GFR Calculator

What This Calculator Does

The GFR Calculator offers a fast, reliable way to estimate your glomerular filtration rate (GFR), which is an essential indicator of kidney function. By entering your age and serum creatinine level, you receive an estimated GFR value along with a kidney disease stage and a clear explanation of your results. This tool supports users in understanding their kidney health and identifying potential risks early, enabling more informed conversations with healthcare providers.

Whether you are monitoring a chronic condition or simply curious about your kidney function, this calculator is designed for easy, accurate, and accessible assessment. It empowers you with actionable insights, helping you make better decisions regarding your health and wellness.

How to Use This Calculator

1. Enter Your Age: Provide your current age in years. Accurate age input is essential for a precise GFR estimate.
2. Input Your Serum Creatinine Level: Enter your most recent serum creatinine value as shown on your lab results.
3. Select the Unit: Choose the unit for serum creatinine (commonly mg/dL or μmol/L) to ensure the formula applies the correct conversion.
4. Click "Calculate": Press the calculate button to generate your estimated GFR, your kidney disease stage, and a descriptive explanation.
5. Interpret Your Results: Review the output fields to understand your kidney function status and read the interpretation guide provided.

Definitions of Key Terms

Age

The number of years since birth. Age is a critical factor in estimating GFR because kidney function naturally changes over time.

Serum Creatinine

A waste product measured by blood tests that indicates how well your kidneys filter blood. Higher levels can signal reduced kidney function.

Unit

The measurement unit for serum creatinine, typically either milligrams per deciliter (mg/dL) or micromoles per liter (μmol/L). The chosen unit ensures the calculation is accurate.

Estimated GFR (eGFR)

The calculated rate at which your kidneys filter waste from the blood, expressed in milliliters per minute per 1.73 m² of body surface area. It is a key marker of kidney health.

Kidney Disease Stage

The classification of kidney function based on your eGFR result, ranging from Stage 1 (normal function) to Stage 5 (kidney failure). Staging guides diagnosis, treatment, and monitoring.

Description

A user-friendly interpretation of your eGFR and kidney disease stage, offering context and recommended next steps.

Calculation Methodology

The calculator uses the 2021 CKD-EPI creatinine equation (race-free), the current guideline-recommended method for estimating GFR in adults. The equation considers your age, serum creatinine level, and sex, and it automatically converts units when needed. Below is the core calculation process:

If serum creatinine unit is μmol/L:
  Convert creatinine to mg/dL by dividing by 88.4

Variables
  Scr   = serum creatinine (mg/dL)
  κ (kappa) = 0.7 for females, 0.9 for males
  α (alpha) = −0.241 for females, −0.302 for males
  Age   = age in years

Indexed GFR (mL/min/1.73 m²):

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

Optional non-indexed GFR:
  BSA = √(height (cm) × weight (kg) / 3600)
  GFR_nonIndexed = GFR × (BSA / 1.73)

Kidney-disease staging (KDIGO):
  Stage 1 (G1): ≥ 90
  Stage 2 (G2): 60 – 89
  Stage 3a (G3a): 45 – 59
  Stage 3b (G3b): 30 – 44
  Stage 4 (G4): 15 – 29
  Stage 5 (G5): < 15

The equation adjusts for the natural decline in kidney function with age and the influence of serum creatinine as an indicator of how well your kidneys are filtering. In clinical settings, further adjustments may be made for sex and ethnicity, but for general use, this version offers a reliable estimate for most adults.

Practical Scenarios

• Routine Health Check: You receive a recent blood test report showing your serum creatinine. Enter your age and this value to quickly check your kidney health and ensure you fall within the normal range for your age.
• Monitoring Chronic Conditions: If you have diabetes or hypertension, use the calculator to track your eGFR over time and catch early signs of kidney impairment. Regular monitoring helps you and your doctor adjust your care plan as needed.
• Family Health History: If you have a family history of kidney disease, use the tool to assess your risk even if you feel healthy. Early detection can prompt lifestyle adjustments or more frequent medical follow-up.
• Evaluating Medication Effects: Certain medications can impact kidney function. After starting a new medication, you can enter updated creatinine results to see if your GFR has changed, supporting more informed discussions with your healthcare provider.

Advanced Tips & Best Practices

• Use Consistent Lab Units: Always double-check the unit of your serum creatinine reading. Mixing up μmol/L and mg/dL can result in incorrect GFR estimates. If in doubt, consult your lab report or healthcare provider.
• Track Trends, Not Just Single Results: Monitoring your GFR over time is more valuable than focusing on a one-off value. Upload or record multiple results to identify patterns, especially if you have risk factors for kidney disease.
• Consider Age and Context: A mildly reduced GFR can be normal in older adults. Always interpret your results in the context of your overall health, symptoms, and medical history.
• Consult Your Doctor for Abnormal Results: If your GFR falls below 60 or you notice a rapid change, seek professional medical advice. Only a healthcare provider can diagnose or manage kidney disease.
• Be Mindful of Limitations: The calculator provides an estimate, not a definitive diagnosis. Factors like extreme muscle mass, pregnancy, or acute illness can affect creatinine levels and GFR results.

Frequently Asked Questions (Optional)

Is this calculator suitable for children or pregnant women?

No, the GFR formula used here is validated for adults only. Children and pregnant women require different equations and clinical judgment for accurate assessment. Please consult a pediatrician or obstetrician for specialized evaluation.

How often should I check my GFR?

For healthy adults, yearly checks are usually sufficient. If you have risk factors like diabetes, high blood pressure, or a family history of kidney disease, your doctor may recommend more frequent monitoring.

Can diet or exercise affect my serum creatinine and GFR?

Yes, intense exercise or a high-protein diet can temporarily increase serum creatinine, which may lower your GFR estimate. For the most accurate results, avoid strenuous activity before your blood test and follow your doctor's instructions.