If you've ever done any sanding, you've likely seen the numbers printed on the back like 80, 120, 220, etc.
And if you're experienced you probably know that the lower numbers correspond with coarser grits and the higher numbers are the finer grits.
But did you ever wonder why that is? And how the numbers are actually determined?
No?
Well we're going to tell you anyway, so buckle up!
For starters, let's clarify what the numbers mean and then we'll get into why and how they are determined.
So, what do the numbers mean in sandpaper?
The numbers on sandpaper represent the grit size, which indicates the coarseness or fineness of the abrasive particles on the disc/sheet.
Specifically:
Lower grit numbers (40, 60, 80, etc) indicate coarser, more abrasive sandpaper that removes material quickly. These are used for heavy-duty sanding, stripping, or heavy material removal.
Higher grit numbers (220, 320, 400, etc) indicate finer, less abrasive sandpaper that leaves a smoother finish. These are used for final finishing and polishing.
And how are the grit numbers determined?
This is where things get interesting and even some industry experts have it wrong!
While the grit numbers refer to the size of the particles, there are differing explanations on how they are actually measured:
Grains per Square Inch
Most articles on the topic say that the grit number is represented by how many grains of a given grit fit in one square inch. The larger the particle, the fewer grains fit in a square inch. So for example, you could fit 40 grains of 40 grit abrasive in a square inch and you could fit 1,000 grains of 1,000 grit grain in that same space.
Holes per Square Inch of Mesh
Steve Ramsay (Woodworking for Mere Mortals), however, says this is a common misconception!
In his Youtube Short video on the topic he says the grit is determined by screens with a certain number of holes per square inch.
So a screen with 80 holes per square inch will fit 80 grit particles, but not 60 or 40 grit particles. And a screen with 400 holes per inch will fit 400 grit particles, but not 320 or anything larger (coarser).
Grains per Linear Inch
Next we have an "Ask the Expert" video from Norton Abrasives EMEA in which Norton Application Engineer Paul Gray explains that the particle size refers to how many grains of a given grit fit side by side in a linear inch.
Referenced in the video below, 36 particles of 36 grit grain lined up side by side would be one inch long.
So where does this leave us?
User "Birdus" from the Practical Machinist Forum summed it up well in his post on the forum:
I've read that sandpaper grit is determined by:
- the number of grains that can fit through a one square inch mesh
- the number of grains that can fit through a one linear inch mesh
- the number of grains on one square inch of sandpaper (maybe not all that different than the mesh)
So what is the truth? How is sandpaper grain size actually measured?
Spoiler alert: none of the explanations above are correct 😱
In order to find out the truth, we need to go to the official source.
There are two organizations that regulate sandpaper grit standards:
- The Federation of European Producers of Abrasives (FEPA)
- The Coated Abrasive Manufacturers Institute (CAMI)
The majority of sandpaper manufacturers use the FEPA standard which is also known as "P" grade because of the P in front of the grit number.
CAMI is only used by US manufacturers of sandpaper (namely 3M) and has looser tolerances for grain size within a given grit.
FEPA's standards have different methods for determining grain size depending abrasive type and the grits being measured but for coated abrasives (sandpaper) in grits P12 - P220, the mass distribution (average grain size) is determined by means of mechanical sieving.
What does that mean?
It means that the grit sizes are determined by the amount of the abrasive material that fits through a series of 5 mechanical sieves with varying mesh sizes per FEPA’s testing parameters.
Still can't visualize it?
Here's our simplified explanation:
A mechanical sieve is essentially a shaker with several pie tins stacked on top of each other. Each pie tin has holes in it and the holes get smaller and smaller from the top tin to the bottom.
Each grain size has specifications for the hole sizes in the pie tins and how many grains should pass through each level. The hole diameters are measured in micrometers (1mm = 1,000 μm).
Here's a video showing the sieve shaker type that FEPA recommends for testing grain sizes:
Now if that ELI5 pie tin version was too basic for you and you want the real stuff, here is a summary from the FEPA standard published in 2006
The size distribution of the grits is determined by the following testing method and parameters:
- No grain retained on the first sieve
- The grain retained in the second sieve doesn't exceed their published maximum level (eg: 7%)
- The total grain retained on sieves 1-3 and 1-4 are within the specified tolerances (eg: 40% for 1-3 and 80% for 1-4).
- The total grains retained on sieves 1-5 is at least the specified minimum value (eg: 92%).
- The remainder in the bottom pan does not exceed the specified maximum value (eg: 8%).
So what does all of that mean?
It means that none of the explanations above were correct!
There is no single size of an 80 grit particle because they have tolerances. That's why there is no 85 grit sandpaper or 90 grit sandpaper. While there may be particles that are slightly larger than the average 80 grit particle, they get grouped with standard sizes based on the tolerances allowed by FEPA.
And to provide some context on how far off the explanations above were, let's look at 80 grit and assume it has an average grain particle size of 200 micrometers (using sieve aperture as a proxy).
Grains Per Square Inch
One inch equals 25,400 micrometers. One square inch = 645,160,000 micrometers.
Using an average particle size of 200 micrometers, if we divide 645,160,000 by 200, we get 3,225,800.
So with this explanation you'd have over 3 million 80 grit particles in a square inch!
Maybe we should rename it 3 million grit?
Holes Per Square Inch of Mesh
This one sounds more like how the grain is actually measured so perhaps it will be closer to the truth.
But right away, we know that the mesh isn't a single size but rather a series of 5 meshes or sieves.
Even still, let's pick one of the sieves and see how many holes it has per square inch.
For this calculation we'll use a No. 70 test sieve which has an aperture of 212 μm and an average wire diameter of 140 μm.
To account for the width of the wire, we'll add those two together and get 352 μm.
At 25,400 micrometers per inch, we'd have 72 of these per linear inch and 5,184 per square inch.
So the average sieve for 80 grit actually has roughly 5,000 holes per square inch.
Closer!
But still not very close at all.
Grains Per Linear Inch
Perhaps this final explanation will be a bit closer to reality?
Let's check.
One inch equals 25,400 micrometers.
With an average grain size of 200 micrometers, if we divide 25,400 by 200 we get 127.
Indeed it is closer...
But still not accurate in either the result or the methodology.
Conclusion
None of these catchy explanations for how sandpaper grain size is measured are accurate.
The actual answer is that the numbers on sandpaper are arbitrary numbers based on standards determined by FEPA and CAMI.
Is that answer so boring and unintuitive that it led to multiple alternative (yet incorrect) explanations?
Who knows.
But if you've made it this far, pat yourself on the back and start telling your friends and family how sandpaper grit is actually measured.
And if you're a real glutton for punishment and want to check our work or learn all about mastergrits and the size of concrete blocks that the test sieves need to be mounted on, here's our source for the FEPA information.