GFR, or glomerular filtration rate, is calculated using a formula that combines your blood creatinine level with your age and sex. The standard formula used today is the 2021 CKD-EPI creatinine equation, which estimates how many milliliters of blood your kidneys filter per minute. Most labs run this calculation automatically when you get a basic metabolic panel, but understanding the formula helps you make sense of your results.
The 2021 CKD-EPI Equation
The current standard for estimating GFR in adults is the 2021 CKD-EPI creatinine equation. It requires three inputs: your serum creatinine level (in mg/dL), your age, and your sex. The formula looks different depending on whether you’re male or female and whether your creatinine falls above or below a specific threshold.
For females with creatinine at or below 0.7 mg/dL:
GFR = 142 × (SCr / 0.7)^(-0.241) × (0.9938)^Age × 1.012
For females with creatinine above 0.7 mg/dL:
GFR = 142 × (SCr / 0.7)^(-1.200) × (0.9938)^Age × 1.012
For males with creatinine at or below 0.9 mg/dL:
GFR = 142 × (SCr / 0.9)^(-0.302) × (0.9938)^Age
For males with creatinine above 0.9 mg/dL:
GFR = 142 × (SCr / 0.9)^(-1.200) × (0.9938)^Age
The result is expressed in mL/min/1.73 m², which represents how much blood your kidneys clean per minute, adjusted for a standard body surface area. A healthy result is 90 or above. The 0.9938 raised to your age means the estimate naturally decreases as you get older, reflecting the normal, gradual decline in kidney function over time.
Why Race Was Removed From the Formula
If you’ve seen older GFR calculations, you may have noticed a race-based adjustment factor. The previous formulas multiplied the result by a coefficient for Black patients, producing a higher estimated GFR. In 2021, a joint task force from the National Kidney Foundation and the American Society of Nephrology recommended immediately dropping race from the equation. Their rationale: race is a social construct, not a biological one, and baking it into clinical formulas risked misclassifying kidney disease and deepening health disparities. The 2021 CKD-EPI equation was specifically developed with a diverse study population so it would perform well across groups without needing a race variable.
A Worked Example
Say you’re a 55-year-old male with a serum creatinine of 1.1 mg/dL. Since 1.1 is above the male threshold of 0.9, you use the fourth equation:
GFR = 142 × (1.1 / 0.9)^(-1.200) × (0.9938)^55
First, divide 1.1 by 0.9 to get 1.222. Raise that to the power of -1.200, which gives approximately 0.786. Then raise 0.9938 to the 55th power, which gives roughly 0.711. Multiply: 142 × 0.786 × 0.711 = about 79.3 mL/min/1.73 m². That places this person in stage G2, a mildly decreased range. In practice, the National Kidney Foundation offers a free online calculator that does this math instantly once you enter creatinine, age, and sex.
What Your GFR Number Means
Your estimated GFR maps to a chronic kidney disease (CKD) stage:
- G1 (90 or above): Normal or high kidney function
- G2 (60 to 89): Mildly decreased
- G3a (45 to 59): Mild to moderate decrease
- G3b (30 to 44): Moderate to severe decrease
- G4 (15 to 29): Severely decreased
- G5 (below 15): Kidney failure
A single GFR reading below 60 doesn’t automatically mean CKD. The diagnosis typically requires that the reduced level persists for three months or more, or that there’s other evidence of kidney damage like protein in the urine. A GFR above 60 paired with abnormal urine findings can also indicate early kidney disease, so the number doesn’t tell the whole story on its own.
Older Formulas You May Encounter
The MDRD Equation
Before CKD-EPI became the standard, many labs used the MDRD (Modification of Diet in Renal Disease) equation:
eGFR = 175 × (SCr)^(-1.154) × (Age)^(-0.203) × 0.742 [if female] × 1.212 [if African American]
This formula uses the same basic inputs but is less accurate when GFR is above 60 mL/min/1.73 m². It tends to underestimate kidney function in relatively healthy people, which can lead to false alarms about mild kidney disease. It also includes the race coefficient that has since been dropped. You may still see MDRD results on older lab reports, but most U.S. labs have switched to the 2021 CKD-EPI equation.
The Cockcroft-Gault Equation
This older formula is still used for one specific purpose: adjusting medication doses based on kidney function. Unlike the other equations, it estimates creatinine clearance (not GFR directly) and factors in body weight:
CrCl = ((140 – age) × weight in kg) / (72 × SCr) × 0.85 [if female]
Many drug prescribing guidelines were originally validated using Cockcroft-Gault, which is why pharmacists and prescribers sometimes still rely on it. The result is not adjusted for body surface area the way CKD-EPI is, and it loses precision in people who are obese or at extreme body weights.
When the Standard Formula Is Less Reliable
All creatinine-based GFR equations share a fundamental limitation: they assume your creatinine production is typical for your age and sex. Creatinine is a waste product from muscle metabolism, so anything that significantly changes your muscle mass throws off the estimate. Bodybuilders and highly muscular athletes will appear to have worse kidney function than they actually do, because their higher muscle mass generates more creatinine. On the other end, people who have lost significant muscle from illness, aging, or amputation may get falsely reassuring GFR numbers.
Diet also matters. A very high-protein diet can temporarily raise creatinine, while a strict vegetarian diet may lower it. Rapidly changing kidney function, like during an acute kidney injury, makes any creatinine-based estimate unreliable because the formulas assume creatinine levels have stabilized.
For situations where creatinine is a poor marker, there’s an alternative blood test called cystatin C. This protein is produced at a steady rate by nearly all cells in the body, so it’s not affected by muscle mass or diet the way creatinine is. The 2021 CKD-EPI system includes both a cystatin C-only equation and a combined creatinine-cystatin C equation. The NKF-ASN task force recommended confirming borderline GFR results with cystatin C testing, especially when creatinine-based estimates might be skewed.
What You Need for an Accurate Calculation
If you want to calculate your own eGFR using an online tool, you need your serum creatinine value from a recent blood test (reported in mg/dL in the U.S. or micromol/L internationally), your age, and your sex. The creatinine assay should be standardized, which virtually all major labs now use. Your lab report will typically list the creatinine value and may already include the calculated eGFR alongside it.
If you’re calculating for medication dosing purposes using Cockcroft-Gault, you’ll also need your body weight in kilograms. Some calculators offer a body surface area adjustment option, which requires both height and weight. For routine monitoring of kidney health, though, the standard CKD-EPI calculation with just creatinine, age, and sex is all that’s needed.