The electromagnetic (EM) bias is the largest source of error in TOPEX/Poseidon and Jason-1 satellite altimeter sea surface height (SSH) estimates. Current operational EM bias models are based on empirical relationships between the bias, wind speed, and significant wave height. These models are limited in their accuracy because wind speed and wave height do not capture enough information about the sea state to uniquely specify the bias. In order to improve EM bias estimation, we have studied the correlation between the EM bias and RMS long wave slope using data from tower-based experiments in the Gulf of Mexico and Bass Straight, Australia. Models based on significant wave height and RMS slope are more accurate than models based on wave height and wind speed by at least 50% in RMS error between predicted and ground truth bias values. Furthermore, models which incorporate wave slope exhibit reduced regional variation between the two widely separated experiment locations.