GFR Calculator

How to Calculate eGFR

This GFR calculator estimates kidney function using the CKD-EPI 2021 equation and the legacy MDRD formula. Estimated Glomerular Filtration Rate (eGFR) measures how well your kidneys filter waste from the blood, calculated from serum creatinine, age, and sex — expressed as mL of blood filtered per minute per 1.73 m² of body surface area.

This calculator uses the CKD-EPI 2021 equation as the default. The 2021 update removed race as a variable, which the National Kidney Foundation and American Society of Nephrology both recommend. The legacy MDRD formula is included for reference but is being phased out in clinical practice.

CKD-EPI 2021 Formula

The CKD-EPI 2021 equation for males: 142 × min(Cr/0.9, 1)^−0.302 × max(Cr/0.9, 1)^−1.200 × 0.9938^Age. For females, the kappa constant is 0.7, the alpha for min is −0.241, and the result is multiplied by 1.012.

The CKD-EPI equation is more accurate than MDRD for eGFR values above 60 mL/min/1.73 m², which is where most patients fall. It performs well across all age groups and is the standard used in major US clinical labs.

CKD Staging by eGFR

The KDIGO (Kidney Disease: Improving Global Outcomes) classification system divides chronic kidney disease into six stages based on eGFR:

• G1 (≥90): Normal or high — kidney damage present but function preserved
• G2 (60–89): Mildly decreased — common in older adults without disease
• G3a (45–59): Mild-to-moderate decrease — monitoring increases
• G3b (30–44): Moderate-to-severe decrease — nephrology referral often needed
• G4 (15–29): Severely decreased — preparation for kidney replacement therapy
• G5 (<15): Kidney failure — dialysis or transplant required

What Affects Serum Creatinine?

Creatinine is a byproduct of muscle metabolism, so people with more muscle mass (typically men, athletes, and younger adults) naturally have higher baseline creatinine levels. People with low muscle mass — elderly patients, those with muscle-wasting diseases — may have falsely low creatinine, overestimating eGFR. A high-protein meal or intense exercise before blood draw can also temporarily elevate creatinine by 10–20%.

eGFR vs. Creatinine Clearance vs. GFR

True GFR requires inulin clearance testing — impractical for routine care. eGFR is an estimation using serum creatinine formulas. Creatinine clearance (from a 24-hour urine collection) is another estimate that slightly overestimates true GFR because tubular secretion adds creatinine to urine. For most clinical purposes, eGFR from CKD-EPI 2021 is the preferred measure. For more context on kidney health, see our corrected calcium calculator which is often ordered alongside kidney function tests.

When to Seek Nephrology Care

Referral to a nephrologist is generally recommended when eGFR falls below 30 mL/min/1.73 m², when there is a rapid unexplained decline of more than 5 mL/min/1.73 m² per year, or when proteinuria, hematuria, or difficult-to-control hypertension accompanies reduced eGFR. The dosage calculator can help calculate adjusted medication doses for patients with reduced kidney function.

Proteinuria: The Other Dimension of CKD Staging

eGFR alone does not fully characterize CKD severity. KDIGO guidelines add albuminuria (protein in urine) to create a 2-dimensional staging system: G1–G5 for eGFR stages, and A1–A3 for albuminuria categories (A1: <30 mg/g; A2: 30–300 mg/g; A3: >300 mg/g). A patient with eGFR of 65 and A3-range proteinuria is at much higher risk of progression than the same eGFR with A1. Urine albumin-to-creatinine ratio (UACR) is the standard screening test for albuminuria. Early UACR abnormalities often precede eGFR decline, particularly in diabetic nephropathy.

Novel Biomarkers Beyond Creatinine

Creatinine-based eGFR has well-known limitations — it overestimates GFR in patients with low muscle mass and underestimates it in very muscular individuals. Cystatin C, a protein produced by all nucleated cells and cleared only by the kidneys, is increasingly used as an alternative or complement to creatinine. CKD-EPI 2021 includes a cystatin C–based equation that may be more accurate in certain populations. KDIGO 2024 recommends using cystatin C to confirm CKD in cases where creatinine-based eGFR seems inconsistent with clinical findings. For clinically assessed metabolic status alongside kidney function, see our corrected calcium calculator.