Accurate magnitude determinations are important issues when producing seismicity catalogs aimed at assessing seismic hazard, especially in moderate seismicity regions. When only a few seismic stations are available, the stability of coda‐wave amplitudes presents great advantages as compared to S/Lg‐wave amplitudes for estimating magnitude. We develop a method for processing coda waves to determine the moment magnitude Mw that is valid both for recent digital short‐period records and old analog short‐period seismograms. Because spectral analyses of paper records are most often not possible, we fit the coda envelopes of the raw time signal. Our empirical representation of the coda envelope is tested with synthetics, and its parameters are determined using 109 earthquakes digitally recorded at 12 stations of the Laboratoire de Détection et de Géophysique (LDG) French velocimetric network between 1997 and 2013. The coda parameters are adapted to the four broad geological zones that can be defined in France. Within each zone, this model matches the observed coda‐wave envelope for Mw between 3.4 and 5.3. Synthetic coda signals computed from the regional quality factors Qc(f) within the [0.3–7 Hz] frequency band of the instrument show that the frequency and regional variation of Qc are enough to explain the coda shape of the raw signal. This empirical coda model is only valid for events with corner frequency larger than the 1 Hz cutoff frequency of the LDG instrument. When applying this raw signal envelope model to actual data, the site effect at each station must be accounted for. From this empirical coda model, we obtain stable corrected coda amplitudes that can be directly converted into Mw by linear regression.