GFR Calculator
Estimate kidney function with the race-free CKD-EPI 2021 equation and stage it against KDIGO.
Your lab result
A single creatinine result is a snapshot. eGFR shifts with hydration, muscle mass, recent meat intake and some medications, and chronic kidney disease is only diagnosed when it stays low for more than three months, usually alongside a urine albumin test. This tool reproduces the equation. It does not interpret your result. That is your doctor's job.
This estimates glomerular filtration rate from serum creatinine, age and sex, using the CKD-EPI 2021 equation.
It shows the eGFR and the chronic kidney disease stage it corresponds to. It is not a diagnosis.
What eGFR measures
Glomerular filtration rate is how much blood your kidneys filter per minute. It is the standard measure of kidney function, and it cannot be measured directly in routine practice, so it is estimated from a blood test.
The estimate uses serum creatinine, a waste product from muscle that healthy kidneys clear. When filtration falls, creatinine rises. Age and sex are included because they affect how much creatinine a body produces in the first place.
One thing worth knowing: the current CKD-EPI 2021 equation removed the race coefficient that older versions applied. A 2021 NKF-ASN task force concluded that including race was not biologically justified and delayed care for Black patients. Some calculators still in circulation use the old equation.
The sex difference reflects average muscle mass, not kidney function: less muscle produces less creatinine, so the same reading implies lower filtration.
What to enter
- Serum creatinine
- From a blood test, in mg/dL in the US or µmol/L elsewhere. Divide µmol/L by 88.4 to convert.
- Age
- Filtration declines with age, and the equation accounts for it.
- Sex
- Included because average muscle mass differs, which changes baseline creatinine production.
Chronic kidney disease stages by eGFR
- G1: 90 or above
- Normal filtration. CKD only if kidney damage is present on other evidence.
- G2: 60-89
- Mildly reduced. Again, CKD only alongside other markers of damage.
- G3a: 45-59 and G3b: 30-44
- Moderately reduced. Usually the point at which specialist input begins.
- G4: 15-29
- Severely reduced. Planning for kidney replacement therapy typically starts here.
- G5: below 15
- Kidney failure.
What this assumes
Creatinine depends on muscle mass, so eGFR is less reliable at the extremes: bodybuilders, amputees, people with very low muscle mass, and during pregnancy.
A single result is a snapshot. CKD is defined by reduced function persisting for at least three months, not by one test.
How to calculate your estimated GFR
The CKD-EPI 2021 equation, with different constants by sex. It is not arithmetic to do by hand.
- Scr
- Serum creatinine in mg/dL
- κ
- 0.7 for women, 0.9 for men
- α
- −0.241 for women, −0.302 for men
Get creatinine in mg/dL. If your lab reports µmol/L, divide by 88.4. Most labs already report eGFR alongside creatinine.
Apply the sex-specific constants. They account for the difference in baseline creatinine production, not for any difference in kidney function.
Adjust for age. The 0.9938^age term reflects the gradual decline in filtration that occurs with normal ageing.
Read the stage, then talk to a doctor. A single low result is not a diagnosis. CKD requires reduced function persisting for at least three months.
See a worked example: why sex changes the answer
- Creatinine
- 1.2 mg/dL
- Age
- 50
Man: eGFR 74, which is stage G2, mildly reduced.
Woman: eGFR 55 from the identical blood result, which is stage G3a.
The difference is muscle mass. Women on average produce less creatinine, so the same reading implies more of it is being retained.
Dropping the man's creatinine to 1.0 raises his eGFR to 92. Small changes in creatinine move eGFR substantially.
74 for a man, 55 for a woman
Frequently asked questions
90 or above is normal filtration for most adults. Below 60 sustained for three months or more indicates chronic kidney disease.
GFR declines gradually with age even in healthy kidneys, so a figure in the 70s in an older adult may be entirely normal for them.
A 2021 NKF-ASN task force concluded that the race coefficient in the older equations was not biologically justified and produced systematically higher eGFR values for Black patients, which delayed referral and transplant listing.
The CKD-EPI 2021 equation removed it. If you are using an older calculator or comparing against an old result, that difference explains part of any discrepancy.
It can be misleading in specific situations, because creatinine depends on muscle mass. Very muscular people can show a falsely low eGFR; people with very low muscle mass, including some older and chronically ill patients, can show a falsely reassuring one.
Amputation, pregnancy, high-protein diets, creatine supplements and some medications also affect it. Where accuracy matters, cystatin C is an alternative marker that does not depend on muscle.
Not necessarily much on its own. Dehydration, illness, recent intense exercise and some medications can all raise creatinine temporarily.
CKD is defined by reduced function persisting for at least three months, usually alongside other markers such as protein in the urine. A repeat test is the normal next step.
The main levers are controlling blood pressure and blood glucose, since hypertension and diabetes cause most CKD. Staying hydrated, limiting NSAIDs and not smoking all help.
Existing damage is generally not reversible, but progression can often be slowed substantially. That is why early detection matters so much.
An alternative blood marker for estimating GFR that does not depend on muscle mass, which is exactly where creatinine is weakest.
Current guidance recommends using it alongside creatinine when a more accurate estimate is needed, particularly at the extremes of body composition. It is more expensive and less widely available.
Problems people actually run into
Reading one result as a diagnosis
Creatinine rises with dehydration, illness, intense exercise and certain medications. A single below-normal eGFR is a prompt for a repeat test, not a conclusion.
CKD requires the reduction to persist for at least three months, usually with other evidence of kidney damage.
Trusting eGFR at the extremes of muscle mass
A heavily muscled athlete produces more creatinine and can show an eGFR that looks concerning while their kidneys are fine. Someone with very low muscle mass can show a reassuring number while function is genuinely reduced.
In both cases cystatin C or a measured clearance gives a much better answer, and is worth asking about if the number does not fit the clinical picture.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Sources
- Estimating Glomerular Filtration Rate · National Institute of Diabetes and Digestive and Kidney Diseases
Last updated: September 4, 2026