Relative intensity noise (RIN) can be used to characterize pulse-to-pulse energy variations of ultrafast lasers, and is a very important performance parameter when considering the suitability of a laser for an application. However, owing to a wide range of measurement and analysis techniques, comparison of RIN values is non-trivial. Here, we clearly layout a definition of RIN as a percentage value for ultrafast laser systems. Furthermore, we analytically describe how the RIN can be measured in the time and frequency domains, and reveal the conditions under which these two widely employed approaches are equivalent. Finally, we experimentally measure the RIN of an ultrafast supercontinuum laser to be 6.57% in the time domain and 6.98% in the frequency domain at 850 nm, and 17.06% in the time domain and 17.08% in the frequency domain at 1000 nm, thus demonstrating the expected strong agreement when the measurements and signal processing are performed appropriately.