CKD-EPI 2021 creatinine equation: 142 × min(Scr/κ,1)^α × max(Scr/κ,1)^−1.200 × 0.9938^age × 1.012 if female, with κ = 0.7 and α = −0.241 for women and κ = 0.9 and α = −0.302 for men.
An estimate for a steady state. It is not valid in acute kidney injury, in pregnancy, at the extremes of muscle mass, or under 18. Staging needs albuminuria and three months of chronicity as well — the value alone gives you the G stage and nothing more.
It is a calculator, not a clinician. Nothing here is medical advice, and the person prescribing owns the decision.