Kidney Function (GFR) Calculator
Estimate your glomerular filtration rate (eGFR) using the MDRD equation to assess kidney function and chronic kidney disease (CKD) stage. Useful for patients and clinicians monitoring renal health.
About this calculator
This calculator uses the four-variable MDRD (Modification of Diet in Renal Disease) equation: eGFR = 175 × creatinine⁻¹·¹⁵⁴ × age⁻⁰·²⁰³ × 0.742 (if female) × 1.212 (if African American). Serum creatinine in mg/dL is the primary input, as creatinine is a waste product filtered by the kidneys — higher levels reflect lower filtration capacity. The age exponent captures the well-documented decline in GFR with aging, approximately 1 mL/min/1.73 m² per year after age 40. The gender multiplier accounts for the fact that women generally have lower muscle mass and therefore lower baseline creatinine production. The race multiplier reflects population-based differences in average muscle mass that affect baseline creatinine levels. Results are reported in mL/min/1.73 m² and mapped to CKD stages G1 through G5, ranging from normal (≥90) to kidney failure (<15).
How to use
Example: A 55-year-old African American woman with serum creatinine of 1.2 mg/dL. Step 1: Creatinine factor — 175 × 1.2⁻¹·¹⁵⁴ = 175 × 0.810 ≈ 141.8. Step 2: Age factor — 55⁻⁰·²⁰³ = 1 / 55⁰·²⁰³ ≈ 1 / 1.887 ≈ 0.530. Step 3: Multiply — 141.8 × 0.530 ≈ 75.2. Step 4: Apply female multiplier — 75.2 × 0.742 ≈ 55.8. Step 5: Apply race multiplier — 55.8 × 1.212 ≈ 67.6 mL/min/1.73 m². This result places the patient in CKD Stage G2 (mildly decreased kidney function, 60–89).
Frequently asked questions
What is a normal eGFR and what do the different CKD stages mean?
An eGFR of 90 mL/min/1.73 m² or above is considered normal kidney function (CKD Stage G1, if other markers of kidney damage are present). Stage G2 (60–89) indicates mildly decreased function, often with no symptoms. Stages G3a (45–59) and G3b (30–44) represent moderate reduction, where complications like anemia and bone disease may emerge. Stage G4 (15–29) is severely decreased function requiring preparation for renal replacement therapy. Stage G5 (below 15) is kidney failure, typically requiring dialysis or transplantation. Regular monitoring is critical because many patients reach advanced stages before experiencing noticeable symptoms.
How accurate is the MDRD equation for estimating kidney function?
The MDRD equation is well-validated and widely used in clinical practice, particularly for detecting moderate-to-severe kidney impairment (eGFR below 60). However, it tends to underestimate eGFR in healthy individuals with normal or near-normal kidney function, which is why the CKD-EPI equation is now preferred in many clinical settings for values above 60. Results can also be affected by non-renal factors influencing creatinine levels, such as high dietary meat intake, muscle-wasting conditions, or certain medications. All eGFR formulas are estimates of true GFR, not direct measurements. For research or highly precise clinical decisions, measured GFR using inulin or iohexol clearance is the gold standard.
Why does race affect the eGFR calculation in the MDRD equation?
The MDRD race multiplier of 1.212 for individuals classified as African American was derived from the original MDRD study population, which found systematic differences in measured versus predicted GFR between racial groups, attributed primarily to differences in average muscle mass and creatinine generation rates. However, this multiplier has been increasingly criticized because race is a social construct, not a biological variable, and using it can perpetuate health disparities by potentially delaying CKD diagnosis in Black patients. Many institutions and the newer CKD-EPI 2021 equation have eliminated the race variable entirely to improve equity and accuracy across populations. If you are using this result clinically, discuss with your physician which equation and population-specific adjustments are most appropriate for your situation.