A method of extrapolating electron densities (Ne) from Incoherent Scatter Radar (ISR) data to high altitudes is presented, and is used to estimate the vertical Total Electron Content (TEC) over Arecibo. Although primarily intended for use on the Ne profiles obtained using the new topside frequency hopping mode at Arecibo which provides measurements to over 2000 km altitude, we show that if the O+-H+ transition altitude is low enough to be observed, then the method may also be applied to the data taken using the standard World Day multi-radar mode, and thus to a great fraction of the World Day data from low solar flux conditions. This method is applied to Arecibo ISR data taken during the January 6--11 1997 CEDAR storm period, and compared with measured TEC from the Global Positioning System (GPS) satellite network. In addition, the TEC is compared to TEC calculated from the Sheffield University Plasmasphere Ionosphere Model (SUPIM) as part of an ongoing project of model validation. ¿ 2000 American Geophysical Union