GFR Calculator: Complete Guide to Glomerular Filtration Rate & Kidney Health
How to calculate GFR. CKD-EPI 2021, MDRD, Schwartz formulas. Trusted for clinical screening. No sign-up—all calculations run locally.
What This Calculator Does & Who It's For
Calculator Purpose & Outputs
- What You'll Get:
Estimated GFR (eGFR): Your kidney filtration rate in mL/min/1.73m² (normalized to body surface area).
CKD Stage: Classification from Stage 1 (normal) to Stage 5 (kidney failure) with color-coded health meter.
Population Mean Comparison: How your GFR compares to age-matched peers (e.g., mean GFR for ages 50-59 is 93).
Formula Comparison: Side-by-side CKD-EPI 2021 (race-neutral) and MDRD (race-adjusted) results for adults.
General Information: Educational information about CKD stages, monitoring frequency, and general next steps.
(Note: This is educational only—always consult a healthcare provider for medical decisions.)
- Calculation Methods:Adults (18+ years): CKD-EPI 2021 formula (race-neutral, preferred by modern guidelines) and MDRD formula (race-adjusted, legacy comparison). Both use serum creatinine, age, and gender. Pediatric (0-18 years): Schwartz formula using height (cm), serum creatinine, age, and gender-specific k values (0.33 for infants, 0.413 for children, 0.7/0.55 for adolescents). Unit Support: Serum creatinine in mg/dL (US standard) or μmol/L (international) with automatic conversion. Height in metric (cm) or US units (ft/in) for pediatric calculations.
- Ideal Users:Individuals with kidney concerns: Educational screening for chronic kidney disease (CKD), understanding lab results, tracking trends over time. High-risk individuals: Diabetes (leading cause of CKD), hypertension, family history of kidney disease, cardiovascular disease, age 60+. Healthcare providers: Quick eGFR calculation reference tool, patient education support. Medication monitoring: Understanding GFR-based dosing requirements (metformin, antibiotics, contrast agents) for discussion with healthcare providers. Pre-surgical screening: Educational assessment of kidney function before procedures requiring contrast or anesthesia. (Note: Always consult healthcare providers for actual medical decisions.)
- Accuracy & Limitations:eGFR formulas are accurate within ±30% for 90% of patients—sufficient for educational screening and general reference. True GFR (measured via clearance studies with inulin, iohexol, or radiolabeled markers) is more precise but expensive, time-consuming, and rarely needed clinically. eGFR may be less accurate in: extreme muscle mass (bodybuilders, athletes), severe muscle wasting (cachexia, amputation), pregnancy (increased GFR), rapidly changing kidney function (acute kidney injury), unusual diets (vegetarian/vegan may have lower creatinine). Important: This calculator is for educational and informational purposes only. It does not provide medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for clinical decisions, diagnosis, and treatment planning.
This GFR calculator estimates your glomerular filtration rate—the rate at which your kidneys filter waste from blood. It provides calculations using standardized formulas (CKD-EPI 2021, MDRD, Schwartz) and interprets results against CKD staging guidelines with age-adjusted context. This tool is for educational and informational purposes only and does not provide medical advice.
Understanding GFR: What It Measures & How It Works
The Filtration Rate Explained
- What GFR Measures:GFR quantifies the rate at which fluid passes through the glomeruli—tiny filtering units within nephrons. Each kidney contains approximately 1 million nephrons, each with a glomerulus that acts as a microscopic sieve. Healthy young adult kidneys filter approximately 120-130 mL of blood per minute, removing waste products (creatinine, urea, uric acid) while retaining essential substances (proteins, red blood cells, electrolytes). The glomerular filtration barrier is semi-permeable, allowing small molecules to pass while blocking larger proteins.
- Why Normalize to 1.73 m² Body Surface Area:GFR is normalized to body surface area (BSA) of 1.73 m²—the average adult BSA. This normalization is critical because larger individuals have more kidney tissue and higher absolute filtration rates, while smaller individuals have less. A 100 lb person and 200 lb person with identical kidney health would have different absolute GFRs but similar normalized GFRs. Normalization ensures accurate CKD staging regardless of body size. The 1.73 m² standard comes from Du Bois' 1916 BSA formula and remains the clinical standard.
- Units Explained: mL/min/1.73m²GFR is reported as milliliters per minute per 1.73 square meters of body surface area. This means: milliliters of blood filtered per minute, adjusted to a standard body surface area. Higher numbers indicate better kidney function. Normal range is 90-120 mL/min/1.73m² for young adults (ages 20-29), declining approximately 1 mL/min/1.73m² per year after age 40. Values below 60 mL/min/1.73m² with evidence of kidney damage indicate chronic kidney disease.
- Why eGFR vs. True GFR (Measured Clearance):True GFR requires measuring clearance of a filtration marker (inulin, iohexol, or radiolabeled compounds like 99mTc-DTPA) via blood and urine collection over 4-24 hours. This is expensive ($500-2000), time-consuming, requires specialized facilities, and is rarely performed clinically. Estimated GFR (eGFR) formulas estimate GFR from serum creatinine—a routine, inexpensive blood test available in all clinical settings. eGFR formulas (CKD-EPI, MDRD, Schwartz) are accurate within ±30% for 90% of patients, which is sufficient for educational screening and general reference. eGFR is the standard screening method worldwide and recommended by all major kidney disease guidelines (KDIGO, NKF, ASN). Healthcare providers use eGFR for clinical decision-making, but this calculator is for educational purposes only.
Glomerular Filtration Rate (GFR) measures how many milliliters of blood your kidneys filter per minute, standardized to a body surface area of 1.73 m². This normalization allows accurate comparison across different body sizes and is essential for CKD staging.
GFR Calculation Formulas
The Three Standard Formulas
- CKD-EPI 2021 Formula (Adults, Preferred):
k = 0.7 (F) or 0.9 (M); a = −0.241 (F) or −0.302 (M); Gender Factor = 1.012 (F) or 1.0 (M). Race-neutral; preferred by modern guidelines.
- MDRD Formula (Adults, Legacy):
2006 formula; race-adjusted. Less accurate at GFR >60. CKD-EPI preferred.
- Schwartz Formula (Pediatric):
k: Infants <1 yr = 0.33; Children 1–13 = 0.413; Adolescents = 0.7 (M) or 0.55 (F).
- Why Different Formulas:Pediatric patients have different muscle mass-to-creatinine relationships than adults, requiring height-based calculations. Adults use creatinine-based formulas because muscle mass stabilizes. CKD-EPI 2021 is preferred over MDRD because it's more accurate, especially at higher GFRs, and removes problematic race adjustments.
Different formulas are used for different patient populations. Understanding which formula applies to you helps interpret results accurately.
CKD Staging: What Each Stage Means
The Six-Stage Classification System
- Stage 1: Normal or High (GFR ≥90):Kidney function is normal, but there may be evidence of kidney damage (protein in urine, imaging abnormalities). Action: Regular monitoring every 1-2 years. Maintain healthy lifestyle (stay hydrated, control blood pressure, avoid excessive protein, limit NSAIDs). Prognosis: Excellent with proper management.
- Stage 2: Mildly Decreased (GFR 60-89):Mild kidney damage with slightly reduced function. May be age-appropriate for older adults. Action: Monitor every 6-12 months. Control blood pressure (<130/80), manage diabetes if present, avoid nephrotoxic medications. Prognosis: Good—progression can often be slowed or stopped.
- Stage 3a: Moderately Decreased (GFR 45-59):Moderate kidney damage. Function is noticeably reduced but usually manageable. Action: Nephrology consultation recommended. Monitor every 3-6 months. Strict blood pressure control, diabetes management, dietary modifications (reduce protein, limit phosphorus/potassium if needed). Prognosis: Variable—depends on underlying cause and treatment adherence.
- Stage 3b: Moderately to Severely Decreased (GFR 30-44):Moderate to severe kidney damage. Function is significantly reduced. Action: Active nephrology care required. Monitor every 1-3 months. Prepare for advanced stages—discuss dialysis access placement, transplant evaluation. Dietary restrictions become more important. Prognosis: Progression to Stage 4-5 likely without aggressive intervention.
- Stage 4: Severely Decreased (GFR 15-29):Severe kidney damage. Function is critically low. Action: Immediate nephrology consultation. Dialysis or transplant evaluation should begin. Monitor monthly or more frequently. Strict dietary restrictions (low protein, phosphorus, potassium, sodium). Prepare for renal replacement therapy. Prognosis: Dialysis or transplant typically needed within months to years.
- Stage 5: Kidney Failure (GFR <15):Kidney failure. Kidneys can no longer maintain health. Action: Dialysis or kidney transplant required for survival. Urgent medical care. Without treatment, uremia (waste buildup) leads to death. Options: hemodialysis, peritoneal dialysis, or kidney transplant. Prognosis: Life-sustaining treatment required. Transplant offers best long-term outcomes.
Chronic Kidney Disease is classified into stages based on GFR and evidence of kidney damage. Understanding your stage guides treatment intensity and monitoring frequency.
Age-Adjusted GFR Interpretation: Why Context Matters
Why Age Matters More Than Absolute Numbers
- Normal Age-Related Decline Pattern:GFR decreases approximately 1 mL/min/1.73m² per year after age 40. This means: A 30-year-old with GFR 90 has mild reduction (expected ~116, loss of ~26 points). A 50-year-old with GFR 85 has lost ~14 points from peak (expected ~99). A 70-year-old with GFR 75 has lost ~25 points but is still within normal age-adjusted range (expected ~75). This is why age is a critical variable in all GFR formulas—it accounts for expected physiological decline. The decline accelerates slightly after age 70, with average GFR dropping to ~65 by age 80.
- Population Mean GFR by Age Group (Reference Values):These population averages help interpret individual results: Ages 20-29: Mean GFR ~116 mL/min/1.73m² (range 90-140). Ages 30-39: ~107 (range 85-125). Ages 40-49: ~99 (range 80-115). Ages 50-59: ~93 (range 75-110). Ages 60-69: ~85 (range 70-100). Ages 70-79: ~75 (range 60-90). Ages 80+: ~65 (range 50-80). These are population averages—individual variation exists based on genetics, health status, and lifestyle. Significant deviation from age norms (more than 20-30 points below expected) suggests kidney disease requiring investigation.
- When to Worry: Red Flags for Kidney Disease:GFR significantly below age-expected values indicates kidney disease, even if the absolute number seems "normal" or is above 60. Examples: A 40-year-old with GFR 70 is concerning (expected ~99, loss of 29 points). A 60-year-old with GFR 60 is borderline (expected ~85, loss of 25 points). A 75-year-old with GFR 50 is concerning (expected ~75, loss of 25 points). Rapid decline (>3-5 mL/min/1.73m² per year) also indicates active disease requiring urgent investigation, regardless of absolute value. Other red flags: protein in urine (albuminuria), elevated blood pressure, diabetes, family history of kidney disease.
- Pediatric GFR Considerations:Children have higher GFRs than adults due to higher metabolic rates, greater kidney efficiency per unit of body mass, and ongoing growth. Normal pediatric GFR ranges from 90-140 mL/min/1.73m², depending on age: Infants (<1 year): 90-120. Children (1-12 years): 100-140. Adolescents (13-18 years): 90-130. The Schwartz formula accounts for age-specific k values (0.33 for infants, 0.413 for children, 0.7/0.55 for adolescents) to provide accurate estimates. Height is critical in pediatric calculations because muscle mass (the source of creatinine) correlates strongly with height in growing children. A child with short stature may have lower GFR estimates even with normal kidney function—this is why height-based formulas are essential for pediatric patients.
GFR naturally declines with age due to reduced nephron number (kidney filtering units), decreased renal blood flow, and age-related structural changes. A GFR that indicates significant kidney disease in a young person may represent normal age-adjusted function in an elderly person. Age-adjusted interpretation prevents misdiagnosis and unnecessary treatment.
Educational Applications: Screening, Monitoring & Medication Dosing Information
When and How Often to Check GFR
- Initial Screening Recommendations:All adults should have GFR checked as part of routine health screening, especially if you have: Diabetes (leading cause of CKD—screen annually), Hypertension (screen every 1-2 years), Family history of kidney disease (screen every 1-2 years), Cardiovascular disease (screen annually), Age 60+ (screen every 1-2 years), Autoimmune diseases (lupus, rheumatoid arthritis—screen annually), Long-term NSAID use (screen every 6-12 months). High-risk individuals (diabetes + hypertension) should be screened annually. The American Diabetes Association recommends annual GFR screening for all diabetic patients.
- Monitoring Frequency by CKD Stage:Monitoring frequency increases as kidney function declines to allow early intervention and treatment adjustments: Stage 1 (GFR ≥90): Every 1-2 years if stable, no kidney damage. Stage 2 (GFR 60-89): Every 6-12 months if stable. Stage 3a (GFR 45-59): Every 3-6 months. Stage 3b (GFR 30-44): Every 1-3 months. Stage 4 (GFR 15-29): Every 1-3 months or more frequently. Stage 5 (GFR <15): Monthly or more frequently, often weekly in dialysis preparation. More frequent monitoring (every 1-3 months) is recommended if GFR is declining rapidly (>3-5 mL/year), if there is protein in urine, or if blood pressure or diabetes are poorly controlled.
- GFR-Based Medication Dosing:Many medications require GFR-based dose adjustments to prevent toxicity and kidney injury. Critical examples: Metformin: Avoid if GFR <30, reduce dose if GFR 30-45 (diabetes medication). Gentamicin/Vancomycin: Dose adjustment required starting at GFR 60, more aggressive reduction below GFR 30 (antibiotics). ACE inhibitors/ARBs: Monitor closely if GFR <60, may need dose reduction (blood pressure medications). NSAIDs: Avoid if GFR <60, especially with CKD (pain medications). Contrast agents: Risk of contrast-induced nephropathy increases below GFR 45, consider alternatives or hydration protocols (imaging). Digoxin: Dose reduction required below GFR 50 (heart medication). Always inform healthcare providers of your GFR when starting new medications or undergoing procedures requiring contrast.
- When GFR Can Improve: Reversible Causes & Treatment:GFR can improve if the underlying cause is reversible or if progression is slowed with early intervention: Acute Kidney Injury (AKI): From dehydration, medications (NSAIDs, antibiotics), infections, contrast agents—can recover with treatment (fluids, stopping nephrotoxic drugs). Early-Stage CKD: Progression can be slowed or stopped with: Blood pressure control (<130/80 mmHg, ACE inhibitors/ARBs preferred), Diabetes management (A1C <7%, SGLT2 inhibitors may protect kidneys), Avoiding nephrotoxic drugs (NSAIDs, certain antibiotics, contrast when possible), Dietary modifications (reduce protein if GFR <60, limit phosphorus/potassium if advanced), Smoking cessation, Weight loss if obese. Important: Once significant damage occurs (Stage 4-5, GFR <30), improvement is unlikely without dialysis or transplant. Early-stage intervention (Stage 1-3a) is most effective—this is why regular screening is critical.
GFR monitoring frequency depends on your CKD stage, risk factors, and treatment goals. Understanding appropriate screening and monitoring schedules ensures timely intervention and prevents complications.
FAQ
? What is a normal GFR for my age?
Normal GFR by age: 20-29 years: ~116, 30-39: ~107, 40-49: ~99, 50-59: ~93, 60-69: ~85, 70-79: ~75, 80+: ~65 mL/min/1.73m². GFR naturally declines ~1 mL/min/year after age 40.
? How is GFR measured?
eGFR is calculated from serum creatinine, age, and gender using CKD-EPI or MDRD formulas. True GFR requires clearance studies (rarely done). eGFR is the standard clinical screening method.
? What happens if my GFR is below 15?
GFR <15 is Stage 5 (Kidney Failure), requiring dialysis or transplant. Without treatment, uremia and death occur. Immediate nephrology care is essential.
? What is the difference between MDRD and CKD-EPI?
MDRD (2006) uses race adjustment and is less accurate at higher GFRs. CKD-EPI 2021 is race-neutral and more accurate. CKD-EPI is now preferred.
? Can GFR improve?
Yes, if reversible (acute injury, early CKD). Chronic Stage 4-5 damage rarely improves without dialysis/transplant. Early intervention is critical.
? What causes low GFR?
Diabetes (leading cause), hypertension, glomerulonephritis, polycystic kidney disease, infections, autoimmune diseases, NSAID overuse, or acute causes (dehydration, medications).
? How often should GFR be checked?
Stage 1-2: Every 6-12 months. Stage 3a: Every 3-6 months. Stage 3b-4: Every 1-3 months. Stage 5: Monthly. Frequency increases with declining function.
? What is a good GFR number?
GFR ≥90 is normal. 60-89 is mildly decreased (may be age-appropriate). <60 with kidney damage indicates CKD. Age-adjusted interpretation is essential.