Use our GFR calculator to quickly estimate your glomerular filtration rate (eGFR) using the CKD-EPI 2021 creatinine equation. Enter your age, sex, and serum creatinine to receive an estimate of kidney function, CKD stage, and—if you add height and weight—an optional absolute GFR not indexed to body surface area.
What is GFR and why it matters
Glomerular filtration rate (GFR) reflects how well your kidneys are filtering blood. Because directly measuring GFR is complex and time-consuming, clinicians commonly use an estimated GFR (eGFR) derived from blood creatinine, age, and sex. eGFR guides the diagnosis and staging of chronic kidney disease (CKD), helps monitor progression over time, and informs safe medication dosing. Higher eGFR indicates better kidney function; lower eGFR suggests impaired filtration and warrants clinical attention in context with other findings such as albuminuria and imaging.
How the GFR calculator works
This tool implements the CKD-EPI 2021 creatinine equation, which estimates GFR from serum creatinine (in mg/dL), age, and sex without using a race variable. The equation was developed from large, diverse cohorts to improve accuracy across a wide range of kidney function. If your laboratory reports creatinine in µmol/L, the calculator automatically converts it to mg/dL to run the equation.
CKD-EPI 2021 at a glance
- Inputs: age, sex (male or female), and serum creatinine.
- Units: creatinine in mg/dL (with automatic conversion from µmol/L).
- Output: eGFR standardized to 1.73 m² body surface area (mL/min/1.73 m²).
- Optional: absolute (non-indexed) GFR when height and weight are provided, using the Mosteller BSA formula.
Units and conversion
Creatinine commonly appears as mg/dL in the United States and µmol/L in many other regions. The tool converts µmol/L to mg/dL by dividing by 88.4 to ensure the correct input for the equation. Always enter values exactly as reported by your lab.
How to use the GFR calculator
- Enter your age in years (adults only).
- Select sex as reported at the time of testing.
- Type your serum creatinine value and choose the correct unit.
- Optionally, add height and weight to view an absolute GFR not indexed to 1.73 m².
- Leave the staging box checked to see your CKD stage based on eGFR.
- Click Calculate to view results instantly.
Interpreting results and CKD staging
Your result appears as eGFR in mL/min/1.73 m². If you provided height and weight, you will also see an absolute GFR. Staging is based on eGFR thresholds and is only one part of CKD classification. Albumin-to-creatinine ratio (ACR) and clinical context are essential for a complete assessment.
- G1: eGFR ? 90 (normal or high, consider ACR and other markers).
- G2: 60–89 (mildly decreased, check for other evidence of kidney disease).
- G3a: 45–59 (mild to moderate decrease).
- G3b: 30–44 (moderate to severe decrease).
- G4: 15–29 (severely decreased).
- G5: < 15 (kidney failure).
Limitations and clinical context
eGFR is an estimate—not a direct measurement. It assumes stable creatinine and typical muscle mass. Accuracy may be reduced in acute kidney injury, pregnancy, extremes of body habitus or diet, neuromuscular diseases, and in patients taking medications that impact creatinine handling. When results do not match the clinical picture, clinicians may confirm kidney function with repeat testing, cystatin C–based equations, or measured GFR. Medication dosing should follow approved labeling, institutional policies, and current guidelines.
Improving your estimate
- Use the most recent lab result, ideally taken when you are well hydrated and clinically stable.
- Always verify units; incorrect units lead to large errors.
- Track trends over time rather than relying on a single eGFR.
- Discuss your results with a qualified healthcare professional, especially if eGFR is below 60 or falling.
Our GFR calculator is designed to be fast, accurate, and easy to use, helping you and your care team translate routine lab values into actionable insights about kidney health.