Cutting a 4×4 post is one of those tasks that seems easy, but unless you have the right tools is actually tricky to get right. Standard circular saws with 7 ¼” blades can only cut 2 ½” deep, so making the cut in a single pass isn’t possible.
That means you’ll need to make multiple passes to cut clean through the 4×4, and this can result in a poor finish unless you take steps to ensure your cuts line up perfectly with each other. There are several methods to accomplish this, so let’s take a look at a few ways to cut a 4×4 with a circular saw.
Tools & Materials
- Circular saw
- Marker or pencil
- Scrap plywood
- Speed square
- Hand saw or reciprocating saw (optional)
- Sandpaper or sander
Cutting a 4×4 with a Circular Saw – Guide
If you need to cut a 4×4 post, then the ideal tool is a large 12” miter saw. This saw has the blade diameter needed to chop through a 4×4 in a single pass, so if this is an option for you it’s the way to go.
A miter saw isn’t always an option though; if you need to cut a 4×4 on the job while you’re installing fence posts then a circular saw is a better option, as it can cut through a 4×4 in 2 or 3 passes, and is far more portable than a 12” miter saw.
Circular Saw Only Method
This method offers a good compromise between speed and accuracy. You only need one saw to complete the cut, and using the initial groove as a guide typically results in a nice clean finish.
Step 1: Mark Cut Line
The first step is to measure and mark your cut line onto the 4×4. Use a speed square and pencil to mark your initial line along the side of the 4×4.
Then flip the 4×4 over and repeat this step on each of the four sides. You want to be careful and deliberate while doing this as these will be your guidelines while cutting through the 4×4.
Step 2: Make the First Cut
First, make sure your circular saw is adjusted to the maximum cutting depth. This will ensure you can complete the cut in without having to cut through all four sides.
Line up your saw’s blade with the cut line you drew in the previous step. To be on the safe side, try to make the cut on the waste or scrap side of the cut. This will ensure that the cut will err on the side of too long instead of too short.
You can always go back and shorten it if needed, but you can’t add back extra material.
Use your speed square as a cutting guide by butting the edge of your saw’s shoe up against it. If you want the cut to be perfect, clamp the speed square to the 4×4 rather than gripping it in your hand.
Then run your saw along the 4×4 in line with the cut line.
Step 2: Rotate the 4×4 and Make the Second Cut
Unclamp the 4×4 from your workbench, and rotate the 4×4 90° (one turn). Your initial cut will leave a 2 ½” deep groove that you can use as a cutting guide for the remaining cuts.
Line up your saw with the cut line from step 1, slide the blade in the existing groove, and complete the second cut. You shouldn’t need a speed square or other guide for this as your basically cutting along the existing groove.
You might be wondering why not flip the 4×4 over to the opposite side (180°) and finish the cut in two passes?
Because you won’t have an existing groove to use as a guide, it’s more likely to result in an uneven finish.
Step 3: Rotate 90° and Make Third Cut
Finally, rotate the 4×4 another 90° and complete the third and final cut. After the first two cuts, there will only be a small piece of lumber remaining to cut through, so this should be effortless.
Simple Jig Method
This method is similar to the above method; it uses a few pieces of scrap plywood as a cutting guide instead of a speed square. If you’re not that familiar with using a circular saw, then this method is a good way to go as it’s more or less foolproof.
Step 1: Mark Cut Line
Just like the above method, the first step is marking your cut line on the 4×4. Use a ruler, square, or scrap piece of wood to mark your desired cut line with a pencil.
Then flip the 4×4 over and repeat this on all four sides.
Step 2: Create Simple C-Shaped Cutting Jig
Next, you’ll want to create a simple C-shaped cutting jig using scrap wood. The jig consists of three pieces of scrap plywood cradling the 4×4 with each one measuring 3 ½” long – the same width as a piece of 4×4 dimensional lumber.
Use your cut line as a starting point, add to it the distance between your saw’s blade and the edge of the shoe and make a second mark in parallel with the first. This is called the offset line – and accounts for the width of your saw’s shoe.
Use a 3 ½” section of plywood to create the first portion of the C-shaped jig. Line it up with your offset line and attach it to the 4×4 using a few wood screws.
Then repeat this on either side of the first section of the jig until you have a C-shaped jig cradling the 4×4.
Step 3: Make Your Cuts
Now, all you need to do is butt the edge of your circular saw against the side of the jig and cut through the 4×4. The jig will ensure you cut completely straight and makes cutting a perfect finish nearly effortless.
Flip the 4×4 over and repeat this cut with your circular saw on all three sides of the 4×4.
Then use a drill to remove the screws holding the jig in place, and you’ve got yourself a perfectly cut 4×4.
Circular Saw & Hand Saw Method
This method is more of a quick and dirty method, making it ideal when you’ve got a bunch of 4x4s that need cutting quickly. It might not result in a perfectly clean finish, but it’s fast and doesn’t require you to rotate the 4×4 at all.
Step 1: Mark Cut Line
Just like the other methods, the first step is to mark your cut line on the 4×4 using a pencil and speed square.
In this case, we are only going to make a single cut line, so there’s no need to flip the 4×4 over and mark additional lines.
Step 2: Make Initial Cut
Next, line up your circular saw’s blade with the cut line, and make your initial cut along the line on the waste side. Use a speed square or piece of scrap wood as a guide if you’re worried about cutting accurately.
Step 3: Use Hand Saw to Finish the Cut
Now you’re left with an approximately 2 ½” deep groove in your 4×4 that needs to be cut all the way through to complete the cut.
Slide your hand saw’s blade into the existing groove, and carefully finish the cut by hand. This will require a little muscle and elbow grease, but as long as the 4×4 is firmly clamped in place it shouldn’t be too difficult.
An alternative to this would be using a reciprocating saw instead of a hand saw. This will make the task a bit faster, but will likely result in a rougher finish than a hand saw will.
How to Cut 4×4 Posts That are Already Mounted?
What do you do if you’ve got a 4×4 that’s already mounted in place vertically?
This is often the case when you’re installing fence posts in concrete, as you don’t know precisely how the post will settle in place, so it’s easier to set them first and cut them afterward.
In this case, the above methods can be a little tricky, especially if you need to hold the saw above your head to reach the cutting area. These methods can still work, but using a c-shaped guide will be easier than using a cut line to guide your cuts.
If that’s not an option, and you don’t need a perfect finish, then a chainsaw can also work well for cutting fence posts to your desired length. If you’re going to put a fence post cap on it afterward, then a perfect finish isn’t necessary anyway.