In Advanced LIGO, detection and astrophysical source parameter estimation of the binary black holemerger GW150914 requires a calibrated estimate of the gravitational-wave strain sensed by the detectors.Producing an estimate from each detector’s differential arm length control loop readout signals requiresapplying time domain filters, which are designed from a frequency domain model of the detector’sgravitational-wave response. The gravitational-wave response model is determined by the detector’s optomechanicalresponse and the properties of its feedback control system. The measurements used to validatethe model and characterize its uncertainty are derived primarily from a dedicated photon radiation pressureactuator, with cross-checks provided by optical and radio frequency references. We describe how thegravitational-wave readout signal is calibrated into equivalent gravitational-wave-induced strain and howthe statistical uncertainties and systematic errors are assessed. Detector data collected over 38 calendardays, from September 12 to October 20, 2015, contain the event GW150914 and approximately 16 days ofcoincident data used to estimate the event false alarm probability. The calibration uncertainty is less than10% in magnitude and 10° in phase across the relevant frequency band, 20 Hz to 1 kHz.