The behavior of an evanescent field sensor is important for design purposes. This work gives an approximate and simple equation to estimate the absorption of light in an optical fiber with lossy cladding. The formula is based on the weakly guiding approximation and estimates the output power fraction of the sensor in terms of the fiber and the surrounding parameters. The formula is then related to the concentration of the surrounding medium. © 2002 Society of Photo-Optical Instrumentation Engineers.