GFR Calculator (eGFR)
Estimate your glomerular filtration rate (GFR) from a blood creatinine result, your age and your sex. The calculator uses the CKD-EPI 2021 equation and shows the matching kidney function stage.
GFR Calculator
What is GFR and why does it matter?
The glomerular filtration rate shows how much blood your kidneys filter each minute. It is the main number doctors use to judge kidney function. Because you cannot measure it directly in routine care, it is estimated (eGFR) from creatinine, a waste product made by your muscles. A falling eGFR can be the first sign of kidney disease, often long before you feel any symptoms.
How is eGFR calculated? CKD-EPI 2021
The CKD-EPI 2021 equation no longer uses race. It uses serum creatinine (Scr, in mg/dL), age and sex:
eGFR = 142 x min(Scr/k, 1)^a x max(Scr/k, 1)^-1.200 x 0.9938^age x 1.012 (if female)
- k = 0.7 for women and 0.9 for men
- a = -0.241 for women and -0.302 for men
- min and max take the smaller or larger of Scr/k and 1
- The result is given in mL/min/1.73 m²
If your lab reports creatinine in µmol/L, divide by 88.4 to get mg/dL.
Worked example
A 50-year-old man with a creatinine of 1.0 mg/dL:
- Scr/k = 1.0 / 0.9 = 1.11, so min(…) = 1 and max(…) = 1.11
- 1.11^-1.200 = 0.881
- 0.9938^50 = 0.733
- eGFR = 142 x 0.881 x 0.733 = about 92 mL/min/1.73 m², which falls in stage G1 if there are other signs of kidney damage, or normal if there are none.
GFR stages and what they mean
| Stage | eGFR (mL/min/1.73 m²) | Description |
|---|---|---|
| G1 | 90 or higher | Normal or high |
| G2 | 60 to 89 | Mildly decreased |
| G3a | 45 to 59 | Mildly to moderately decreased |
| G3b | 30 to 44 | Moderately to severely decreased |
| G4 | 15 to 29 | Severely decreased |
| G5 | below 15 | Kidney failure |
Chronic kidney disease is diagnosed when eGFR stays below 60 for more than three months, or when there are other markers of kidney damage such as albumin in the urine. A single result is never enough for a diagnosis. Eating a lot of cooked meat just before the test, dehydration or a hard workout can change a single value.
Common mistakes when calculating GFR
- Mixing up units. Creatinine in µmol/L entered as mg/dL gives a wildly wrong result.
- Ignoring muscle mass. Very muscular people can have higher creatinine with healthy kidneys, and people with low muscle mass can look better than they are.
- Creatine supplements and heavy meat meals can raise creatinine temporarily.
- Using it during acute illness. The equation assumes a stable situation, so it is unreliable during dehydration, acute kidney injury or hospital stays.
- Using it for children or pregnancy. Other equations are used in those cases.
When to see a doctor
Talk to your doctor if your eGFR is below 60, if it drops quickly between tests, or if you have diabetes, high blood pressure or a family history of kidney disease. Foamy urine, swelling in the legs or face, or a big change in how much you urinate also need medical attention. This calculator is informational and does not replace a medical diagnosis.
Frequently Asked Questions
What is a normal GFR?
A normal eGFR is 90 mL/min/1.73 m² or higher. It naturally declines slowly with age, and values between 60 and 89 can be normal for older adults if there are no other signs of kidney damage.
Which formula does this GFR calculator use?
It uses the CKD-EPI 2021 creatinine equation, which does not include race. It is the formula recommended by major kidney societies for adults.
Can I have a low GFR and healthy kidneys?
Sometimes. Low muscle mass, dehydration, a creatine supplement or a recent meat-heavy meal can shift creatinine, so doctors repeat the test and look at other markers before diagnosing kidney disease.
How can I improve my GFR?
Control blood pressure and blood sugar, avoid regular use of anti-inflammatory painkillers unless prescribed, stay hydrated, do not smoke and follow your doctor’s advice. Ask your doctor before changing any treatment.