How this estimate is calculated
This calculator turns a serum creatinine result, your age and your sex into an estimated glomerular filtration rate (eGFR) using the CKD-EPI 2021 creatinine equation, published by Inker and colleagues in the New England Journal of Medicine. It replaced an earlier 2009 version of the same equation that included a race adjustment; the 2021 update removed that term after a National Kidney Foundation and American Society of Nephrology task force concluded race was being used as an unreliable proxy for something it does not actually measure, and that a race-free formula produced fairer, similarly accurate results across everyone.
Scr is serum creatinine in mg/dL. κ (kappa) is 0.7 for women and 0.9 for men; α (alpha) is −0.241 for women and −0.302 for men. The min(...) and max(...) terms are a mathematical way of applying one exponent below the κ threshold and a different, steeper one above it, since the relationship between creatinine and true filtration rate is not a straight line across its whole range. Take this page's own defaults: a 45-year-old man with a creatinine of 0.90 mg/dL. Because 0.90 ÷ 0.9 (κ) equals exactly 1.0, both the min and max terms equal 1, leaving 142 × 0.9938^45, which comes to about 106.8 mL/min/1.73m². Change any input and the worked substitution above the chart updates to show your own arithmetic in full.
Why sex changes the equation
Both κ and α differ by sex, and women get an extra ×1.012 multiplier on top. This is not an arbitrary adjustment: it corrects for the fact that, on average, women have less muscle mass than men at the same age and creatinine level, so the same reading implies a proportionally higher filtration rate for a woman than for a man. The equation was fitted separately by sex against measured GFR from pooled research studies, so the constants reflect an averaged relationship across large populations rather than any individual's actual muscle mass.
Reading the CKD stage bands
The band chart above places your estimate on the five stages KDIGO (Kidney Disease: Improving Global Outcomes) uses to describe GFR: G1 is 90 or above, G2 is 60 to 89, G3a is 45 to 59, G3b is 30 to 44, G4 is 15 to 29, and G5 is below 15. Read the boundary as inclusive on the higher stage: an eGFR of exactly 90 counts as G1, while 89.9 falls into G2.
Here is the part worth repeating: a GFR stage on its own does not diagnose chronic kidney disease. KDIGO's actual definition combines this GFR category with an albuminuria category, A1 to A3, based on a separate urine albumin-to-creatinine ratio test, and requires abnormalities in one or both to persist for at least three months before CKD is diagnosed. G1 and G2 specifically do not count as CKD at all unless there is other evidence of kidney damage, such as albuminuria, alongside them. This calculator only has your GFR side of that picture; it cannot see your ACR or your history, so treat the stage shown as a starting point for a conversation with a doctor, never as a finished diagnosis.
What creatinine is, and why muscle mass skews it
Creatinine is a waste product made at a fairly steady rate by breaking down creatine in muscle tissue, then filtered out of the blood almost entirely by the kidneys. Because the kidneys normally clear it efficiently, a rising blood creatinine level usually means filtration has slowed down, which is why it is used to estimate GFR in the first place.
The catch is that creatinine production depends on muscle mass, not just kidney function, so anything that changes how much muscle someone carries changes their creatinine independently of their kidneys. A very muscular athlete or bodybuilder can run a higher creatinine than someone the same age and sex with less muscle, purely from making more of it, which can make this equation understate their true filtration rate. The opposite happens with very low muscle mass: someone with a limb amputation, a long illness, or the general muscle loss common in frail elderly people can show a low, reassuring-looking creatinine that actually understates how much kidney function has been lost, because there is less muscle generating it in the first place. None of this is a flaw unique to this calculator; it applies to every creatinine-based GFR equation, and is one of the reasons doctors interpret a result alongside the person in front of them, not the number alone.
GFR and normal ageing
Kidney filtration rate drifts down with ordinary ageing, separately from any disease. Research following healthy kidney donors found GFR declining by roughly 1 mL/min/1.73m² a year on average from somewhere around the age of 40 onward, as the kidneys gradually lose nephrons, the tiny filtering units they are built from. That is why a stable G2 reading in an otherwise well 75-year-old is a very different situation to the same reading appearing suddenly in a healthy 30-year-old: the equation already factors age into its output, but it cannot tell the difference between filtration that has always tracked normally for someone's age and a genuinely new problem. Trend and context matter as much as the single number.
Hydration, meat meals and other things that shift a single reading
A single creatinine result can move for reasons that have nothing to do with long-term kidney health. Dehydration concentrates the blood and briefly raises creatinine; drinking plenty of water beforehand can lower it a little. Eating a large meat meal in the hours before a blood draw adds a modest amount of creatine and creatinine from the meat itself, nudging the result upward temporarily. Intense exercise shortly before testing, certain medications, and even which lab equipment processed the sample can all move the figure by a small but real amount. None of this means a single test is worthless, only that one reading is a snapshot, not a trend, and a result that looks slightly off is often worth simply repeating under normal conditions before reading too much into it.
The older MDRD and Cockcroft-Gault equations
Before CKD-EPI, the main formula in routine use was MDRD (Modification of Diet in Renal Disease), published in 1999; it is less accurate than CKD-EPI at higher GFR values, so most laboratories and this calculator have moved on from it. Cockcroft-Gault, an older 1976 formula that estimates creatinine clearance in mL/min rather than GFR in mL/min/1.73m² and additionally needs body weight, is not used here either, but it has not disappeared: many drug dosing guidelines were originally written and validated against it, so pharmacists and prescribers still reach for Cockcroft-Gault specifically to check medication doses, even as CKD-EPI has become the standard for diagnosing and staging kidney disease itself. For children and teenagers under 18, none of the adult equations apply; the bedside Schwartz equation, which uses height rather than sex, is the recognised paediatric alternative and is not implemented on this page.
When to talk to a doctor
Book an appointment, rather than waiting for a routine check-up, if a GFR estimate comes back under 60 on two occasions three months or more apart, since that is the point at which persistence starts to matter for a possible CKD diagnosis. The same goes for any sudden, sizeable drop from your own previous results, even if the new figure is still in a "normal" band for your age, since a rapid change is generally more informative than where a single reading happens to land. Seek medical attention promptly, rather than booking a routine slot, for an estimate under 15, for a sharp drop alongside symptoms such as swelling, much-reduced urination, breathlessness or confusion, or for any reading you cannot explain that worries you. This calculator cannot see your history or your symptoms, so when in doubt, ask a doctor rather than relying on the number alone.
Questions people ask
Can this calculator diagnose kidney disease?
No. It estimates GFR from one creatinine result, your age and your sex. Diagnosing chronic kidney disease needs a urine albumin-to-creatinine ratio test alongside this figure, and confirmation that any abnormal results persist for at least three months. Treat this page as a starting point for a conversation with a doctor, not a diagnosis.
Why does the equation need my sex?
The κ and α constants, and an extra 1.012 multiplier for women, correct for the fact that women have, on average, less muscle mass than men at the same age and creatinine level. Since creatinine production depends on muscle mass, the same blood result implies a different estimated filtration rate depending on sex.
Why did the 2021 equation remove race?
The earlier 2009 CKD-EPI equation included a coefficient for Black patients. A National Kidney Foundation and American Society of Nephrology task force concluded race was being used as an unreliable proxy for muscle mass and other biology it does not actually measure, and recommended the race-free 2021 version used on this page, which performs comparably across groups without that adjustment.
My eGFR is 65. Do I have kidney disease?
Not necessarily, and often not at all. G2 (60 to 89) does not meet the definition of chronic kidney disease by itself; it needs other evidence of kidney damage, such as an abnormal urine albumin result, and persistence over three months. A single G2 reading, especially in someone older, is frequently unremarkable ageing rather than disease.
Why is this calculator not built for children?
CKD-EPI was developed and validated using adult data, so it is not reliable in children and teenagers under 18. The recognised alternative for that age group is the paediatric bedside Schwartz equation, which estimates GFR from height and creatinine rather than sex and age. This page blocks the calculation under 18 rather than showing a misleading number.
Can dehydration or a big meal change my result?
Yes. Dehydration can concentrate the blood and raise creatinine temporarily, and a large meat meal shortly before a blood draw adds a small amount of creatinine from the meat itself. Both effects are usually modest and short-lived, which is one reason a single unusual reading is often worth simply repeating rather than acting on immediately.
Is Cockcroft-Gault still used anywhere?
Yes, mainly for drug dosing. Many medication guidelines were originally validated against Cockcroft-Gault's creatinine clearance figure, so pharmacists and prescribers sometimes still use it specifically to check doses, even though CKD-EPI is now the standard for diagnosing and staging kidney disease.
This tool estimates GFR from the published CKD-EPI 2021 equation using one creatinine result; it is not a diagnosis and is not medical advice. It cannot see your urine albumin result, your test history or your symptoms, all of which matter to a real assessment of kidney health. Do not start, stop or change any medication or treatment based on this page. If you have concerns about your kidney function, a low or rapidly changing result, or symptoms such as swelling, much-reduced urination, breathlessness or confusion, contact a doctor.