GAC stands for granular activated carbon, which is carbon supplied as loose granules packed into a cartridge rather than compressed into a solid block. The difference between the two forms is not chemical at all: it is about how water moves through the media, and that determines where each one belongs.
The advantage: flow
Loose granules leave far more open space between them than a compressed block does, so water passes with much less resistance. That means a GAC cartridge delivers higher flow at lower pressure drop than a block of the same size, which matters wherever volume is high: whole house carbon duties, pre filtration on systems that need to keep up with a household, and any position where the alternative is a house that has noticeably lost pressure.
The drawback: channelling
Because there are gaps between granules, water can find a path of least resistance and take it, passing through with far less contact than the bed volume suggests. Channelling is worse in poorly packed cartridges, worse after the bed has settled, and worse at high flow. A carbon block cannot channel because there is no gap to exploit, which is why blocks are preferred where guaranteed contact matters more than flow, particularly in drinking water and ahead of a membrane.
The other difference: mechanical filtration
A block filters particles as well as adsorbing, often finely enough for cyst reduction claims on absolute rated versions, so one cartridge does two jobs. GAC does very little mechanically and can shed fine carbon particles, which is why a new GAC cartridge produces grey water for the first few litres and why a sediment stage after it is sometimes specified. On a whole house feed that shedding is inconsequential; ahead of a fine membrane it is not.
Choosing between them
GAC for whole house carbon beds and high flow duties. Block for under sink drinking water and reverse osmosis pre filtration, where contact and mechanical filtration matter more than pressure drop. Catalytic carbon in either form where the utility doses chloramine rather than free chlorine, since standard carbon handles chloramine poorly. And change either on stated volume, not on appearance, since chlorine capacity expires invisibly. Our replacement cartridge guide covers intervals.
Questions people ask about gac filter cartridge
What does GAC stand for?
Granular activated carbon: carbon supplied as loose granules rather than compressed into a solid block.
What is its advantage over a carbon block?
Much higher flow at lower pressure drop, which matters on whole house duties and anywhere a block would restrict the supply.
What is channelling?
Water finding a low resistance path between granules and passing with far less contact than the bed volume implies.
Why is my new GAC cartridge producing grey water?
Fine carbon particles shedding from the loose media. Run several litres through and discard them before use.