What does 9H mean?

9H refers to the hardest rating on the Wolff-Wilborn pencil hardness scale.

"A pencil is held at an angle of 45º and pressed hard into the surface until the end of the pencil breaks off.  The coating is rated for the hardest pencil you can use without the surface marking."

This may sound a little odd, but it is a genuine scientific measurement. Of course, you can't use just any old pencil, you have to use very expensive scientifically calibrated pencils which comply with DIN ISO 15184. The pencils themselves are calibrated using the Mohs scale, which its self may use an "H" suffix as 1H to 10H, to determine hardness. However, it is important not to confuse the two, 9H on the Mohs scale is far harder than 9H on the Wolff-Wilborn scale. A calibrated 9H pencil seems to be somewhere around 2.5 on Mohs scale, which is about as hard as your fingernails.

Different industries appear to use this test in different ways, some have a mouse which holds the pencil at 45º which is then dragged across the surface of the coating, others hold the pencil and press down, which may be more suitable for field testing on upright surfaces. 

When ceramic coatings first took the world by storm, the marketeers used the fact that their product was rated 9H as a selling point. However, as all the ceramic coatings are rated 9H, this does little to help one brand differentiate it from the others, and so it is something that is bragged about a little less now.

What this question is really about

People asking what 9H means are usually trying to compare ceramic coatings and work out which one is “stronger” or more scratch-resistant. The confusion increases when they see some products advertised as 10H and assume this must be a meaningful upgrade.

What 9H actually refers to

9H is not a measurement of real-world scratch resistance. It comes from a pencil hardness test, a laboratory method originally designed to assess coatings on flat industrial materials.

• The test uses pencils of increasing hardness pressed against a surface
• 9H is the hardest standard pencil grade commonly used
• If the surface resists marking, it is labelled 9H

Why this test is limited on cars

Pencil hardness tests do not reflect how paint is damaged in real life.

• Car paint is flexible, not rigid like lab panels
• Scratches come from grit, dirt, and washing - not pencil tips
• Different brands test at different pressures and angles

What 9H does not mean

• It does not mean the coating is scratch-proof
• It does not mean stone chips will be prevented
• It does not guarantee longer lifespan on the road

So what about 10H coatings?

Claims of 10H hardness are where the terminology becomes misleading. The Wolff-Wilborn industrial test kit produced by Koh-I-Noor Hardtmuth does contain a 10H pencil, but this falls outside the DIN ISO 15184 standard.

• There is no standard pencil hardness scale above 9H
• 10H is not a recognised test result
• It is usually a marketing claim rather than a measurable upgrade

In practical terms, a “10H” coating cannot be meaningfully compared to a 9H coating because the reference scale has already ended. If you were to take 10H as legitimate, it still wouldn't be a fair comparison, as other brands have only been tested up to 9H. Had they also been tested with  Koh-I-Noor 10H, they too may have passed.

Why does the scale only go up to 9H?

The Wolff-Wilborn test is a relative scratch resistance test using pencils of increasing hardness.

Pencil cores are a controlled mix of graphite and clay. As more clay is added, the pencil becomes harder. In industrial manufacturing, that progression reliably tops out at 9H. Beyond that point:

• The “lead” becomes too brittle and inconsistent
• It chips instead of producing a controlled scratch
• Results stop being repeatable between batches and brands

So the scale ends not because 9H is the hardest thing that exists, but because 9H is the hardest usable, standardised pencil.

The test method is defined around what can be produced consistently, not around the ultimate hardness of materials.

Why is Wolff-Wilborn considered adequate for industrial coatings?

Because most industrial coatings are:

• Thin films (microns thick)
• Organic or hybrid polymer systems
• Applied over much softer substrates (paint, plastic, aluminium, steel)

The real-world failure mode for these coatings is not “bulk hardness”, it is:

• Marring
• Light scratching
• Abrasion from dust, washing, wiping and handling

The pencil test is adequate because it:

• Directly tests surface scratch resistance
• Is cheap, fast and highly repeatable
• Is sensitive in exactly the range where coatings actually sit
• Correlates well with real-world damage mechanisms

For paints and ceramic-polymer coatings, once you reach the top end of the pencil scale, you are already beyond what everyday contaminants (road dust, wash media, fingernails, plastics) can easily mark.

At that point, further increases in true material hardness bring diminishing practical benefit, while brittleness, adhesion, flexibility and chemical resistance become more important than “being harder”.

Why higher numbers don’t equal better protection

• Harder coatings can still scratch if washing technique is poor
• Flexibility, bonding, and chemistry matter more than hardness labels
• A well-maintained coating outperforms a neglected “harder” one

What to focus on instead of H ratings

• Surface preparation quality before coating
• Coating chemistry and real-world durability
• How the car is washed and maintained
• Suitability for how the car is actually used

Best-practice takeaway

• 9H comes from a pencil test, not real-world driving
• 10H is not a recognised or meaningful standard
• Hardness numbers are marketing shorthand, not guarantees
• Preparation and maintenance matter far more than labels

Written by Danny Argent. Last updated 13/01/2026 17:16