Helium tank cylinders are normally made of aluminum. Cylinders made with aluminum are around 40 percent lighter than steel manufactured tanks. Westwinds is a common brand of helium cylinders and weighs around 14 to 40 LBS.

Westwinds Aluminum CylindersWeight
27 Cubic Foot Tank14 LBS
55 Cubic Foot Tank26 LBS
125 Cubic Foot Tank40 LBS