glomerular filtration calculator

What is glomerular filtration?

Glomerular filtration is the process your kidneys use to clean blood. Tiny structures called glomeruli act like selective filters, removing wastes and extra water while keeping important proteins and cells in circulation. The speed of this process is called the glomerular filtration rate (GFR).

Because direct GFR measurement is complex, clinicians typically use an estimated GFR (eGFR) based on blood tests, especially serum creatinine. eGFR is one of the most common kidney function markers in routine lab reports.

How this calculator works

This page uses the 2021 CKD-EPI creatinine equation. It estimates eGFR using:

• Age
• Sex assigned at birth
• Serum creatinine concentration

The result is reported in mL/min/1.73 m², which is standardized to a body surface area of 1.73 square meters. If you provide height and weight, the tool also estimates body surface area and calculates an absolute (non-indexed) GFR.

Equation summary

The CKD-EPI 2021 creatinine equation uses sex-specific constants and age scaling. In simplified form:

• eGFR = 142 × min(Scr/K,1)^a × max(Scr/K,1)^-1.200 × 0.9938^Age × (1.012 if female)
• Scr = serum creatinine in mg/dL
• K and a differ by sex

This equation removed race from the older model and is now widely used in many health systems.

How to interpret your eGFR result

eGFR should be interpreted with context, trends, and other tests (especially urine albumin). A single result is useful, but repeated values over time are often more meaningful.

Common GFR categories

• G1: ≥ 90 (normal or high, if other kidney markers are normal)
• 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)

Chronic kidney disease (CKD) is generally defined by abnormalities lasting at least 3 months, so persistence matters.

Why eGFR can change

Kidney estimates can move up or down for many reasons that are not always permanent kidney damage. Examples include hydration status, recent illness, medication effects, dietary protein intake, muscle mass, and laboratory variation.

That is why clinicians look at trends, urine testing, blood pressure, diabetes control, and the overall clinical picture before making conclusions.

When to talk to a healthcare professional

• You have repeated low eGFR results.
• You see a rapid decline compared with prior labs.
• You have swelling, fatigue, changes in urination, or uncontrolled blood pressure.
• You have diabetes, heart disease, or a family history of kidney disease.

A clinician may add urine albumin-to-creatinine ratio (uACR), cystatin C testing, imaging, or medication review to better evaluate risk.

Important limitations

Creatinine-based eGFR is an estimate and may be less accurate in some settings, such as:

• Very high or very low muscle mass
• Pregnancy
• Acute kidney injury
• Extremes of diet or body size

For medication dosing or critical decisions, clinicians may use additional methods and clinical judgment.

Quick FAQ

Is a single low eGFR always chronic kidney disease?

No. CKD typically requires abnormalities present for at least 3 months. One isolated value should be rechecked and interpreted in context.

Why include age in the formula?

Average filtration tends to decrease with age, so equations include age to better estimate expected renal clearance.

What if my creatinine is reported in µmol/L?

Select µmol/L in the calculator. The tool converts to mg/dL automatically before applying the equation.

Should I make treatment changes based on this tool?

No. Use this as an educational reference and discuss all treatment decisions with your healthcare professional.