We present the in-flight effective area calibration of the Low Energy Transmission Grating Spectrometer (LETGS), which comprises the High Resolution Camera Spectroscopic readout (HRC-S) and the Low Energy Transmission Grating (LETG) aboard the Chandra X-ray Observatory. Previous studies of the LETGS effective area calibration have focused on specific energy regimes: 1) the low-energy calibration for which we compared observations of Sirius B and HZ 43 with pure hydrogen non-LTE white dwarf emission models; and 2) the mid-energy calibration for which we compared observations of the active galactic nuclei PKS 2155-304 and 3C 273 with simple power-law models of their seemingly featureless continua. The residuals of the model comparisons were taken to be true residuals in the HRC-S quantum efficiency (QE) model. Additional in-flight observations of celestial sources with well-understood X-ray spectra have served to verify and fine-tune the calibration. Thus, from these studies we have derived corrections to the HRC-S QE to match the predicted and observed spectra over the full practical energy range of the LETGS. Furthermore, from pre-flight laboratory flatfield data we have constructed an HRC-S quantum efficiency uniformity (QEU) model. Application of the QEU to our semi-empirical in-flight HRC-S QE has resulted in an improved HRC-S on-axis QE. Implementation of the HRC-S QEU with the on-axis QE now allows for the computation of effective area for any reasonable Chandra/LETGS pointing.