Wearable heat flux devices are disclosed that can detect heat flux based on evaporative cooling for determining a core body temperature of a user, and that can heat or cool a surface of a user for reaching a steady-state heat flux to determine the core body temperature of the user. Exemplary heat flux devices can include a heat flux sensor and a wicking layer. The heat flux sensor can be configured to detect heat flux at a location on a user. The wicking layer can be configured to absorb moisture at the location and to transport the moisture above the heat flux sensor. The heat flux subsequently detected by the heat flux sensor includes the evaporative cooling from the evaporation of the moisture.