What is calculated. Two different numbers, deliberately shown together:

• eGFR (CKD-EPI 2021, creatinine only). eGFR = 142 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^−1.200 × 0.9938^age × 1.012 (if female), with κ = 0.7 female / 0.9 male and α = −0.241 female / −0.302 male, Scr in mg/dL. This is the race-free equation recommended by the NKF and ASN (Inker et al., New England Journal of Medicine, 2021) and the one UK labs now report. Result is normalised to a body surface area of 1.73 m².
• Creatinine clearance (Cockcroft-Gault, 1976). CrCl = ((140 − age) × weight in kg) ÷ (72 × Scr in mg/dL) × 0.85 if female — the same equation expressed in SI units as ((140 − age) × kg × 1.23) ÷ Scr in µmol/L for men, ×1.04 for women. It is not normalised to body surface area, which is exactly why many drug licences and SPCs still quote it.

Unit conversion. Creatinine in µmol/L is divided by 88.4 to give mg/dL.

Worked example. A 65-year-old woman, creatinine 130 µmol/L (= 1.471 mg/dL), 70 kg. CKD-EPI: Scr/κ = 1.471/0.7 = 2.101, so the min term is 1 and the max term is 2.101^−1.2 = 0.410; the age factor is 0.9938⁶⁵ = 0.667; so 142 × 0.410 × 0.667 × 1.012 = 39 mL/min/1.73 m² — KDIGO stage G3b. Cockcroft-Gault: ((140 − 65) × 70) ÷ (72 × 1.471) × 0.85 = 42.1 mL/min. The SI shortcut ((140 − age) × kg × 1.04) ÷ Scr in µmol/L gives 42.0; the 0.1 mL/min difference is just the rounding of the 1.04 constant, and this tool uses the unrounded mg/dL form. The 3 mL/min gap between 39 and 42 is normal and is not a discrepancy to reconcile.

Staging. KDIGO G-stages are G1 ≥90, G2 60–89, G3a 45–59, G3b 30–44, G4 15–29, G5 <15 mL/min/1.73 m². A single eGFR does not diagnose CKD: the definition also needs markers of kidney damage (usually albumin-to-creatinine ratio ≥3 mg/mmol) persisting for more than three months. This tool has no albuminuria input, so it reports a G-stage, never a CKD diagnosis.

Where it misleads. Both equations assume creatinine is at steady state — they are wrong in acute kidney injury, where a rising creatinine makes GFR look better than it is. Creatinine is a function of muscle mass, so both overestimate function in frail or sarcopenic patients, amputees, and people on long-term steroids, and underestimate it in very muscular people and in people eating a lot of cooked meat. Neither is validated under 18 years (use the bedside Schwartz equation for children) or in pregnancy, where GFR rises substantially and creatinine falls. In obesity, Cockcroft-Gault on actual body weight overestimates clearance — many formularies ask for adjusted or ideal body weight there, and this tool always uses the weight you enter. If a drug dose sits right on a threshold, or the patient is at a muscle-mass extreme, a cystatin C-based eGFR or a measured clearance is the better answer.

Disclaimer. Informational arithmetic for qualified clinicians — not medical advice, and not a substitute for your laboratory's reported figure or your local formulary. Methodology: CKD-EPI 2021 (Inker et al., NEJM 2021; National Kidney Foundation) · Cockcroft & Gault, Nephron 1976 · KDIGO 2024 CKD guideline. Last reviewed: 2026-09-21.