gfr calculator

What is GFR?

GFR stands for glomerular filtration rate, which is a measure of how well your kidneys are filtering blood. In practice, most people see an estimated value called eGFR on their lab report rather than a directly measured GFR. eGFR helps clinicians screen for kidney function changes and monitor chronic kidney disease (CKD) over time.

How this GFR calculator works

This calculator uses the CKD-EPI 2021 creatinine equation, a commonly used formula that estimates kidney filtration rate from serum creatinine, age, and sex. The result is reported as:

mL/min/1.73 m² (indexed to a standard body surface area).

If you also enter height and weight, the calculator will estimate body surface area and provide an absolute GFR (not indexed), which can be useful in specific clinical contexts.

Equation summary (CKD-EPI 2021)

• Uses serum creatinine in mg/dL (µmol/L is converted automatically).
• Adjusts for age-related decline in filtration.
• Uses sex-specific coefficients.
• Does not include race in the 2021 version.

How to interpret your eGFR result

eGFR is usually interpreted alongside urine albumin testing, blood pressure, and clinical history. A quick stage guide:

• G1: eGFR ≥ 90 — Normal or high (if other kidney-damage markers are absent).
• G2: 60–89 — Mildly decreased.
• G3a: 45–59 — Mild to moderate decrease.
• G3b: 30–44 — Moderate to severe decrease.
• G4: 15–29 — Severe decrease.
• G5: < 15 — Kidney failure range.

Important: A single low value does not automatically mean CKD. Kidney disease is typically defined by abnormalities present for 3 months or longer.

What can affect eGFR accuracy?

Creatinine-based estimates are useful but imperfect. Results can be influenced by non-kidney factors:

• Very high or very low muscle mass
• Recent intense exercise
• Hydration status
• Certain medications or supplements
• Acute illness (where kidney function may be rapidly changing)

In some cases, clinicians order cystatin C or directly measured clearance testing when more precision is needed.

When to seek medical advice

Talk with your healthcare professional if:

• Your eGFR is persistently below 60.
• Your result changes significantly between tests.
• You have diabetes, high blood pressure, swelling, or foamy urine.
• You have a family history of kidney disease.

Frequently asked questions

Is eGFR the same as measured GFR?

No. eGFR is an estimate based on blood markers and demographics. Measured GFR uses specialized tests and is less common in routine care.

Can I improve my kidney health?

In many cases, yes. Blood pressure control, diabetes management, avoiding nephrotoxic drugs when possible, reducing excess sodium, regular activity, and smoking cessation can help protect kidney function.

Why is body surface area used?

Lab eGFR values are indexed to 1.73 m² so results can be compared across adults. For medication dosing or special populations, clinicians may look at absolute (non-indexed) GFR as well.

Bottom line

A GFR calculator is a practical screening tool, not a final diagnosis. Use your result as a starting point, then combine it with urine testing, repeat labs, and clinician interpretation for the full picture of kidney health.