I usually buy it in 10 ft. lengths. This gives me a little extra to work with when I need to move the CO2 cylinder around for cleaning or replacing it. My diffusers are in my aquariums. If you are using an outside the aquarium reactor, you may be able to get by with a little less. You can measure how much you need easy enough. CO2 tubing is cheap and I would rather have a little more than not enough.