How the eGFR Calculator Estimates Kidney Function
Estimated glomerular filtration rate, or eGFR, is a number that describes how well your kidneys are filtering waste from your blood, reported in millilitres per minute per 1.73 square metres of body surface area. It is the single most used marker of kidney health because a full measured GFR is slow and expensive, whereas eGFR can be estimated from one blood test. This calculator is aimed at people who have seen an eGFR on a lab report and want to understand what it means, at students learning nephrology, and at anyone comparing an old MDRD result with the current standard. It reports both the eGFR and the chronic kidney disease stage, and it treats the choice of measurement unit as a safety-critical decision rather than a detail.
The primary equation is the CKD-EPI 2021 creatinine formula, the current standard recommended by the National Kidney Foundation and American Society of Nephrology joint task force. It reads eGFR = 142 x min(Scr/kappa, 1)^alpha x max(Scr/kappa, 1)^-1.200 x 0.9938^age x 1.012 if female, where kappa is 0.7 for females and 0.9 for males, alpha is -0.241 for females and -0.302 for males, and Scr is serum creatinine in mg/dL. The important change from the older 2009 version is that the 2021 equation deliberately removed the race coefficient, so a single equation now applies to everyone. The tool also shows the older MDRD result, because records from earlier years used it and patients often want to compare. Creatinine reported in micromoles per litre is divided by 88.4 to convert to mg/dL before any calculation.
See the numbers move with a worked case. A 50-year-old woman with a serum creatinine of 0.9 mg/dL produces an eGFR of about 78 mL/min/1.73 m squared, which sits in stage G2, mildly decreased. A 60-year-old man with a creatinine of 1.2 mg/dL comes out near 69, also G2. A younger 40-year-old man with a normal creatinine of 1.0 mg/dL lands around 98, which is stage G1, normal or high. Notice how age pulls the result down even when creatinine is identical, which is why the equation includes an age term rather than reading creatinine alone.
The result guides real conversations rather than replacing them. A patient managing type 2 diabetes watches their eGFR over the years to catch a slow decline early, because falling numbers change how some medicines are dosed. A pharmacist checks eGFR before dispensing a drug that the kidneys clear, since a stage G4 result near 20 means a much lower dose or a different medicine. Someone whose lab still reports the old MDRD value uses the side-by-side view to understand why their newer CKD-EPI number looks a little different. A person converting an overseas report in micromoles per litre uses the prominent unit selector, because entering 80 as if it were mg/dL instead of µmol/L would produce a wildly wrong and frightening answer.
The honest limits matter here more than on most tools. eGFR is an estimate, not a diagnosis, and it is least reliable in people with unusual muscle mass, in acute kidney injury, in pregnancy, and at very high filtration rates where CKD-EPI and MDRD diverge the most. A single value is a snapshot; nephrologists look at the trend and confirm chronic kidney disease over at least three months. This is why the calculator labels every result as an estimate and directs you to a nephrologist. Nothing here is a diagnosis or a treatment decision, so discuss any result, especially a low or falling one, with your doctor. The calculation runs entirely in your browser, so your age, sex and creatinine value are never uploaded, saved or shared.
