What is the formula for calculating abrasive grit size?

You need a specific surface finish, but abrasive grit numbers are confusing. Picking the wrong grit wastes time, ruins parts, and costs you money, directly impacting your production efficiency.

There is no simple mathematical formula to calculate grit size. Instead, grit size is determined by physically sorting abrasive particles through a series of standardized mesh sieves. The grit number corresponds to the sieve that grades the particles.

You might be looking for a simple equation, but the truth is more physical than mathematical. At our factory in Henan, we don’t ‘calculate’ grit size¹; we grade it according to international standards². This process ensures the consistency our customers need for predictable results. For nearly 30 years, this consistency has been the foundation of our business. Understanding this physical process is key to choosing the right abrasive for your job every single time. It is about precision in manufacturing, not just in numbers.

How to calculate grit size?

You want to predict your surface finish, but how is the grit number itself found? Making a wrong assumption about what the number means leads to inconsistent results and costly rework.

Grit size is not calculated; it is physically graded using a stack of calibrated sieves. The abrasive particles are sorted by passing through meshes with different hole sizes, and the grit number corresponds to the sieve that captures them.

The process of determining grit size is one of separation and classification. In our production lines, we have large, vibrating sieve towers³ that do this on a massive scale. Think of it like sorting rocks, pebbles, and sand. We start with a large batch of crushed abrasive grain, which contains particles of many different sizes. This mix is poured onto the top of a stack of sieves. The sieve at the top has the largest holes, and each sieve below it has progressively smaller holes. A machine then shakes the entire stack. Large particles are caught by the top sieves, while smaller particles fall through to lower levels until they are caught by a sieve they cannot pass through. This meticulous process is why you can trust that a P60 wheel from us will perform the same as the last one.

What is the formula for grit?

You are still searching for a formula to connect the grit number to the particle size. This confusion can lead to purchasing the wrong product, even if the number seems right.

There is no single universal formula for grit. Different standards, like ANSI for the US and FEPA for Europe, define grit sizes. The general rule is: the higher the grit number, the smaller the abrasive particle and the finer the finish.

While there is no mathematical equation, there is a clear inverse relationship: big number, small particle; small number, big particle. For example, a 36-grit wheel is very coarse and used for rapid material removal. A 600-grit wheel is very fine and used for polishing. The complication comes from different global standards. The two most common are ANSI⁴ (American National Standards Institute) and FEPA⁵ (Federation of European Producers of Abrasives). FEPA is further divided into "F" for bonded abrasives and "P" for coated abrasives⁶. While their numbers are often similar, a P80 grit is not identical to an F80 or an ANSI 80. As a global supplier expanding from China, we manufacture our RL brand products and provide OEM services to both standards to meet the needs of all our clients.

What is the formula for grit to micron conversion?

You need to compare an abrasive from Europe (FEPA) with one from the US (ANSI). The different grit numbers for a similar particle size make direct comparison impossible and risky.

There is no simple calculation, but standard conversion charts are used. Microns are a universal unit of physical measurement, making them the best way to accurately compare grit sizes across different standards like ANSI and FEPA.

This is where things get truly practical for a purchasing manager or technical director. Instead of relying on grit designations, which are just names for a grade, you can use the actual particle size measured in micron⁷s. A micron is one-thousandth of a millimeter (µm). It is a precise, physical measurement. This allows you to compare an apple from one standard to an apple from another. For example, a buyer in Germany looking at a spec sheet from a US engineer can use a chart to see that the required ANSI 240 grit corresponds to about 58 microns, which is very close to a FEPA P320 grit. This understanding prevents errors in procurement and ensures the technical result is what you expect.

What is the unit of measure for grit?

Is "grit" a real unit like an inch or a gram? This fundamental misunderstanding can lead to communication errors with suppliers, causing costly delays and incorrect orders.

"Grit" is not a physical unit of measure; it is a grade or designation. It originates from the number of mesh lines per linear inch in the screen used to sort the particles. Therefore, a higher grit number means a finer mesh.

The term itself gives you the best clue to its meaning. A "60 grit" abrasive was historically graded with a screen that had 60 wires, or lines, crossing every linear inch. This created a specific pattern of openings. The more lines you have per inch, the smaller the openings, and therefore the finer the particles that are graded. This is why a 400-grit particle is much smaller than a 40-grit particle. It is helpful to think of it like clothing sizes (S, M, L). "Medium" is not a unit of measurement like a centimeter, but it represents a standard range of measurements. A "medium" shirt from one brand might be slightly different from another, just as a P80 grit is slightly different from an F80. The micron, on the other hand, is the actual measurement. It tells you the physical size of the particle, not just the name of its size category.

Conclusion

Grit size is a grade determined by physical sieving, not a formula. For true precision, convert grit numbers to microns to compare standards and ensure the correct finish for your application.