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Haircut Lengths: 18mm, 21mm, 25mm [With Pics]

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Knowing your haircut lengths is easier said than done. Let’s just say – they don’t make it easy. Clipper numbers were previously almost a “code” used by barbers and stylists. But in an age of DIY trims men are out looking for the answers. 

Here’s what you’re about to learn. 

You’ll see what three popular haircut lengths actually look like; specifically the 18mm, 21mm, and 25mm haircuts. 

Knowing what they look like should take some of the guesswork out of it. Actually grabbing for that clipper and running it over your luscious locks can be a daunting process. 

At least you’ll know what to expect before you do it. 

You’ll also learn what clipper grade number you’ll need in order to trim it. No more confusion – straight to the point. 

Let’s get to it. 

The 18mm Haircut

To trim an 18mm haircut length you’ll need to use a number 6 clipper grade. 

This is true for all major clipper brands including Wahl, Andis, Oster, Babyliss, and Remington. 

Although it more accurately amounts to ¾ inch or 19mm, at this length, one millimeter longer or shorter is not going to be a visible or noticeable difference. 

So, what does an 18mm haircut look like? 

It’ll look like this. 

Example of an 18mm haircut
From Shutterstock

As you can see, it does carry some benefits. 

It’s still very short, but it may just be long enough to style. Nothing fancy – but potentially a very short quiff. Try working some clay in and see what you’re able to do with it. 

If you want to make an 18mm haircut a little more interesting, change up the sides and back. Tapers and fades will always be an option. 

Unlike the number 2 or number 3, it’s short but not so short that you’ll be able to see the scalp. 

The beauty of it is that you’ll still be able to keep it looking sharp and sleek with very little effort. 

It won’t immediately come across as looking buzzed. It’ll simply look like a neat and tidy men’s haircut. 

The 21mm Haircut

To trim a 21mm haircut length you’ll need to use a number 7 clipper guard. 

This clipper guard is available for most major clipper brands except BabylissPro which ends at a #6. 

It’ll trim down to ⅞ inch which although more accurately amounts to around 22mm, at this length, that 1mm difference isn’t noticeable. 

Here’s what it’ll look like: 

Example of a 21mm haircut
From Shutterstock

21mm’s worth of hair does give you more options. Although it’s only a few millimeters longer than the 18mm cut, you do have a wide range of styles to choose from. 

You’ll be able to style a short quiff a little easier  – there’s just more of it to work with. You could even try experimenting with faux hawks to see if the texture of your hair would allow for it. 

At 21mm, you can have more fun with styling products as you’d be able to make better use of them. Stick to matte or low shine clays and pastes as heavier, shinier products such as pomades and gels may reveal too much of the scalp. 

When you’ve got more length like this, you can also have more fun with the sides and back. As the hair on top would be longer, trimming the sides down shorter (with or without fades/tapers) would lead to a more striking contrast. 

The 25mm (1-Inch) Haircut

To trim a 25mm (1-inch) haircut length you’ll need to use a number 8 clipper guard. 

This is generally the longest clipper guard length you can get for most major clipper brands, except for Wahl which does go up to 38mm. 

25mm amounts to 1 inch in length; a nice round number that a lot of men are drawn toward. 

This is what it’ll look like: 

Example of a 25mm haircut
From Shutterstock

The benefits of the 25mm haircut are similar to those of the 21mm haircut. You’ve got flexibility and versatility when it comes to styling. 

It’ll be easier to get texture or some volume if this is what you want. 

You can experiment with crew cuts as well, where you’ve got longer hair on top (25mm in this instance) and buzzed down shorter at the sides and back. 

The hair on top is longer at the front of the top before tapering shorter as you go back towards the crown. 

It’s unlikely you’d be able to get this done properly without a barber’s assistance, but it’s just an example of what you can achieve with this length of hair. 

Scissors Or Clippers For 18mm, 21mm, And 25mm Haircuts? 

The main debate when it comes to “longer” clipper guards like these is whether or not they’re the best way of achieving these lengths. 

Would you be better off going for a scissor cut instead? Ultimately, aren’t clippers meant for trimming down short (i.e number 4 and lower)? 

There’s no “right” answer to this, but here are my thoughts. 

It’s perfectly acceptable to use a clipper guard to trim down to lengths such as these, but you should be aware that it won’t look as natural or textured as a scissor-cut. 

Clippers are great because they’re able to trim down to very specific lengths; quick and perfectly even, with every hair being trimmed down to the exact same length. 

While this is great for trimming short buzz cuts, when it comes to longer haircut lengths, you may not want this effect. 

Having said this, scissor-cuts can be very tricky to DIY. Clippers are easy for home haircuts and a good option for men who don’t want to have to trim down as short as a #4 or shorter. 

Plus, it’s really the only way of knowing exactly how short you’re trimming down to. If you’re intent on trimming down to exactly 18mm, 21mm, or 25mm, using clipper guards with specific lengths is the only way you’ll achieve it precisely. 

Feel free to try it out. If you find that the end result looks a little too unnatural or you’d just like more texture and variation in length, consider a scissor cut instead. 

How To Trim An 18mm, 21mm, or 25mm Haircut

Here’s a brief, step-by-step routine for trimming a basic men’s haircut that’s the same length all over. Nothing fancy, but there are some solid tips here for DIY trims. 

Once you’ve got your hands on a pair of clippers with the necessary clipper guards (listed above), it’s time to get to work. 

1. Wash The Hair

Clean hair is easier to clip down. Grease, dirt and excessive natural oils will obstruct the trim and prevent an even cut. 

A shampoo wash would be best, but rinsing with water alone would be fine too.

2. Prepare Your Clipper

You’ll want to make sure your clipper is charged before trimming. A well-charged clipper will glide through your hair effortlessly, while a poorly-charged clipper will tug and pull. 

Clean and oil the clipper blades beforehand as well. It will most likely have come with a tiny little brush that can be used to scoop and sweep away any old hairs clogging up around it. 

A drop of clipper oil will also go a long way. Your device should have come with a little bottle of one, but if not there are alternative oils that can be used instead. 

Well-oiled clipper blades will work seamlessly, with minimal friction and tugging. 

3. Attach The Correct Clipper Guard

Of course, this will depend on the length you’re trimming down to. 

  • 18mm – #6
  • 21mm – #7
  • 25mm – #8

You may find that the clipper you’ve bought doesn’t come with the necessary guard length. Cheaper clippers often provide a narrower range of clipper guards. 

Have a look online. You’ll be able to find individual clipper guards being sold for the larger brands. 

Just make sure it’ll fit the specific device you’ve got. 

4. Start Trimming The Top And The Sides

The back is the hardest part, so leave it until last. 

Trim against the grain. On the top, this means trimming backward, and on the sides, it’ll mean trimming upwards. 

Trimming against the grain (against the direction of growth) is quicker, more effective, and leads to a more even cut. 

Making the sides and back shorter than the top is often a good idea at these longer hair lengths. It slims down the face and prevents the haircut from growing out like a fuzzball. 

For example, if you’re trimming the top down to 25mm, try trimming the sides and back down to 21mm or 18mm. 

This is effective and isn’t difficult to do yourself. We’re not talking about fancy tapering or fading – just trimming down a little shorter than you otherwise would have in order to add some contrast. 

5. Trim The Back

As I mentioned, the back is tricky. This is because it’s harder to see. If you’ve got a buddy that can help you out with this part, definitely take them up on the offer. 

If you’re doing it yourself, don’t worry – just be patient. A handheld mirror would allow you to see the back of your head as it reflects off your bathroom mirror. 

Try your best to trim against the grain. For the most part, this will be upward, but around the crown area hair can grow in awkward and varied directions. 

Have a feel around the crown to try and figure out what’s going on and then do your best to trim against the grain for as even a cut as possible. 

6. Line Up The Neck

Again, this bit is hard to do yourself. But thanks to the innovation of modern grooming technology, it’s possible. 

A neckline shaving template tool (Amazon Link) can help you trim a straight neckline yourself. Just use the lone blade of the clipper with no guard attached. 

You can then follow this up by shaving beneath it to clear up any neck stubble left behind.  

At these lengths, you’ll be left with a pretty hard and harsh line at the bottom of the neck. If you wanted to soften this up, try trimming the very bottom edge of the neck (i.e just above the neckline) a clipper grade shorter. 

This should add a mild fading effect at the neckline and prevent the line from looking too harsh. 

Conclusion

This should hopefully have cleared up any glaring questions you may have had about these common haircut lengths. 

If you do decide that you’d rather use clippers to achieve these lengths rather than regular old scissors, this guide should serve you well. 

Enjoy.

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