Fiber optic loss is calculated in two parts: cable loss and connector loss. Cable loss (dB) = cable length (km) × attenuation coefficient (dB/km). 2 dB/km for single-mode fiber at 1550nm and 0. Guidelines On What Loss To Expect When Testing Fiber Optic Cables To be able to judge whether a fiber optic cable plant is good, one does a insertion loss test with a light source and power meter and compares that to an estimate of what is a reasonable loss for that cable plant. Contractors often install, terminate, and certify cabling without knowing the client's specific requirements.