FM Deviation & Bandwidth Calculator
Read an FM signal's modulation index and occupied bandwidth from its peak deviation, and check whether it fits a channel.
Peak deviation is compared with the highest modulating frequency to give the modulation index, and Carson's rule puts the bandwidth at twice their sum: the carrier plus its two nearest sideband groups, ignoring the small far-out tails. A single tone spreads the spectrum into sidebands whose amplitudes follow Bessel functions, so the number of strong sideband pairs is given as the usual index plus one and is an estimate, not a measured spectrum. Channel fit compares that bandwidth with the full spacing and allows no guard band; the largest deviation that still fits is half the spacing minus the top audio frequency. Narrowband FM is taken as an index of 0.3 or less. Stereo pilot, pre-emphasis, splatter filters and the exact occupancy definition of a particular service are not modelled. Everything runs in your browser with no storage, uploads or network access.