Pellet hardness refers to the mechanical force required to crush or break an individual pellet, typically measured using a hardness tester that applies increasing compressive force to a single pellet until it fractures, with the result recorded in units of force such as kilograms or Newtons. While related to pellet durability index, hardness measures resistance to a single applied crushing force, whereas PDI measures resistance to the cumulative abrasive and impact stresses of tumbling.

Pellet hardness is influenced by similar factors to overall pellet quality — formulation, conditioning, die specification and cooling — but is sometimes tracked as a separate or complementary quality measure to PDI, particularly where a specific minimum hardness is specified to ensure pellets survive a particular handling or feeding system without excessive breakage.

Hardness testing equipment ranges from simple manual or spring-loaded testers suitable for quick floor checks to more precise, motorized testing instruments capable of generating a detailed force-versus-deformation curve for each tested pellet, with the more sophisticated instruments often used in formal quality assurance laboratories rather than routine production floor monitoring.

Because hardness is measured on individual pellets rather than a bulk sample as in PDI testing, hardness results can show meaningful variation between individual pellets even within the same batch, and most testing protocols specify testing a minimum number of pellets per sample to obtain a statistically meaningful average hardness value rather than relying on any single pellet's result.

Excessively hard pellets are not always desirable in every application; in some aquafeed or starter feed applications, for example, overly hard pellets may be less readily consumed by the target species, so target hardness ranges are often set based on both durability requirements and animal feeding behavior considerations, rather than simply maximizing hardness as a universal quality goal.

Hardness and durability do not always move in perfect lockstep — it is possible for a pellet to test as quite hard under a single crushing force test while still generating significant fines under the cumulative abrasive stress of a durability tumble test, which is one reason many quality programs track both measurements rather than relying on either one alone as a complete picture of pellet mechanical quality.