GFR Calculator: Estimate Your Kidney Function (CKD-EPI)

How to Use Our GFR Calculator

To calculate your estimated GFR (eGFR), you will need the results from a recent blood test, specifically your serum creatinine level. Enter the following information for the most accurate result.

• Serum Creatinine: Enter the creatinine value from your lab report. Creatinine is a waste product from muscle metabolism, and healthy kidneys filter it out of the blood. Make sure to select the correct unit of measurement: milligrams per deciliter ($mg/dL$) or micromoles per liter ($mumol/L$).

• Age: Enter your age in years. Kidney function naturally declines with age, so this is a critical variable in the GFR equation.

• Gender: Select Male or Female. The formula accounts for differences in average muscle mass between males and females, which directly impacts baseline creatinine levels.

• Race: Select whether you identify as Black/African American. The most current GFR equations (like the 2021 CKD-EPI Refit) no longer use race as a variable, but older equations did. Our calculator uses the latest 2021 CKD-EPI equation which has removed the race coefficient to provide a more equitable and accurate assessment. You can simply proceed without this input.

Understanding Your Results

The main result of this calculator is your estimated Glomerular Filtration Rate (eGFR). This number is expressed in units of mL/min/1.73m2.

In simple terms, your eGFR represents how many milliliters ($mL$) of blood your kidneys filter each minute ($min$), standardized to a typical body surface area of 1.73 square meters (m2). It’s the key indicator of your overall kidney function. A higher number generally means better kidney function, while a lower number indicates that your kidneys are not filtering waste as effectively as they should.

The Stages of Chronic Kidney Disease (CKD)

Your eGFR result is used by medical professionals to determine if Chronic Kidney Disease (CKD) is present and to classify it into one of five stages. It’s important to note that a diagnosis of CKD also requires signs of kidney damage (like protein in the urine) or a low eGFR for more than three months.

Important Disclaimer: This calculator is an educational tool, not a substitute for professional medical advice. Your eGFR result should always be interpreted by a qualified healthcare provider who understands your full medical history.

Frequently Asked Questions

What exactly is Glomerular Filtration Rate (GFR)?

The Glomerular Filtration Rate (GFR) is a measure of how efficiently your kidneys are filtering waste and excess fluid from your blood. Your kidneys contain millions of tiny filtering units called glomeruli. GFR represents the total volume of fluid that these filters process per minute. A direct GFR measurement is complex, so doctors use a blood test to calculate an estimated GFR, or eGFR, which is a reliable indicator of kidney health.

Why is serum creatinine used to calculate eGFR?

Creatinine is a chemical waste product generated from normal muscle wear and tear. Healthy kidneys consistently filter creatinine from the blood and excrete it into the urine. When kidney function declines, they are less able to clear this waste product, causing its level in the blood (serum creatinine) to rise. Because its production and clearance are relatively stable, serum creatinine serves as a reliable proxy marker for kidney function. A higher creatinine level in the blood generally corresponds to a lower GFR.

Why did old GFR formulas include a “race” variable, and why is it gone now?

Previous GFR equations, like the original CKD-EPI and MDRD formulas, included a variable for Black/African American individuals. This was based on studies that observed, on average, higher serum creatinine levels in people who identified as Black compared to those who identified as White, even at the same measured GFR. It was assumed this was due to differences in muscle mass.

However, this practice was problematic because race is a social construct, not a biological one. Using a race-based coefficient could lead to inaccuracies, systematically overestimating GFR in Black patients and potentially delaying their diagnosis and access to specialized care or transplant lists. In 2021, the National Kidney Foundation and the American Society of Nephrology Task Force recommended and released a new CKD-EPI Refit equation that removes the race variable to provide a more equitable and accurate estimation for everyone. Our calculator uses this modern, race-free formula.

What causes a low GFR?

A low GFR is a sign that the kidneys are not functioning properly. The most common causes of damage that lead to low GFR and Chronic Kidney Disease are:

• Diabetes: High blood sugar can damage the glomeruli over time. This is the leading cause of kidney failure.

• High Blood Pressure (Hypertension): Uncontrolled high blood pressure can damage the blood vessels in and leading to the kidneys.

• Glomerulonephritis: Inflammation of the kidney’s filtering units.

• Polycystic Kidney Disease (PKD): A genetic disorder causing cysts to grow in the kidneys.

• Obstructions: Conditions that block the urinary tract, like an enlarged prostate, kidney stones, or tumors.

• Certain Medications: Long-term use of some drugs, like NSAIDs (e.g., ibuprofen, naproxen), can harm the kidneys.

### Can I improve my GFR?

Whether GFR can be improved depends on the cause and extent of the kidney damage. If the low GFR is due to an acute kidney injury (e.g., from dehydration or a specific medication), function may be fully restored once the underlying issue is resolved.

For Chronic Kidney Disease (CKD), the damage is usually irreversible. However, you can take crucial steps to preserve existing kidney function and slow the rate of decline. This often involves:

• Strictly controlling blood pressure and blood sugar.

• Eating a kidney-friendly diet (often lower in sodium, protein, and phosphorus).

• Avoiding medications that can harm the kidneys (NSAIDs).

• Not smoking.

• Maintaining a healthy weight.

What is the difference between eGFR and a 24-hour urine collection?

• eGFR (estimated GFR): This is a calculation based on a simple blood test for creatinine. It’s fast, convenient, and the standard screening method. It provides a very good estimate of kidney function.

• 24-Hour Urine Collection (for Creatinine Clearance): This test involves collecting all of your urine over a 24-hour period. It directly measures how much creatinine your body is clearing. It can sometimes provide a more accurate picture than eGFR, especially in people with unusual body compositions (e.g., amputees, bodybuilders) or on specific diets. However, it’s cumbersome and prone to collection errors.

Your doctor will decide which test is most appropriate for your situation.

At what GFR level should I see a nephrologist?

Generally, a referral to a nephrologist (a kidney specialist) is recommended when a person’s eGFR falls below 30 mL/min/1.73m² (Stage 4 CKD). However, a referral may be made earlier, especially if you have Stage 3 CKD ($eGFR \< 60$) along with complications like significant proteinuria (protein in the urine), rapidly declining GFR, or hard-to-control high blood pressure.

Does a single low eGFR reading mean I have kidney disease?

Not necessarily. Your eGFR can fluctuate. A temporary drop can be caused by dehydration, certain medications, a high-protein meal before the test, or a urinary tract infection. A diagnosis of Chronic Kidney Disease requires a low eGFR (below 60) that persists for three months or longer, or evidence of kidney damage regardless of GFR level. Your doctor will likely order a repeat test to confirm the result.

Concrete Example: Interpreting a GFR Result

Let’s look at a hypothetical case:

• Patient: A 65-year-old male.

• Lab Result: Serum Creatinine is $1.5,mg/dL$.

Plugging these values into the GFR calculator:

1. Inputs: Creatinine = 1.5 mg/dL, Age = 65, Gender = Male.

2. Calculation (using the CKD-EPI 2021 formula): The formula is complex, but the calculator processes it instantly.

3. eGFR Result: Approximately 49,mL/min/1.73m2.

Interpretation:

• An eGFR of 49 places this individual in Stage 3a CKD (Mildly to Moderately Decreased function).

• This is not a reason to panic, but it is a clear signal that action is needed.

• A doctor would likely confirm the result with a repeat test, check for protein in the urine, and then develop a plan to manage blood pressure, review medications, and recommend dietary changes to protect the remaining kidney function.

Can my diet affect my creatinine levels and GFR calculation?

Yes, to a small extent. Since creatinine comes from muscle, eating large amounts of cooked meat (which contains creatine that converts to creatinine) can temporarily raise your blood creatinine levels and slightly lower your eGFR result. For this reason, some doctors advise avoiding large meat meals for 12 hours before your blood test. Similarly, creatine supplements, popular among athletes, can also increase serum creatinine and lead to a misleadingly low eGFR. It’s important to tell your doctor about any supplements you are taking.

Monitoring your kidney function is often part of a broader health assessment. Since kidney function is indexed to body size, understanding your Body Surface Area (BSA) Calculator can provide additional context. Furthermore, as high blood pressure is a primary risk factor for kidney disease, keeping track of your numbers with a Blood Pressure Chart is a critical step in preserving your health.