PEI and Mohs. What's the difference?

PEI ratings and Mohs both measure hardness, so what's the difference?

PEI (Porcelain Enamel Institute) ratings have been used since the 1930s to help determine the hardness and durability of tile. This is crucial because not all tile can be used in all areas. PEI ratings act as a kind of shortcut to decide where the tile can be installed. Tile that experiences much foot traffic should be harder and denser than tile that receives no foot traffic such as wall tile. From a technical perspective PEI ratings are actually determined by a measurement of Abrasion Resistance on a machine which counts the number of revolutions under a standard abrasive load. When the tile shows damage the revolutions are counted which gives the PEI rating.

Mohs ratings (named after geologist Friedrich Mohs in 1812) specifically measure the hardness of the tile glaze on a scale from 1 to 10 with 1 being soft as talc and 10 being as hard as diamond. Mohs is tested using scratching picks with alloy tips that match the traditional minerals of the Mohs Hardness Scale. 

The table below gives common objects that can be used to provide similar results in your own shop.
The PEI method was developed on the basis that the degree of deterioration of a floor should be determined by the visible difference in appearance between the worn surface and the unworn surface, assessed at a defined distance under standard conditions of lighting. It is important to stress that, for the same degree of abrasion, wear is more noticable on dark surfaces. 

PEI 0 - Tiles technically unsuitable for floors.
PEI 1 - Residential and commercial wall and barefoot traffic. < 154 revolutions.
PEI 2 - Wall and residential bath floor, and soft soled traffic. 300, 450, 600 revs.
PEI 3 - All residential floors and light commercial floors. 750, 900, 1200, 1500  revs.
PEI 4 - Medium commercial, light industrial and institutional, moderate soiling. 1500+ revs.
PEI 5 - Extra heavy traffic, abrasive dirt, chemically more resistant. Up to 12000 revs.

The particular number or PEI rating is a measure of the durability of the tile surface only. It does not determine the slip resistance, overall strength, moisture absorption, or quality of the tile itself, only the quality and durability of the surface or glaze. For most residential flooring applications it is always best to go with a PEI rating of three or higher, although two is suitable for some applications.

So, PEI and Mohs both measure hardness but in different ways. PEI measures resistance to visible wear, Mohs measures actual mineral hardness. A PEI number can be a bit vague so look at the actual abrasion resistance in revolutions to see whether the rated tile is at the high or low end of the PEI number.

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