Your eGFR (estimated glomerular filtration rate) is calculated using a mathematical equation that combines your blood creatinine level, age, and sex. The current standard formula, called the 2021 CKD-EPI creatinine equation, produces a number representing how many milliliters of blood your kidneys filter per minute. A normal result for a young adult is around 116 mL/min, declining naturally with age.
What the Equation Actually Looks Like
The formula used by most labs today is:
eGFR = 142 × min(SCr/κ, 1)α × max(SCr/κ, 1)-1.200 × 0.9938Age × 1.012 [if female]
That looks intimidating, but what it does is straightforward. It takes your serum creatinine (a waste product your muscles produce), adjusts it based on whether you’re male or female, then factors in your age. The Greek letters are just preset constants: κ is 0.7 for females and 0.9 for males, while α is -0.241 for females and -0.302 for males. The “min” and “max” functions split the calculation depending on whether your creatinine is above or below the expected threshold for your sex.
In practice, the equation works out to four simpler versions. A female with creatinine at or below 0.7 mg/dL uses one version, while a female above 0.7 uses another. Same split for males, with the dividing line at 0.9 mg/dL. Your lab’s computer runs the appropriate version automatically. You never need to calculate it yourself.
The Three Inputs and Why They Matter
Creatinine is the core measurement. It’s a byproduct of normal muscle metabolism that your kidneys filter out of your blood at a steady rate. When your kidneys lose filtering power, creatinine builds up. Higher creatinine generally means lower kidney function, so the equation translates your creatinine level into a filtration estimate.
Age matters because kidney function naturally declines over time. The equation includes 0.9938 raised to the power of your age, which slightly reduces the estimate for each year you’ve lived. A healthy 70-year-old averages an eGFR around 75, compared to about 116 for someone in their twenties. That decline is normal, not a sign of disease.
Sex is included because males and females typically differ in muscle mass, which directly affects how much creatinine the body produces. The equation uses different constants for each sex to account for this. Males generally produce more creatinine at baseline, so the formula adjusts accordingly.
Why Race Was Removed From the Formula
Older versions of the equation included a race coefficient that produced different results for Black and non-Black patients. In 2021, the National Kidney Foundation and American Society of Nephrology jointly recommended removing race from the calculation entirely. Their task force concluded that race is a social construct, not a biological one, and that the race coefficient could delay diagnosis and treatment for Black patients. The updated 2021 CKD-EPI equation, now the recommended standard, uses the same formula for everyone regardless of race.
The Role of Cystatin C
Creatinine isn’t the only blood marker that can estimate kidney function. Cystatin C is a small protein produced by nearly all cells in the body, and it offers a second way to gauge filtration. Unlike creatinine, cystatin C levels aren’t heavily influenced by muscle mass, making it useful for people whose creatinine might be misleading: bodybuilders, people with amputations, those with muscle-wasting conditions, or anyone on a very low-protein diet.
Cystatin C has its own blind spots, though. Chronic inflammation, obesity, smoking, and thyroid problems can falsely elevate it. For this reason, an eGFR calculated from both creatinine and cystatin C together is generally considered more accurate than either marker alone. Guidelines recommend cystatin C as a confirmatory test when a creatinine-based result doesn’t seem to match the clinical picture.
Older Formulas Still in Use
The 2021 CKD-EPI equation is the current gold standard, but you may encounter two older formulas. The MDRD (Modification of Diet in Renal Disease) equation was the previous standard and is still used in some lab systems. It performs similarly to CKD-EPI for people with significantly reduced kidney function but is less accurate at higher filtration rates, which is why many labs used to simply report “greater than 60” rather than giving a precise number.
The Cockcroft-Gault equation is older still and includes body weight as a variable. It was developed in the 1970s using a small study population and is now considered methodologically outdated. Research comparing all three formulas found that Cockcroft-Gault consistently underestimates actual kidney function, while MDRD and CKD-EPI produce more accurate results. Some pharmacists still reference Cockcroft-Gault for drug dosing because certain medications were originally studied using that formula, but for diagnosing and staging kidney disease, CKD-EPI is preferred.
How eGFR Maps to Kidney Disease Stages
Once your eGFR is calculated, it falls into one of six categories used to classify chronic kidney disease:
- G1 (eGFR 90 or above): Normal or high filtration. Kidney disease is only diagnosed at this level if there’s other evidence of damage, such as protein in the urine.
- G2 (eGFR 60 to 89): Mildly decreased. Often normal for older adults and not necessarily a concern on its own.
- G3a (eGFR 45 to 59): Mild to moderate decrease.
- G3b (eGFR 30 to 44): Moderate to severe decrease.
- G4 (eGFR 15 to 29): Severely decreased.
- G5 (eGFR below 15): Kidney failure.
These thresholds are the same regardless of age. That means a healthy 75-year-old with an eGFR of 75 is technically in the G2 category, even though that number is perfectly average for their age group. Context matters: your doctor will interpret the number alongside your age, trends over time, and other markers like urine albumin.
Average eGFR by Age
Because the formula includes an age component, expected values shift over a lifetime. According to Cleveland Clinic, average eGFR by decade looks like this:
- Ages 20 to 29: 116
- Ages 30 to 39: 107
- Ages 40 to 49: 99
- Ages 50 to 59: 93
- Ages 60 to 69: 85
- Age 70 and older: 75
A gradual decline of roughly 8 to 10 points per decade is typical. A sudden drop, or a result significantly below average for your age, is what prompts further evaluation.
What Can Throw Off Your Result
Because the equation relies on creatinine, anything that changes your creatinine level without reflecting actual kidney function will skew the result. Muscle mass is the biggest variable. People with very high muscle mass (bodybuilders, for instance) may show a falsely low eGFR because their muscles produce more creatinine. Conversely, people with low muscle mass from malnutrition, cancer, neuromuscular disorders, or paralysis may show a falsely high eGFR.
Diet plays a role too. A large meat-heavy meal the day before your blood draw can temporarily raise creatinine. Vegetarian and low-meat diets do the opposite, producing lower baseline creatinine and a potentially inflated eGFR. Certain medications can also interfere, either by affecting kidney filtration directly or by altering how creatinine is measured in the lab. High blood glucose and ketone bodies in people with poorly controlled diabetes can falsely elevate creatinine readings when measured with certain lab methods.
The formula also performs poorly when kidney function is changing rapidly. Hospitalized patients, people with acute kidney injury, and pregnant women all have unstable creatinine levels that make a single eGFR snapshot unreliable.
How eGFR Is Calculated in Children
The CKD-EPI equation is designed for adults 18 and older. For children and adolescents, labs use the Bedside Schwartz equation, which relies on the child’s height and serum creatinine. This formula accounts for the fact that children’s bodies produce creatinine at different rates depending on their size and stage of development. It’s currently considered the best method for estimating kidney function in pediatric patients.