📻 Modulation Index Calculator (AM)
Calculate the AM modulation index from carrier and signal voltage levels.
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Amplitude Modulation Index
The modulation index describes how strongly a carrier wave’s amplitude is varied by the message signal in AM radio. Measured from the modulated waveform’s envelope, it’s calculated as m = (Vmax − Vmin) ÷ (Vmax + Vmin), where Vmax and Vmin are the envelope’s peak and trough amplitudes.
An index of 0 means no modulation at all, while 1 (100%) is the maximum before the signal starts to distort (over-modulation). Broadcast AM transmitters are typically designed to run close to but not exceeding 100% modulation for the strongest usable signal.
Last reviewed August 2026