FAA Part 103 is the federal rule set for ultralight vehicles in the United States. It is one reason pilots are drawn to simple, lightweight aircraft, but it is not a shortcut around responsibility.
The basic idea
Part 103 is aimed at sport and recreational operation. A qualifying powered ultralight is generally discussed around single-occupant use, very low empty weight, limited fuel capacity, limited speed, and daytime recreational operation. Final answers depend on the current regulation and the specific aircraft configuration.
Common powered ultralight limits buyers ask about
Commonly discussed powered ultralight numbers include 254 pounds empty weight, 5 gallons of fuel capacity, 55 knots maximum level-flight speed, and 24 knots power-off stall speed. These details should always be verified against the current rule text and final aircraft documentation.
What Part 103 does not do
Part 103 does not remove the need for training, conservative decisions, suitable fields, weather awareness, airspace awareness, or respect for local airport rules. A pilot still has to understand where they can fly, when they can fly, and what conditions are appropriate.
Official resources
Use official sources before making operating decisions. Start with eCFR 14 CFR Part 103 and the FAA ultralight vehicles page.