Relation between the gas-phase metallicity measured using the direct T_e−method and the R3 ratio. We compare our estimates with several strong-line metallicity calibrations from literature. We show the calibration by Bian et al. (2018), which uses local analogues that occupy similar regions to z ∼ 2 star-forming galaxies on the BPT diagram, the calibration based on local galaxies with EW(Hβ) ≥ 200 derived by Nakajima et al. (2022), Hirschmann et al. (2023) calibration for z > 4 galaxies from the TNG100 simulation, and Sanders et al. (2024) and Chakraborty et al. (2025) calibrations for high-z galaxies between 2 < z < 10.