Hearing research requires a calibrated sound source, traditionally as lab equipment. Online research is quicker and more inclusive, but most participants lack calibration equipment and their sound sources are uncalibrated and diverse. This article explains how the open-source EasyEyes.app calibrates loudspeakers online. A library of smartphone-microphone profiles allows EasyEyes to use the participant's phone to calibrate their computer's loudspeaker in three minutes. Participants select their phone model, which is verified by screen size. Calibration employs the Novak et al. nonsynchronous maximum-length-sequence (MLS) algorithm. The computer's loudspeaker is corrected by convolving its input with the inverse of its impulse response. Researchers can contribute to the open-access library by calibrating phones with a measurement microphone. In the library, each profile is linked back to the profile used to produce it, back to the manufacturer profile of a measurement microphone. Correction accuracy is such that playing the flat-spectrum MLS through the corrected loudspeaker produces a nearly flat spectrum, with standard deviation less than 3 dB. A survey shows that a library of 94 phone models from major brands will support most participants in the USA (87%) and UK (80%). This method facilitates efficient and inclusive online hearing research.