cm (3′)||30cm (12”) x 30cm (12”)||1 and 3/4 bags|
|1.2m (4′)||30cm (12”) x 45cm (18”)||2 bags|
|1.8m (6′)||30cm (12”) x 60cm (24”)||2 and 3/4 bags|
Safety Concerns When Digging Fence Post Holes
Before you start digging any fence post holes, first consider any potential dangers that may be in the ground. In many cases electric cables, water pipes, gas pipes and fibre cables are present below the ground and can easily be damaged by shovels and post hole diggers.
Although any buried electric cables should be at least 600mm below ground level, and gas pipes around a similar depth this isn’t always the case so it’s well worth checking if there could potentially be any pipes or lines where you’ll be digging.
Most utility companies will keep records of where they’re supply lines are run so it’s worth contacting the utility suppliers for your area to see if they can provide information on where cables and pipes are likely to run.
To find out who your local PES supplier (Public Electricity Supplier) see here and to find out who your gas supplier and gas transporter is see the Ofgem site here.
One other very useful service is linesearchbeforeudig.co.uk that can supply utility asset information and help you identify where any utilities may be within the grounds of your property.
One final point to note when it comes to digging fence post holes is the hole itself. Never leave any post holes that have been already dug unattended. If you need to leave the area you’re working in for any time, cover all of your post holes over with a suitable piece of ply board or similar.
Unattended holes, especially in the dark, can easily cause broken legs and ankles if anyone falls down them, so keep them covered!
Tools Needed for Digging Fence Post Holes
Digging a simple hole may seem like a pretty easy job. You just need a shovel right? Essentially yes, but as with many DIY and construction jobs, there should be a tool made specifically for the job at hand.
Technically you can dig a hole with a standard shovel, but the hole will always be as wide as the shovel itself and this may be too wide or too narrow, depending on the shovel.
Also, trying to remove earth from a hole that’s fairly deep using a shovel is back-breaking work. If you’ve only got one hole to dig, it’s bearable, but if you have several holes to do, you’ll be shattered by the end of the second.
With this in mind, use the correct tools for the job, in this case the following:
- Trenching shovel (also known as a tile shovel, drainage shovel, rabbiting spade or fencing shovel)
- Post hole diggers
- Reciprocating saw, multi tool or small handsaw such as a compass saw (for cutting through and tree roots)
- Club hammer (for hitting in post markers)
- String line (for keeping posts inline)
- Tape measure (for measuring size of hole)
- Wooden stakes (to mark post positions)
- Bucket or tarpaulin (for removing or placing earth from holes)
- Prying bar
- Hole tamper
For the purposes of this guide we are going to use a post hole digger (or clamshell digger as it’s also known). As they’re alternate name suggests, they are a simple clamshell-like tool that you shove down into the hole, close together and then lift, removing any waste earth.
They are both cheap to hire, cheap to buy and readily available from most DIY stores, online or from your local builders merchant. There are however a few other options that also need mentioning:
Manual Fence Post Auger
These tools resemble a giant cork screw and to use them you essentially position the tip where you want to form your hole and then twist the handles to “screw” it down into ground. As you screw the auger down, the thread (or flighting) bites into the soil and loosens and removes it.
As you turn the auger you need to apply some downward pressure, to force it down into the ground.
If you have loamy, peaty, sandy or silty soil, this soil-type is fairly light and easy to work with and in turn fairly easy to dig through, but if you have a heavy soil such as clay or a chalky soil that’s full of stones, using a manual auger will be hard work.
Petrol Power Fence Post Auger
The petrol fence post auger is in essence pretty much the same as its manual counterpart with the only difference being that instead of have manually screwing the thread down into the ground, there’s a petrol engine to do the work for you – fantastic you might be thinking! But hold on a sec, this isn’t as ideal as it sounds.
The first issue is that for a full sized power auger you need 2 strong and fully able people to operate it. When rotating, the shaft and thread generate quite a bit of toque and it takes a fair amount of strength just to hold on to the tool and it’s even worse if you have a heavy clay soil.
You can also get smaller “1 person” power fence post augers or ground drills that perform fairly well in soft and loose soil, but do tend to struggle in something more substantial.
Secondly, if there are lots of stones or tree roots present in the ground you’re working in, power augers can easily bind on objects like these and when they do, if you’re not fully balanced it can easily send you flying!
On the whole, power augers are great and can enable you to dig upwards of 25 post holes in a day in good soil conditions, but if there is any debris in the ground, this not too pleasant job can turn into a hellish nightmare!
How to Dig a Fence Post Hole
Now that we know what tools we need to use and how big our fence post hole is going to be in respect to the size of fence posts we are using it’s time to get our post holes dug.
Step 1 – Mark out fence post holes
Before any digging commences, first you need to lay out your string line and then knock in a fence post marker (wooden stake) at each point where a fence post is going to be placed.
Ensure your string line is straight and inline with where you want your posts to go and that the front face of each stake it flush with the line.
Step 2 – Measure and mark post hole dimensions
Using your tape measure, measure out from each face of the fence post marker the correct distance for the size of fence post you’re using.
For example, if you’re using 4 inch posts we know that your hole needs to be 12 inches square so measure out from each of the 4 faces of your fence post marker, 6 inches to create your 12 inch square.
Once you’ve measured out 6 inches with your tape at the centre of the face your measuring out from, use your shovel to score through the turf and mark a line. Once you’ve worked your way around all 4 faces you should then be left with a 12 inch square piece of turf to remove.
If you’re digging your hole in soil, dig a small channel with your space to define the hole and if you’re working with something tougher such as concrete or asphalt, you can you a suitable marker or chalk to define your hole.
Repeat for any further holes.
Step 3 – Remove turf and loosen top layer of soil
Remove the divot of turf that just you cut and put it to one side, you will need this later to place back around the post. Slide it up and over the top of the post marking stake, being careful not to move it too much.
Before you start digging, unhook your string line from one end and move it out of the way so that it doesn’t get damaged.
Using your trenching shovel, start digging down and around the inside perimeter of the hole to loosen the soil. Loosen as much soil as you can.
It’s a lot easier to loosen the soil first before digging it out with your post hole diggers.
Step 4 – Remove loose earth with post hole diggers
With a good amount of soil loosened up you can now start to remove it using your post hole diggers. Hold them upright near the top of the handles and close the handles together.
Next, pick them up and then use a little force to force the blades down into the loose soil and then pull the handles apart to grab the soil and then pull them upwards out of the hole
Position your load over your bucket or tarp and close the handles together to release the load.
Repeat step 3 and this step and work your way down to the required depth, occasionally stopping to measure your depth with the tape measure.
Remember: You will have to line the base of the hole with around 6 inches of aggregate to allow drainage so remember to account for this.
Step 5 – Remove any rocks or tree roots
During digging you are likely to encounter a whole host of objects including rocks, tree roots, old bricks and blocks etc.
If you find solid objects such as rocks etc, use your prying bar to level them out.
If you come across any tree roots, if you have a reciprocating saw, use this to cut them out and if not, a small narrow saw such as a compass saw.
Step 6 – Tamp base of hole down flat
Once you have reached your desired post hole depth, tamp down the entire base of the hole with your tamping bar to pack the soil down until its solid. This will provide a nice flat and level surface for your posts.
Depending on how loose the soil is, it may compact down a fair amount so make sure to measure the depth again.
If your hole is now too deep, add some soil back in and tamp it down until your back to the required depth.
Before getting your fence posts fixed in place, remember to put your line level back so that you have a line to level your posts against.
The above information covers how to dig out a fence post hole, for more information on how to fix fence posts in place, see our project here.
Once your posts have been set, see our project here for information on how to fix fence fence panels to fence posts to get your whole fencing project completed.