The Hardness Rating on Your Screen Protector Might Not Mean What You Think

When purchasing a screen protector, there are certain specifications that can help you determine the reliability of the new film or glass. Hardness is one of these ratings, primarily used for tempered glass protectors to describe the durability of the material. 9H is a common listing, often presented in a way that implies a screen protector is pretty durable, if you take it literally. If you explore deeper, you’ll realize that the rating doesn’t mean what you think, even on some top-rated screen protectors.

You see, the true toughness scale is called the Mohs mineral scale or the Mohs hardness scale. The weakest mineral is talc at level 1, gypsum at level 2, and the strongest material being diamond at level 10. Most people understand, however, that diamonds are very strong. A knife or glass plate is somewhere in the middle, around 5.5. The Mohs scale is not linear, so an object at level 2 is not twice as hard as an object at level 1.

Now, you would think that the 9H number puts tempered glass protectors pretty high on the list, close to diamond. But that’s not true. 9H hardness is not actually a measurement that belongs to the Mohs scale. Hardness, as it applies to films and thin materials, such as tempered glass, follows a different scale, standardized by the American Society for Testing and Materials (ASTM). Specifically, ASTM D3363 establishes a testing method for determining the hardness of a coating: a pencil is passed over the surface at a 45-degree angle and, based on the marks left or not left, it is given a score. They use different pencil leads, from 9B, the softest, to 10H, the hardest. A hardness of 10H is roughly equivalent to about 3 on the Mohs scale, although there is no precise conversion between the scales. However, 9H really isn’t particularly difficult.

The hardest pencil has a rating of 10H, so a 9H screen protector sits a little lower. Even the hardest 10H pencil lead ranks relatively low on the Mohs scale, around 1 to 1.5. So a rating of 9H is clearly not comparable to a Mohs rating of 9. The pencil test only tells you that the surface resisted scratches from a 9H pencil under the test conditions. But the rough level on the Mohs scale means that anything stronger than this type of lead can scratch the surface, including keys, coins, styluses and perhaps even fingernails, which are 2.5 on the Mohs scale.

This is where things get more confusing. The glass actually has a rating on the Mohs scale, around a level 5.5. Tempered glass is a bit higher, around a level 6 or 7. Technically, in terms of mineral hardness, tempered glass is tough. But while higher hardness ratings can mean greater scratch resistance, they also correlate with a higher risk of breakage. This is precisely why 9H screen protectors resist some scuffs and scratches, but often break completely after a sudden drop. That being said, tempered glass screen protectors are still better than plastic ones at protecting modern devices from physical damage.

What we can take away is that the 9H hardness rating, when used to describe a tempered glass screen protector, is not all it claims to be. The film is not totally protected from scratches, especially those from harder objects or materials, and for true durability it falls between 5 and 7 on the Mohs scale – it can still be broken. The 9H hardness does not tell you that it is a Mohs 9. It tells you that the film has passed a 9H pencil hardness test. Manufacturers don’t lie; it happened. But they often use linguistic and marketing tricks to make the feat seem better than it really is.

In reality, when shopping for screen protectors, rather than looking at a single feature or measurement, like 9H hardness, you need to consider several factors. The first, and most important thing to consider is how you will use your device. What kind protection you need? 9H hardness can be great if your phone or device spends a lot of time in your purse near objects that won’t harm it. But if you’re outdoors, working on a construction site, for example, you’ll need a thicker, scratch-resistant protector paired with a sturdy phone case.

You should also consider the type of material the protector is made from. Tempered glass is not always the best option, regardless of its level of hardness. Thermoplastic polyurethane (TPU) will not break and although it feels rubbery and softer, it provides great protection against knocks, bumps, drops and even most objects, although it may show more scratches and abrasions over time; they can somehow heal. Polyethylene terephthalate (PET) protectors or materials provide a happy medium between soft plastic and glass. They offer excellent protection against scratches but virtually no protection against impacts. Then, of course, there’s the question of whether you should choose a matte screen protector for your phone, a glossy protector, or a reflective protector. This has nothing to do with sturdiness though, and everything to do with outward appearance and experience.

If your phone doesn’t already have an oleophobic coating to keep moisture out, this is also another feature you should always look for. Don’t buy a screen protector for your iPhone without one. In fact, even if your device has one, you should still look for it in a protector.