GFR / eGFR

Calculating Estimated Glomerular Filtration Rate (eGFR)

eGFR (estimated Glomerular Filtration Rate) is a screening measure of how well the kidneys are filtering blood, estimated from a routine blood test result (serum creatinine) plus age and gender. It’s the single most commonly used lab value for tracking kidney function over time. Enter your gender at birth, age, and serum creatinine (from a recent blood test — it will be printed on your lab report), and this calculator estimates your eGFR and which standard Chronic Kidney Disease (CKD) stage it falls into.

This calculator uses the 2021 CKD-EPI Creatinine Equation, the current formula endorsed by the National Kidney Foundation and American Society of Nephrology. It replaced an earlier version of the same equation that included a race coefficient — the current, race-free version is now the standard used across U.S. clinical practice.

The Formula

The equation is centered on a per-gender creatinine “pivot point” (kappa): 0.7 mg/dL for women, 0.9 mg/dL for men. At or below that pivot, a gentler exponent applies; above it, a steeper one always applies:

$$\text{eGFR} = 142 \times \min\left(\frac{\vA{\text{Scr}}}{\vB\kappa}, 1\right)^{\vC\alpha} \times \max\left(\frac{\vA{\text{Scr}}}{\vB\kappa}, 1\right)^{-1.200} \times 0.9938^{\vD{\text{Age}}}\times \text{S}$$

where $\vA{\text{Scr}}$ is serum creatinine in mg/dL, $\vB\kappa$ is 0.7 (women) or 0.9 (men), $\vC\alpha$ is −0.241 (women) or −0.302 (men), $\vD{\text{Age}}$ is in years, and $\text{S}$ is a fixed multiplier of 1.012 for women or 1 for men. In plain terms: the formula compares your creatinine to your gender’s typical range, then adjusts down for age, since kidney function naturally declines slowly over a lifetime.

The result is then compared against the standard CKD staging system:

Worked Example

A female, age 50, with a serum creatinine of 0.8 mg/dL (above her 0.7 mg/dL pivot point, so the steeper −1.200 exponent applies):

1. $\vA{0.8} \div \vB{0.7} \approx 1.1429$, raised to the power of $-1.2 \approx 0.8519$.
2. $0.9938^{\vD{50}} \approx 0.7328$.
3. $142 \times 0.8519 \times 0.7328 \times 1.012 \approx 89.7$.

An eGFR of about 89.7 mL/min/1.73 m² falls right at the boundary between G1 (Normal) and G2 (Mildly Decreased) — landing in G2.

Key Factors to Consider

• A single eGFR result is a screening snapshot, not a diagnosis. Kidney function is normally tracked with repeated testing over time, since a single creatinine measurement can be affected by short-term factors like hydration, recent exercise, or a high-protein meal — a clinician interprets trends and other factors together, not one isolated number.
• Muscle mass affects creatinine levels independent of actual kidney function. Creatinine is a byproduct of muscle metabolism, so a very muscular person can have a higher creatinine level (and therefore a lower calculated eGFR) than their true kidney function would suggest, and vice versa for someone with low muscle mass — this is a known limitation of any creatinine-based eGFR formula.
• Additional kidney function markers exist beyond creatinine-based eGFR. A cystatin-C-based or combined creatinine-cystatin-C equation is sometimes used alongside or instead of the creatinine-only formula, particularly when muscle mass might be skewing the creatinine-based result — your healthcare provider may use one of these alternatives if relevant to your situation.
• CKD staging also considers other markers, like proteinuria, not eGFR alone. A complete CKD assessment typically combines the eGFR-based G-stage with an A-stage based on albumin levels in urine — this calculator estimates the eGFR/G-stage component only, one piece of a fuller clinical picture.

Common Mistakes

• Treating a single eGFR result as a diagnosis rather than a screening snapshot. As covered above, one creatinine measurement can be affected by short-term factors like hydration or recent exercise — a real assessment of kidney function tracks eGFR over repeated tests, not one isolated result.
• Interpreting a low eGFR the same way regardless of muscle mass. As covered above, creatinine is a byproduct of muscle metabolism, not solely of kidney filtration — a very muscular person can show a lower calculated eGFR than their actual kidney function reflects, and a person with low muscle mass can show a falsely reassuring one.
• Comparing an eGFR calculated with the older race-adjusted equation directly against a race-free result from this calculator. The two equations can produce different numbers for the same lab values — a meaningful comparison needs both results from the same version of the formula, not one of each.
• Treating the eGFR-based G-stage alone as a complete CKD diagnosis. As covered above, a full clinical assessment also factors in an A-stage based on urine albumin — a G-stage from this calculator is one input to that picture, not the whole picture.

Useful to Know

• The formula’s separate “pivot point” for each gender (0.7 mg/dL for women, 0.9 mg/dL for men) exists because typical creatinine levels differ by usual body composition — it reflects a statistical reference range, not a judgment about kidney health being inherently better or worse for either gender.
• Because muscle mass is one of this formula’s known limitations, the BMI Calculator and Body Fat Calculator can add useful body-composition context when discussing an eGFR result with a healthcare provider — particularly for someone notably more or less muscular than average.
• eGFR values naturally trend downward with age even in healthy kidneys, which is why the formula includes an age-based adjustment — a single lower-than-90 reading in an older adult isn’t automatically a sign of disease on its own.