An earthquake catalog containing a uniform size estimate is important for long-term seismic hazard assessment in regions of low-to-moderate seismicity. During the update of the Earthquake Catalog of Switzerland (ecos), we performed regression analyses to convert all earthquake size information in ecos to physically meaningful moment magnitude Mw. For 34 events in and near Switzerland, we determined seismic moment (thus Mw) by regional waveform inversion. Independent Mw estimates for the same events do not exist; however, Mw from European-Mediterranean events, obtained in the same way, agree with Mw from Harvard CMT solutions. All other size estimates, ML, MD, mb, MS, and intensities, are calibrated relative to these 34 events. Teleseismic MS and mb from international data centers are directly regressed against Mw. Most observations in ecos consist of local magnitudes (ML, MD) and intensities. For local magnitudes, we first calibrated the Swiss Seismological Service’s ML. Then we calibrated magnitudes from observatories in neighboring countries (France, Germany, Italy) using only events in the border region (e.g., France–Switzerland). Modern instrumental records exist only since the mid-1970s. We calibrated the macroseismic dataset, which represents by far the largest period in the catalog, by determining surface wave magnitude MS for stronger twentieth century Swiss earthquakes from analog seismograms. These MS, which were converted to Mw, connect intensities and Mw. After calibration, all 20,300 events in ecos have a unified Mw, including a class-type uncertainty estimate based on the original magnitude scale. ecos covers the period 250–2001, from 44° N to 51° N and 4° E to 13° E. The largest event in ecos is the 1356 Mw 6.9 Basle earthquake.