The raw output of the Advanced Laser Interferometer Gravitational-Wave Observatory (Advanced LIGO) needs to be calibrated to produce the dimensionless strain. Accurate and precise calibration is of crucial importance for detecting gravitational waves and measuring the properties of astrophysical sources. In this talk, I will explain the basics of Advanced LIGO detector calibration, and describe the procedure of calibrating the strain data and quantifying the systematic errors and statistical uncertainties in the third observing run (O3). The final calibration products in the first half of O3 will be presented. Major improvements in O3 and future perspectives will be briefly discussed.