Estimated glomerular filtration rate (eGFR) is calculated using a blood test for creatinine combined with your age and sex. The standard formula used in the United States since 2021 is the CKD-EPI creatinine equation, which produces a number measured in mL/min/1.73 m². A healthy young adult typically has an eGFR between 100 and 110. You don’t need to calculate it yourself: labs report it automatically whenever they measure creatinine. But understanding how the number is generated helps you interpret what it means.
The CKD-EPI 2021 Formula
The equation currently recommended by the National Kidney Foundation looks like this:
eGFR = 142 × min(Scr/K, 1)α × max(Scr/K, 1)-1.200 × 0.9938Age × 1.012 [if female]
Here’s what each piece means:
- Scr is your serum creatinine level in mg/dL, pulled from a standard blood draw.
- K is 0.7 for females and 0.9 for males. This adjusts for average differences in muscle mass between sexes, since creatinine is a byproduct of muscle metabolism.
- α is -0.241 for females and -0.302 for males.
- min(Scr/K, 1) means you divide your creatinine by K, and if the result is less than 1, you use it. If it’s 1 or higher, you plug in 1 instead.
- max(Scr/K, 1) works the opposite way: you use the result only if it’s greater than 1, otherwise you use 1.
- 0.9938Age accounts for the natural decline in kidney filtration as you get older.
The formula no longer includes a race variable. In 2021, a joint task force from the National Kidney Foundation and the American Society of Nephrology recommended removing race from the calculation, citing that the updated equation was developed with a diverse population and performs without disproportionately affecting any group. Labs across the U.S. were directed to adopt this version immediately.
A Quick Example
Say you’re a 55-year-old male with a serum creatinine of 1.1 mg/dL. You’d divide 1.1 by 0.9 (the K value for males), getting about 1.22. Since that’s greater than 1, the “min” portion becomes 1 raised to the -0.302 power (which equals 1), and the “max” portion becomes 1.22 raised to the -1.200 power (about 0.79). Multiply 142 × 1 × 0.79 × 0.993855, and you get roughly 79 mL/min/1.73 m². That falls in the mildly decreased range.
In practice, you’ll never need to do this by hand. The National Kidney Foundation and other organizations offer free online calculators where you enter your creatinine, age, and sex. Your lab report will almost always include the eGFR alongside your creatinine result.
What the Units Mean
The result is expressed in mL/min/1.73 m², which tells you how many milliliters of blood your kidneys filter per minute, normalized to an average adult body surface area of 1.73 square meters. That normalization matters because a larger person naturally has bigger kidneys that filter more blood. Adjusting to a standard body size lets doctors compare your result against population reference ranges regardless of whether you’re 5’2″ or 6’4″.
For people at the extremes of body size, such as those with obesity, amputations, or very high muscle mass, failing to account for actual body surface area can reduce the accuracy of the estimate.
What Your eGFR Number Means
Chronic kidney disease is classified into stages based on your eGFR:
- G1 (90 or above): Normal or high kidney function.
- G2 (60 to 89): Mildly decreased. Common with aging and not necessarily a sign of disease on its own.
- G3a (45 to 59): Mild to moderate decrease.
- G3b (30 to 44): Moderate to severe decrease.
- G4 (15 to 29): Severely decreased kidney function.
- G5 (below 15): Kidney failure.
A single eGFR below 60 doesn’t automatically mean you have chronic kidney disease. The diagnosis requires the low result to persist for at least three months, often alongside other markers like protein in the urine. Context matters: an otherwise healthy 72-year-old with an eGFR of 55 is in a different situation than a 40-year-old with the same number.
How eGFR Changes With Age
Kidney filtration naturally declines as you age, even in completely healthy people. In a large Dutch study of healthy adults, the 5th percentile (the low end of normal) was around 94 mL/min/1.73 m² for men in their late 20s, dropped to about 67 by their early 50s, and reached roughly 44 by their early 70s. Women showed a similar trajectory, with values of 85, 70, and 51 at those same ages. This is one reason an eGFR of 55 in a 75-year-old may not carry the same clinical weight as it would in a younger person.
When Creatinine Alone Isn’t Enough
Creatinine comes from muscle breakdown, so anything that changes your muscle mass can skew the result. Bodybuilders, people with amputations, those with spinal cord injuries, very frail older adults, people with muscle-wasting conditions, and those who eat a vegetarian or very low-meat diet can all get misleading creatinine-based eGFR values. Pregnancy and acute illness also make creatinine unreliable because kidney function is changing rapidly rather than sitting at a stable baseline.
In these situations, a second blood marker called cystatin C can be measured. Cystatin C is produced at a steady rate by nearly all cells in the body, so it’s not influenced by muscle mass or diet the way creatinine is. The 2024 KDIGO guidelines recommend using a combined creatinine-plus-cystatin-C equation for people with class III obesity, elderly patients, cancer patients, and anyone whose creatinine is likely to be misleading due to altered muscle mass, prolonged hospitalization, or creatine supplement use. The combined equation provides greater accuracy and less bias than either marker alone.
Cystatin C has its own blind spots, though. Steroid use, thyroid problems, significant inflammation, and high body fat can all push cystatin C levels up independently of kidney function.
Preparing for an Accurate Test
Because eGFR depends on a clean creatinine measurement, your provider may ask you to fast before the blood draw, skip certain medications, or avoid eating meat for a day or two beforehand. Eating a large steak the night before your test, for example, can temporarily raise creatinine and make your eGFR look worse than it actually is. Following prep instructions closely gives you the most reliable result.
The Formula for Children
The CKD-EPI equation is designed for adults. For children, the standard is the bedside Schwartz formula, which is simpler:
eGFR = 0.413 × (height in cm ÷ serum creatinine in mg/dL)
This works because in growing children, height correlates strongly with kidney size and muscle mass. It’s a quick bedside calculation that gives a reliable approximation of kidney function in pediatric patients.
eGFR and Medication Dosing
Many medications are cleared by the kidneys, so your eGFR directly affects what dose is safe for you. An older formula called Cockcroft-Gault was once the standard for adjusting drug doses, but the National Kidney Foundation no longer recommends it because it was never updated to use modern standardized creatinine measurements. It tends to overestimate kidney function, which creates a real risk of overdosing drugs that have a narrow margin of safety, particularly chemotherapy agents. Current guidance favors using the CKD-EPI equation for dosing decisions.