GFR Calculator
Enter your serum creatinine, age, sex, and weight to estimate your GFR using the CKD-EPI 2021 equation. See your CKD stage, understand what the number means clinically, and learn what factors affect accuracy.
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Enter your values above to see the results.
Tips & Notes
- ✓Always look at your eGFR trend over time rather than a single value — a stable eGFR of 55 over 2 years has very different implications than an eGFR that has dropped from 85 to 55 over the same period.
- ✓Serum creatinine is affected by protein intake in the 24 hours before testing. For the most consistent results, avoid high-meat meals (especially cooked red meat) the day before a creatinine blood draw.
- ✓eGFR naturally declines with age — approximately 0.75–1.0 mL/min/1.73m² per year after age 40. An eGFR of 65 in a healthy 75-year-old may reflect normal aging rather than kidney disease.
- ✓The Cockcroft-Gault equation is used specifically for drug dosing decisions (kidney-cleared medications), while CKD-EPI is the preferred equation for CKD diagnosis and staging.
- ✓A low eGFR requires two readings at least 3 months apart to diagnose CKD — acute illness, dehydration, or NSAID use can temporarily lower eGFR without indicating true chronic kidney disease.
Common Mistakes
- ✗Diagnosing CKD from a single eGFR reading — one low result may reflect dehydration, recent heavy exercise, high protein intake, or acute illness rather than true chronic kidney damage.
- ✗Comparing creatinine values across different laboratories without accounting for assay differences — creatinine measurement methods can differ between labs, producing slightly different absolute values.
- ✗Ignoring eGFR in the context of age — an eGFR of 60 is within the stage G2 CKD range but may be entirely appropriate for a 75-year-old without other kidney damage markers.
- ✗Using creatinine-based eGFR alone in very muscular or very frail individuals where creatinine does not accurately reflect kidney function — cystatin C provides a more accurate estimate in these populations.
- ✗Not recognizing that proteinuria (protein in urine) can indicate kidney disease even when eGFR is above 60 — CKD staging requires both eGFR and albuminuria/proteinuria assessment together.
GFR Calculator Overview
eGFR from serum creatinine is the most widely used clinical tool for assessing kidney function — it is calculated automatically by most laboratories whenever creatinine is tested, and it is the primary metric for diagnosing and staging Chronic Kidney Disease.
CKD-EPI 2021 equation — current standard:
CKD-EPI 2021 Equation (current standard — race-neutral): eGFR = 142 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^(−1.200) × 0.9938^Age × (1.012 if female) Where: κ = 0.7 (female) or 0.9 (male) α = −0.241 (female) or −0.302 (male) Scr = serum creatinine in mg/dL Cockcroft-Gault (estimates creatinine clearance, commonly used for drug dosing): CrCl = [(140 − age) × weight kg × (0.85 if female)] ÷ (72 × serum creatinine mg/dL)
EX: Male, age 55, serum creatinine 1.4 mg/dL, weight 80 kg CKD-EPI: κ = 0.9, α = −0.302 eGFR = 142 × min(1.4/0.9, 1)^(−0.302) × max(1.4/0.9, 1)^(−1.200) × 0.9938^55 Note: 1.4/0.9 = 1.556 > 1, so min term = 1^(−0.302) = 1.0, max term = 1.556^(−1.200) 0.9938^55 = 0.712 (age factor) eGFR ≈ 142 × 1.0 × 0.601 × 0.712 ≈ 60.7 mL/min/1.73m² CKD Stage G2 (60–89) — borderline; repeat in 3 months before confirming diagnosis
When eGFR results may be inaccurate:
CKD requires BOTH: eGFR < 60 for ≥ 3 months OR markers of kidney damage (proteinuria, hematuria, imaging abnormalities) for ≥ 3 months A single low eGFR is NOT sufficient to diagnose CKD — it requires confirmation over time. Natural age-related decline: eGFR decreases approximately 0.75–1.0 mL/min/1.73m² per year after age 40 A 75-year-old with eGFR of 65 may have no kidney disease — just age-related decline.
EX: Two people, both eGFR = 55 mL/min/1.73m² Person A: Age 78, no proteinuria, no other kidney markers → likely age-related decline, possibly G2 CKD Person B: Age 42, protein in urine (proteinuria), diabetes → G3a CKD, warrants nephrology referral Same eGFR number, very different clinical significance. This is why interpretation always requires clinical context beyond the number itself.
CKD staging by eGFR — complete classification:
| CKD Stage | eGFR (mL/min/1.73m²) | Description | Recommended action |
|---|---|---|---|
| G1 | 90 or above | Normal or high — kidney damage markers present if CKD | Treat underlying cause; monitor annually |
| G2 | 60–89 | Mildly decreased | Identify risk factors; monitor every 6–12 months |
| G3a | 45–59 | Mildly to moderately decreased | Nephrology referral for most; monitor every 6 months |
| G3b | 30–44 | Moderately to severely decreased | Nephrology care; prepare for kidney replacement planning |
| G4 | 15–29 | Severely decreased | Active kidney replacement therapy planning (dialysis or transplant) |
| G5 | Below 15 | Kidney failure | Kidney replacement therapy (dialysis or transplant) or conservative care |
Factors that affect creatinine-based eGFR accuracy:
| Factor | Effect on creatinine-based eGFR | More accurate alternative |
|---|---|---|
| High muscle mass (athletes, bodybuilders) | Overestimates creatinine → underestimates eGFR | Cystatin C-based eGFR |
| Very low muscle mass (elderly, malnutrition) | Underestimates creatinine → overestimates eGFR | Cystatin C-based eGFR |
| High meat diet | Temporarily elevates creatinine → appears lower GFR | Test fasting or plant-based meal day |
| Pregnancy | eGFR increases due to higher GFR — standard values do not apply | Obstetric nephrologist guidance |
| Acute kidney injury | Creatinine lags behind actual GFR changes by 24–48 hours | Serial creatinine + clinical assessment |
The most important limitation of creatinine-based GFR estimation is that it reflects muscle mass as much as kidney function. Creatinine is a breakdown product of muscle creatine — someone with very high muscle mass will have higher baseline creatinine and appear to have lower eGFR than their true kidney function warrants. Conversely, a frail elderly person with minimal muscle mass may have a creatinine level that looks normal even with significantly impaired kidney function. Cystatin C, a protein filtered by the kidneys that is independent of muscle mass, provides a more accurate eGFR in these populations and is increasingly used alongside creatinine for a combined CKD-EPI estimate.
Frequently Asked Questions
A normal GFR for a young adult is approximately 90–120 mL/min/1.73m², meaning the kidneys are filtering 90–120 mL of blood per minute per standardized body surface area. This represents the combined filtration capacity of approximately 2 million nephrons across both kidneys. GFR naturally declines with age — the average 70-year-old has an eGFR of approximately 70–80 mL/min/1.73m², which may be entirely healthy for their age without kidney disease. The clinical significance of any given eGFR depends on age, the presence of proteinuria, the trend over time, and other kidney damage markers.
Low eGFR can result from acute or chronic causes. Acute causes include dehydration (reduced blood flow to kidneys), acute kidney injury from infections, medications, or contrast agents, and conditions that reduce cardiac output like heart failure. These acute causes are potentially reversible with treatment. Chronic causes that progressively damage nephrons include diabetes mellitus (the leading cause of CKD worldwide), hypertension (second most common), glomerulonephritis, polycystic kidney disease, and chronic use of nephrotoxic medications like NSAIDs. The key distinction between acute and chronic kidney disease is whether the eGFR normalizes with treatment and time, or remains persistently below 60 for more than 3 months.
CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) and Cockcroft-Gault estimate slightly different things using different mathematical models. CKD-EPI, developed in 2009 and updated in 2021 (removing race as a variable), estimates GFR standardized to body surface area (mL/min/1.73m²) and is the recommended equation for CKD diagnosis and staging. It performs well across a wide range of GFR values. Cockcroft-Gault estimates creatinine clearance (mL/min), which approximates but is not identical to GFR, and uses body weight in the formula. It remains widely used for drug dosing calculations because most pharmacokinetic studies used Cockcroft-Gault for their recommendations. For most patients, the two equations give similar results, but they diverge in patients with obesity or unusual body composition.
Diabetes is the leading cause of chronic kidney disease worldwide, accounting for approximately 35–40% of CKD cases. Chronically elevated blood glucose damages the small blood vessels (microvasculature) of the glomeruli — the filtration units within each nephron. Early diabetic kidney disease actually causes a temporarily elevated GFR (hyperfiltration) as the kidneys compensate, which can mask emerging damage. As the disease progresses, the GFR declines and protein begins appearing in urine (albuminuria) — the earliest detectable marker of diabetic nephropathy, often appearing years before GFR declines. This is why annual urine albumin testing alongside eGFR is recommended for all people with diabetes, regardless of whether their GFR appears normal.
Yes — several lifestyle and dietary interventions have evidence for slowing the rate of eGFR decline. Blood pressure control is the most important: maintaining blood pressure below 130/80 mmHg significantly reduces the pressure-related damage to glomeruli. For people with diabetes, maintaining near-normal blood glucose reduces the risk of diabetic nephropathy progression. Dietary protein restriction (0.6–0.8 g/kg/day) in advanced CKD reduces the metabolic burden on remaining nephrons and slows decline, though the evidence for protein restriction in early CKD is less clear. Reducing sodium intake also helps by controlling blood pressure. SGLT2 inhibitor medications (originally developed for diabetes) have demonstrated strong kidney-protective effects in both diabetic and non-diabetic CKD and are now recommended across a wide range of CKD patients.
Nephrology referral is generally recommended when eGFR falls below 45 mL/min/1.73m² (Stage G3b), for any patient with rapidly declining eGFR (loss of more than 5 mL/min/1.73m² per year), when significant proteinuria is present (urine albumin-to-creatinine ratio above 300 mg/g), when CKD etiology is unclear or complex, when difficult-to-control hypertension accompanies CKD, or when complications of CKD are present (anemia, electrolyte disorders, secondary hyperparathyroidism). Earlier referral — at eGFR below 60 — is appropriate for patients whose CKD is progressing, has unusual features, or is associated with conditions like glomerulonephritis that require specialist management. Your primary care provider should guide the decision based on your specific clinical situation.