How much has sea level risen in the past 100 years?

As estimated by tide gauges, global mean sea level, or the average height of the ocean surface, has risen 6 - 8 inches (15 - 20 centimeters) since 1920. In the continental U.S., relative sea level has risen about 10 - 12 inches (25 - 30 centimeters) over the same period. In the past three decades, satellites have measured an increase in global mean sea level of about 10 cm. In addition, observational data from tide gauges and satellites also show that sea level rise, both globally and along the continental U.S., is accelerating (see NOAA and NASA portals for altimeter-based global rates and NOAA for local tide gauge rates).

See SeaLevel 101: How we measure? or the the National Sea Level Explorer: Present for any location for more information.