If a blood test ever came back with a creatinine number and a line saying “eGFR”, you may have wondered what it means. The short version is that it estimates how well your kidneys are cleaning your blood.
This GFR calculator uses the same equation that most labs use, the 2021 CKD-EPI creatinine equation, to turn your creatinine, age and sex into an estimated GFR and a category. It’s for understanding your results, not for diagnosing anything.
How to use the GFR Calculator#
- Enter your age. The equation is for adults, aged 18 and over.
- Choose the sex used by the equation. The formula has separate constants for male and female.
- Enter your serum creatinine. Pick mg/dL or µmol/L to match your lab report.
- Read the result. You get an estimated GFR and its category.
What GFR is#
The glomerular filtration rate is how much blood your kidneys filter each minute, measured in millilitres per minute for a standard body size (1.73 square metres). Measuring it directly is difficult, so doctors estimate it from creatinine, a waste product from muscle that healthy kidneys clear. When your kidneys slow down, creatinine builds up in your blood.
A healthy young adult typically has a GFR somewhere around 90 to 120. Kidney function naturally drifts down with age.
The equation#
The 2021 CKD-EPI creatinine equation, which dropped the old race adjustment, is:
Scr is creatinine in mg/dL. For women, κ is 0.7 and α is −0.241. For men, κ is 0.9 and α is −0.302. The calculator does the sum, including the conversion from µmol/L, where you divide by 88.4 to get mg/dL.
The categories#
| Category | eGFR | Meaning |
|---|---|---|
| G1 | 90 or more | Normal or high |
| G2 | 60 to 89 | Mildly decreased |
| G3a | 45 to 59 | Mild to moderately decreased |
| G3b | 30 to 44 | Moderately to severely decreased |
| G4 | 15 to 29 | Severely decreased |
| G5 | Below 15 | Kidney failure range |
For comparison, a 60-year-old man with 1.6 mg/dL comes out at about 49, in category G3a, and a 70-year-old woman with 2.0 mg/dL gets about 26, in G4.
Why a number isn’t a diagnosis#
- Chronic kidney disease needs time. It’s usually diagnosed when GFR stays below 60 for at least three months, or when there are other signs of kidney damage, such as protein in the urine.
- Creatinine depends on muscle. Very muscular people can read low on kidney function, and people with little muscle can read high. Cooked meat and creatine supplements can raise the number too.
- One bad day. Dehydration, illness and some medicines change the result temporarily.
- Pregnancy and extremes of size. The equation is less reliable in these cases.
Doctors often add a urine test, and sometimes a second blood marker called cystatin C, to get a clearer picture.
What to do with your result#
If your number looks lower than you expected, don’t panic and don’t ignore it. Take the lab report to your doctor, who can say whether it needs repeating or follow-up. This calculator is a general information tool and doesn’t replace medical advice. For more on kidney health, the National Kidney Foundation is a good place to start.
Two worked examples#
A 45-year-old man has a serum creatinine of 1.0 mg/dL. The equation gives an eGFR of about 95 mL/min/1.73 m², which falls in category G1, normal or high, assuming there are no other signs of kidney disease. A 60-year-old woman with a creatinine of 1.1 mg/dL has an eGFR of about 58, in category G3a, mildly to moderately decreased. For both, the result alone does not mean kidney disease, because a mild reduction is common with age.
What can nudge the number#
Creatinine comes from muscle, so a very muscular person can have a higher level without kidney problems, and a frail older person with little muscle can have a low level even when the kidneys are not working well. A big meal of cooked meat or creatine supplements can raise creatinine for a few hours, and dehydration can too. Some medicines affect the reading. Doctors confirm kidney disease with repeat tests over at least three months, together with a urine test for protein, which the eGFR alone does not show. In some cases a second marker, cystatin C, is used. Do not stop medicines or change your diet because of a calculator; speak to your doctor. This page is for information only.
Frequently asked questions#
For a healthy adult, roughly 90 or above is considered normal or high, and it falls gradually with age. A single result needs a doctor to interpret it.
Divide by 88.4. A creatinine of 88.4 µmol/L is 1.0 mg/dL.
The equation uses different constants for male and female kidney function, because average creatinine levels differ. It doesn’t change how you should be treated.
GFR is the true rate, which is hard to measure. eGFR is the estimate worked out from a blood test with an equation like this one.
No. This equation is for adults aged 18 and over. Children use different equations.
It is a formula that estimates kidney function from serum creatinine, age and sex. The 2021 version removed the race coefficient used in earlier versions. Many laboratories report the eGFR from it with your blood test, and the calculator lets you check the same figure.