The Solar Low Energy X-ray Spectrometer (SoLEXS) on board India’s Aditya-L1 mission was launched on September 2, 2023, and commenced solar observations on December 13, 2023, following successful aperture cover deployment. Operating from the sun–Earth L1 Lagrange point, SoLEXS has been providing continuous sun-as-a-star soft X-ray spectroscopy across 2 to 22 keV with 170 eV resolution at 5.9 keV and 1-s temporal cadence since January 6, 2024. The instrument employs two silicon drift detectors with aperture areas of 7.1 and 0.1 mm2 to accommodate the full dynamic range of solar activity from A-class to X-class flares. We present comprehensive ground and on board calibration procedures that establish SoLEXS’s quantitative spectroscopic capabilities. Ground calibration encompassed energy-channel relationships, spectral resolution characterization, instrument response functions, and collimator angular response measurements, with thermo-vacuum testing validating performance stability across operational temperature ranges. On board calibration utilizing an internal Fe55 source demonstrated preserved post-launch spectral resolution (164.9 to 171.2 eV), whereas cross-calibration with GOES-XRS and Chandrayaan-2/XSM confirmed radiometric accuracy and flux agreement. The instrument’s 100% observational duty cycle at L1 enables unprecedented continuous monitoring of solar flare evolution across all intensity classes, providing calibrated data for advancing coronal heating mechanisms, flare energetics, and flare-coronal mass ejection relationship studies through soft X-ray spectroscopy.