GFR Calculator — Free Online Kidney Function Estimator

How to Use the GFR Calculator

1. Enter your serum creatinine level: Find your serum creatinine value on your most recent blood test results (basic metabolic panel or comprehensive metabolic panel). Enter the value in mg/dL. Normal ranges are approximately 0.6-1.2 mg/dL for males and 0.5-1.1 mg/dL for females, but this varies by laboratory.
2. Enter your age: Type your current age in years. Kidney function naturally declines with age — a healthy 70-year-old typically has a lower GFR than a healthy 30-year-old. The CKD-EPI formula accounts for this age-related change.
3. Select your biological sex: Choose male or female. Males typically have higher creatinine levels due to greater muscle mass, so the formula applies different coefficients for each sex to produce an accurate GFR estimate.
4. Review your results: The calculator displays your estimated GFR in mL/min/1.73 m², your CKD stage and its description, a color-coded visual scale showing where your GFR falls, and a reference table of all six CKD stages for comparison.

Always discuss your GFR results with your healthcare provider. A single eGFR value provides a snapshot, but trends over time are more clinically meaningful. Two or more readings below 60 mL/min/1.73 m² at least 90 days apart typically lead to a CKD diagnosis.

GFR Formula (CKD-EPI 2021 Creatinine Equation)

Variables Explained

• Scr (Serum Creatinine): Your blood creatinine level in mg/dL. Higher creatinine generally indicates lower kidney function because the kidneys are not filtering it out effectively.
• k (kappa): A sex-specific constant: 0.7 for females, 0.9 for males. This normalizes the creatinine value relative to typical levels for each sex.
• a (alpha): A sex-specific exponent: -0.241 for females, -0.302 for males. Applied when creatinine is below the sex-specific kappa value.
• 142: The base coefficient of the equation, representing the estimated GFR for a reference individual.
• 0.9938^Age: The age factor. Each year of age reduces GFR by approximately 0.62%, reflecting the natural age-related decline in kidney function.
• 1.012: The sex coefficient for females, accounting for typically lower creatinine production in females compared to males of the same size.

Step-by-Step Calculation Example

Suppose a 50-year-old male has a serum creatinine of 1.2 mg/dL:

1. k (male) = 0.9, alpha (male) = -0.302
2. Scr/k = 1.2 / 0.9 = 1.333
3. Since 1.333 > 1: min(1.333, 1) = 1, max(1.333, 1) = 1.333
4. 142 x 1^(-0.302) x 1.333^(-1.200) = 142 x 1 x 0.714 = 101.4
5. Age factor: 0.9938^50 = 0.730
6. eGFR = 101.4 x 0.730 x 1.0 (male) = 74.0 mL/min/1.73 m²
7. CKD Stage: Stage 2 (mildly decreased function)

An eGFR of 74 indicates mildly decreased kidney function. For a 50-year-old, this may be within normal age-related variation. The doctor would likely recommend monitoring with repeat testing in 6 to 12 months and managing any risk factors like blood pressure.

Practical Examples

Example 1: Robert's Routine Checkup

Robert is a 62-year-old male whose annual blood work shows a serum creatinine of 1.0 mg/dL:

• eGFR: approximately 83 mL/min/1.73 m²
• CKD Stage: Stage 2 (mildly decreased)
• Previous year's eGFR: 85 mL/min/1.73 m²

Robert's eGFR of 83 is normal for his age. The 2-point decline from last year is within normal variation and not a cause for concern. His doctor recommends continuing annual monitoring and maintaining good blood pressure control.

Example 2: Maria Managing Diabetes

Maria is a 55-year-old female with type 2 diabetes. Her creatinine has been gradually rising over 3 years: 0.9, 1.1, 1.3 mg/dL:

• eGFR Year 1 (Scr 0.9): approximately 73 mL/min/1.73 m²
• eGFR Year 2 (Scr 1.1): approximately 55 mL/min/1.73 m²
• eGFR Year 3 (Scr 1.3): approximately 43 mL/min/1.73 m²

Maria's declining eGFR from Stage 2 to Stage 3b over three years indicates diabetic kidney disease progression. Her nephrologist adjusts her diabetes management, starts an SGLT2 inhibitor (which has kidney-protective effects), and increases monitoring to every 3 months.

Example 3: Young Adult Screening

Chen is a 28-year-old male who recently had blood work done for a life insurance application. His serum creatinine is 0.85 mg/dL:

• eGFR: approximately 113 mL/min/1.73 m²
• CKD Stage: Stage 1 (normal or high function)

Chen's eGFR of 113 indicates excellent kidney function, typical for a healthy young adult. No follow-up is needed beyond routine annual checkups unless risk factors develop.

CKD Stage Reference Table

Tips and Complete Guide to Understanding GFR

Understanding Your Results in Context

A single GFR reading is a snapshot, not a definitive diagnosis. GFR naturally declines with age — losing about 1 mL/min/1.73 m² per year after age 30 is considered normal. A 70-year-old with a GFR of 65 may have perfectly healthy kidneys for their age, while the same value in a 30-year-old would warrant further investigation. Context matters: your doctor considers GFR alongside other markers like albumin in urine (albuminuria), blood pressure, and your complete medical history to make clinical decisions.

Risk Factors for Kidney Disease

The leading causes of chronic kidney disease are diabetes (responsible for approximately 38% of CKD cases) and hypertension (approximately 26%). Other significant risk factors include family history of kidney disease, cardiovascular disease, obesity, smoking, frequent use of NSAIDs or other nephrotoxic medications, recurrent kidney infections, and autoimmune diseases like lupus. If you have any of these risk factors, regular GFR monitoring is especially important for early detection. Early-stage CKD is often asymptomatic, making screening through blood tests the primary means of detection.

Protecting Your Kidneys

Evidence-based strategies for kidney protection include maintaining blood pressure below 130/80 mmHg (the most impactful single intervention), controlling blood sugar if diabetic (HbA1c below 7%), staying well hydrated, eating a balanced diet moderate in protein and low in sodium, exercising regularly, avoiding excessive NSAID use, and not smoking. For patients already diagnosed with CKD, SGLT2 inhibitor medications and ACE inhibitors or ARBs have been shown to slow progression. Work with your healthcare provider to develop a kidney-protective plan tailored to your specific situation.

Common Mistakes to Avoid

• Self-diagnosing based on a single test: One abnormal GFR reading does not confirm CKD. Laboratory error, dehydration, recent intense exercise, or a high-protein meal before the test can all temporarily affect results. Diagnosis requires at least two readings below 60 at least 90 days apart.
• Ignoring mildly reduced GFR: Stage 2 CKD (GFR 60-89) is often dismissed as insignificant, but it may indicate early kidney damage that can progress if risk factors are not managed. Use it as motivation to address modifiable risk factors.
• Using the wrong units: This calculator expects creatinine in mg/dL (the standard unit in the US). If your lab reports creatinine in micromol/L (common in many countries), divide by 88.4 to convert to mg/dL.
• Comparing your GFR to others: GFR depends heavily on age, sex, and body composition. A GFR that is normal for one person may be abnormal for another. Focus on your own trend over time rather than comparing to others.
• Taking nephrotoxic medications without monitoring: If you regularly use NSAIDs, certain antibiotics, or other medications that affect kidney function, ensure your GFR is monitored regularly. Discuss kidney-safe alternatives with your doctor if your GFR begins to decline.

Frequently Asked Questions