GFR Calculator (Kidney Function)

Glomerular Filtration Rate (GFR) is how fast the kidneys filter waste out of the blood. Measuring it directly means infusing a marker and timing its clearance, which almost nobody does outside research, so what gets reported is eGFR, an estimate calculated from a creatinine blood test.

CKD-EPI 2021 (the current standard, and the equation this page uses)

CKD-EPI stands for Chronic Kidney Disease Epidemiology Collaboration. The 2021 revision dropped the race coefficient that earlier versions applied to Black patients, which is the change most labs made between 2021 and 2023.

For creatinine in mg/dL:

eGFR = 142 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^(−1.200) × 0.9938^Age × 1.012 if female

Where:

• Scr = serum creatinine (mg/dL). If your lab reports µmol/L, switch the unit next to the input. The conversion is µmol/L ÷ 88.4.
• κ (kappa) = 0.7 for females, 0.9 for males
• α (alpha) = −0.241 for females, −0.302 for males
• The 1.012 female factor is small but real. Leave it out and every woman’s result comes back 1.2% low.

Note what the two terms do. When creatinine sits at or below κ, the min term does the work and the max term is 1. Above κ, the min term is 1 and the steep −1.200 exponent takes over. That is the knee in the curve, and it is why eGFR falls off a cliff once creatinine climbs past normal.

MDRD, the older equation you may still see

MDRD stands for Modification of Diet in Renal Disease. Plenty of labs still report it:

eGFR = 175 × Scr^(−1.154) × Age^(−0.203) × 0.742 if female

It reads lower than CKD-EPI at good kidney function, sometimes by 20 or more, which is the usual reason two calculators disagree about the same blood test. This page shows both so the gap is visible instead of confusing. CKD-EPI is the one to go by.

Worked example

Male, age 55, serum creatinine 1.3 mg/dL. Scr/κ = 1.3 / 0.9 = 1.444, which is above 1, so the min term is 1 and the max term carries it. eGFR = 142 × 1.444^(−1.200) × 0.9938^55 = 142 × 0.6432 × 0.7103 = 64.9 mL/min/1.73 m², stage G2. The same man on MDRD comes out at 57.3, which would read as G3a. Same blood test, different stage, and that is exactly why the equation used gets named on the report.

CKD staging by eGFR

Two things the staging table does not say. CKD (chronic kidney disease) is not diagnosed from one number: it needs the abnormality to persist for at least three months, and full KDIGO (Kidney Disease: Improving Global Outcomes) staging pairs the eGFR stage with an albuminuria stage, which no creatinine equation can see. And eGFR drifts down with age anyway, roughly 1 mL/min/1.73 m² per year after 40, so a G2 result in a healthy 75-year-old means something different from the same number in a 30-year-old.

The equation is for adults with stable kidney function. It is unreliable in pregnancy, in acute kidney injury where creatinine is still moving, at the extremes of muscle mass (bodybuilders, amputees, people with advanced muscle wasting), and in anyone on creatine supplements. Creatinine comes from muscle, so the estimate inherits every assumption about how much muscle a person has.