What GFR Measures and Why eGFR Exists
Glomerular filtration rate (GFR) is a core measure of kidney function: it reflects how much fluid your kidneys filter through the glomeruli each minute. A true “measured GFR” can be obtained using specialized clearance tests, but these are more involved than routine bloodwork. That’s why most day-to-day kidney monitoring relies on eGFR, an estimated GFR calculated from serum creatinine and basic patient characteristics.
Creatinine is a breakdown product of muscle metabolism that is cleared primarily by the kidneys. When kidney filtration falls, creatinine often rises. However, creatinine is not a perfect marker. It varies with muscle mass, diet, hydration, medications, and measurement conditions. eGFR equations were built to reduce those confounders by using creatinine together with age and sex to estimate the filtration rate you would likely have if creatinine were the only signal available.
Adult eGFR: CKD-EPI 2021 Creatinine Equation
Many laboratories have moved toward the CKD-EPI 2021 creatinine equation for adults. It is a widely used, race-free formula that estimates GFR indexed to a standard body surface area of 1.73 m². “Indexed” means the result is normalized for body size to make comparisons across people more consistent.
This calculator uses your serum creatinine, age, and sex to produce an indexed eGFR value in mL/min/1.73 m². If you choose BSA adjustment, it also estimates an absolute value (mL/min) using your height and weight. Absolute values can be useful in selected clinical situations (for example, certain medication dosing approaches), but your local guidance should decide when to use indexed versus absolute results.
Pediatric eGFR: Why Children Use Different Equations
Children are not simply “small adults.” Creatinine generation changes with growth, and kidney size and physiology evolve throughout childhood. For that reason, pediatric practice commonly uses equations designed and validated in children. A commonly referenced creatinine-based option is the bedside Schwartz equation (2009), which uses height and serum creatinine to estimate eGFR indexed to 1.73 m².
If you select the pediatric method in this calculator, height becomes required. The calculator will then estimate eGFR using bedside Schwartz. For anyone near the adult/pediatric boundary, follow the approach your clinician or lab report specifies.
Creatinine Clearance (CrCl) and the Cockcroft-Gault Formula
Creatinine clearance (CrCl) is another way to estimate kidney function. It is often estimated using the Cockcroft-Gault equation, which uses age, body weight, sex, and serum creatinine to generate a result in mL/min (not indexed to 1.73 m²). Although eGFR is commonly used for CKD staging, Cockcroft-Gault remains relevant because some medication labels and dosing guidelines still reference CrCl rather than eGFR.
Weight choice matters. Some institutions use actual body weight, while others use ideal or adjusted body weight in specific situations. This calculator uses the standard weight input you provide and clearly labels results as an estimate, not a dosing order. If you are calculating kidney function for a high-risk medication, use your local dosing protocol and consider pharmacist or clinician review.
How to Interpret Results with CKD Categories
eGFR becomes more useful when you interpret it in categories. One widely used approach defines GFR categories G1 through G5: G1 is normal or high (≥90), while G5 is kidney failure (<15), with intermediate categories reflecting progressively reduced filtration. Importantly, G1 or G2 does not automatically mean chronic kidney disease unless there is evidence of kidney damage (such as persistent albuminuria, abnormal imaging, or structural abnormalities).
This calculator labels your G category using the common cut points: G1 (≥90), G2 (60–89), G3a (45–59), G3b (30–44), G4 (15–29), and G5 (<15). The number alone should be interpreted in context of your age, symptoms, medical history, and whether findings persist over time.
Albuminuria (ACR) Adds Risk Information Beyond eGFR
Urine albumin-to-creatinine ratio (ACR) helps detect kidney damage by measuring how much albumin is leaking into the urine. Even when eGFR is preserved, albuminuria can signal increased risk and may alter management. CKD frameworks often pair GFR categories (G1–G5) with albuminuria categories (A1–A3).
In many clinical references, ACR categories are: A1 (normal to mildly increased), A2 (moderately increased), and A3 (severely increased). Thresholds are typically expressed as mg/g or mg/mmol depending on region. This calculator accepts either unit and labels your A category accordingly. If your ACR is high, repeating the test and evaluating causes (including diabetes, hypertension, infection, and exercise effects) are common next steps.
Indexed vs Absolute GFR and When BSA Adjustment Might Matter
Most eGFR results are reported per 1.73 m² to standardize kidney function for body size. This is helpful when comparing results across people and for CKD staging. However, some clinical decisions may consider absolute filtration in mL/min, especially when body size is very different from the “standard” body surface area or when guidance explicitly requires an absolute value.
If you enable BSA adjustment in this calculator, it estimates BSA using the Mosteller method and then converts indexed eGFR to an absolute estimate: absolute GFR ≈ eGFR × (BSA / 1.73). This conversion is an estimate; it does not replace measured clearance when precision is required.
Why Two People with the Same Creatinine Can Have Different eGFR
Age and sex influence creatinine generation and the relationship between creatinine and true filtration. As people age, average muscle mass and kidney function tend to change, so the same creatinine may imply a different filtration rate in a younger versus older person. Sex-based coefficients exist because creatinine generation differs on average between males and females.
This is one reason it is usually better to follow your eGFR trend over time rather than anchoring to a single creatinine value. If you are monitoring a chronic condition, consistent timing, consistent hydration status, and standardized lab methods make trends easier to interpret.
When eGFR Can Be Misleading
Creatinine-based estimates assume stable kidney function. In acute kidney injury, creatinine may be rising or falling rapidly, and equations can lag behind the real physiologic change. eGFR can also be biased when creatinine production is unusually low (very low muscle mass, amputations, severe chronic illness) or unusually high (very high muscle mass, heavy supplementation, intense exercise).
Diet and medications can also matter. High meat intake shortly before testing can raise creatinine transiently. Some medications can alter creatinine handling or lab interpretation. If your result is surprising or conflicts with symptoms, clinicians often repeat testing, use urine markers like ACR, consider cystatin C, or pursue measured clearance testing depending on the situation.
Using Trend Calculations Without Over-Interpreting Noise
Kidney labs naturally vary. Hydration status, lab variation, and short-term physiologic changes can move creatinine and eGFR modestly even if underlying kidney function is stable. That’s why clinical guidelines typically rely on persistence over time (for example, abnormal findings present for at least three months) to define chronic kidney disease.
The Trend tab in this calculator lets you paste multiple creatinine results and estimate the corresponding eGFR values to see directionality over time. Use it to spot patterns, not to self-diagnose. If your numbers change quickly, or if you have swelling, reduced urine output, severe fatigue, shortness of breath, or other concerning symptoms, medical review is appropriate.
Practical Tips for Better Home and Clinic Conversations
- Track both eGFR and ACR: the combination is more informative than either alone.
- Repeat abnormal results: one-off values can be misleading.
- Compare like with like: use the same lab method when possible and note creatinine units.
- Bring context: hydration, recent illness, workouts, supplements, and medication changes can shift creatinine.
- Ask about the equation used: labs may report CKD-EPI 2021 or another method; consistency matters for trends.
A calculator is most useful when it improves communication. Use the outputs here to ask targeted questions: Is this persistent? Does my ACR change the risk interpretation? Should dosing guidance use eGFR or Cockcroft-Gault CrCl? Should we confirm with cystatin C or repeat labs?
FAQ
GFR Calculator – Frequently Asked Questions
Quick answers about eGFR equations, CKD categories, albuminuria, BSA adjustment, and limitations of creatinine-based estimates.
GFR (glomerular filtration rate) describes how much blood the kidneys filter each minute. Because measuring true GFR is complex, most routine results report eGFR, an estimate based on lab values and basic patient details.
eGFR is an estimated GFR commonly calculated from serum creatinine, age, and sex using validated equations. This calculator includes the CKD-EPI 2021 creatinine equation for adults and the bedside Schwartz equation for children.
CKD-EPI is intended for adults. For patients under 18, a pediatric equation such as bedside Schwartz is commonly used. If you are unsure which applies, confirm with your lab report or clinician.
eGFR is typically indexed to a standard body surface area (1.73 m²) and is used for CKD staging. Creatinine clearance (CrCl), often estimated with Cockcroft-Gault, produces an unindexed result in mL/min and is still used for dosing some medications.
Normal depends on age and context. Many references classify G1 as ≥90 mL/min/1.73 m², but G1 or G2 alone does not confirm CKD unless other markers of kidney damage are present.
CKD staging commonly uses GFR categories: G1 (≥90), G2 (60–89), G3a (45–59), G3b (30–44), G4 (15–29), and G5 (<15 mL/min/1.73 m²). This tool labels your category and explains what it usually implies.
ACR (albumin-to-creatinine ratio) is a urine test that reflects albumin leakage. Albuminuria categories (A1–A3) are used with eGFR categories to describe CKD risk more accurately than eGFR alone.
Many eGFR equations index results to a standard body size (1.73 m²) to make comparisons easier. In some situations, clinicians may adjust indexed eGFR to an “absolute” value using your body surface area (BSA).
Creatinine-based estimates can be less reliable during acute kidney injury, pregnancy, very unusual muscle mass (very low or very high), rapidly changing creatinine, and some special diets or medications. If accuracy matters, clinicians may use additional tests or measured GFR methods.