Taste the Difference
Pellets are produced by shredding dried whole hops and then extruding them through a die. The stickiness of the hops resins naturally holds the pellets together, so no additional binding agents are needed. Pellets are favored from a storage perspective because they occupy just a fraction of the space needed for an equivalent amount of whole leaf hops, but because pellets disintegrate in the kettle and fermentor, whole hops are often preferred for dry hopping and in the hopback.
T-90 pellets are the type most commonly available to hobbyists, either by the pound (454 g) or in 1- or 2-ounce (28- or 57-g) packages, but T-45 pellets are seen on occasion. T-45 pellets have had much of the vegetal material removed, producing a more concentrated product: The numbers simply tell you what fraction of the original dried hops material makes it into the pellet. One hundred kilograms of whole leaf hops will yield 90 kilograms of T-90 or 45 kilograms of T-45 pellets, respectively. Thus, if a homebrew recipe calls for 2 ounces (57 g) of whole leaf Cascade, you could substitute 1.8 ounces (51 g) of T-90 Cascade pellets (90 percent of 2 ounces) or 0.9 ounces (26 g) of T-45 pellets (45 percent of 2 ounces), assuming the same alpha-acid percentage.