This GFR calculator estimates glomerular filtration rate in mL/min/1.73m² using the race-free CKD-EPI 2021 equation, the current standard for adults. Enter age, sex, and serum creatinine to read your estimated GFR and the matching CKD stage.
What Is GFR?
Glomerular filtration rate (GFR) is the volume of blood the kidneys filter each minute, the single most useful number for describing overall kidney function. It is reported in mL/min/1.73m² — millilitres filtered per minute, standardised to an average adult body surface area of 1.73m² so values compare fairly between a small and a large person. A healthy adult filters roughly 90 to 120 mL/min/1.73m², and the rate declines gradually with age. Because GFR cannot be measured directly in routine care, it is estimated from a blood marker, almost always serum creatinine.
How This GFR Calculator Works
This tool applies the race-free CKD-EPI 2021 creatinine equation, the equation the National Kidney Foundation recommends for adults. It takes three inputs: age in years, sex, and serum creatinine. Sex sets two coefficients (κ and α) that adjust for the lower average creatinine production in women, and an age term applies the expected decline in filtration over the lifespan. The equation deliberately omits any race coefficient. For the full formula and a worked example, see the eGFR calculator.
Serum Creatinine Units
Enter serum creatinine in mg/dL. If your report uses µmol/L, divide by 88.4 to convert (for example, 88 µmol/L ÷ 88.4 ≈ 1.0 mg/dL) — the unit converter does this for you. The wrong unit produces a result that is off by nearly a hundred-fold, so always confirm the unit printed on the laboratory report before reading the result.
Estimated GFR vs Measured GFR
Estimated GFR (eGFR) is calculated from serum creatinine; measured GFR (mGFR) uses an external filtration marker, such as iohexol or inulin, and is the gold standard but is slow, costly, and rarely needed in practice. For everyday clinical use, eGFR from the CKD-EPI 2021 equation is sufficient and reproducible. Measured GFR is reserved for situations where an exact value matters — for example, evaluating a potential living kidney donor or dosing a narrow-therapeutic-index drug when the estimate is in doubt.
How to Read Your GFR Result
An estimated GFR maps to the KDIGO categories used to stage chronic kidney disease:
| GFR | Category | Interpretation |
|---|---|---|
| ≥90 | G1 | Normal filtration |
| 60–89 | G2 | Mildly reduced |
| 45–59 | G3a | Mild-to-moderate reduction |
| 30–44 | G3b | Moderate-to-severe reduction |
| 15–29 | G4 | Severely reduced |
| <15 | G5 | Kidney failure |
A value at or above 60 with no other sign of kidney damage is generally normal kidney function. A value under 60 that persists for three months or longer points to chronic kidney disease and is staged from G3a to G5.
GFR vs Creatinine Clearance
GFR estimates filtration and stages kidney disease; creatinine clearance estimates clearance and guides drug dosing. They use different units and reference points and are not interchangeable without converting the indexed eGFR back to absolute mL/min. See CrCl vs eGFR for which to use and when.
Limitations and Edge Cases
Because GFR here is estimated from creatinine, it can mislead whenever creatinine production departs from the average:
- Low muscle mass (frailty, amputation, paralysis): eGFR reads falsely high.
- High muscle mass or high-protein diet: eGFR may read falsely low.
- Acute kidney injury or any non-steady state: the equation assumes a stable creatinine and is not valid while the level is changing rapidly.
- Pregnancy: filtration rises physiologically and the equation is not validated.
- Extremes of body size: the per-1.73m² value must be de-indexed before comparing it with a Cockcroft–Gault CrCl.
What Changes GFR
Glomerular filtration rate falls when fewer or less-efficient glomeruli are filtering blood, which can happen gradually in chronic kidney disease or abruptly in acute kidney injury. The most common drivers of a sustained decline are diabetes and high blood pressure, which together account for most chronic kidney disease. Age also matters: filtration declines slowly across adulthood, so a value that would be low in a young adult can be expected in an older one. Short-term swings in the estimated value, by contrast, often reflect changes in serum creatinine rather than true filtration — for example, a high-protein meal or strenuous exercise can nudge creatinine up and the estimate down without any real change in kidney function.
How to Use Your GFR Result
Treat a single estimate as a snapshot. If the value is below 60 mL/min/1.73m², the usual approach is to confirm it on a repeat test after about three months, check urine albumin to grade any kidney damage, and review medications and contributing conditions with a clinician. A result at or above 60 with no albuminuria is generally reassuring. Tracking the trend over time is more informative than any one reading: a stable GFR is very different from one that is falling steadily, even when both sit in the same band. Use the number to open a conversation with a clinician, not to self-diagnose.
Estimated GFR and Albuminuria Together
Glomerular filtration rate tells only half the story of kidney health. Modern staging combines the GFR category with a measure of kidney damage — urine albumin, reported as the albumin-to-creatinine ratio. Two people can share an identical GFR yet face very different risk if one has heavy albuminuria and the other has none. That is why a GFR result is most useful alongside a urine test rather than on its own, and why a normal GFR does not by itself rule out early kidney disease. When you review a GFR value, ask both how well the kidneys are filtering and whether there is any sign of damage, then bring both to a clinician for interpretation.