GFR Calculator: What Your Kidney Function Number Means
What an eGFR result measures, why muscle mass distorts it, and when a single low number is worth a repeat test.
Quick answer: Estimated GFR is a calculation of how fast your kidneys filter blood, reported in mL/min per 1.73 m2. It comes from a blood creatinine result plus your age and sex. Above 90 counts as normal, 60 to 89 is mildly reduced, and a result below 60 that persists for three months or more points to chronic kidney disease.
Most people meet their eGFR by accident, at the bottom of a routine blood panel, sitting next to numbers they were not expecting to think about. Worth understanding before you decide whether to worry.
What the number actually measures
Glomerular filtration rate is the volume of fluid your kidneys filter per minute. Measuring it directly means injecting a tracer and timing how fast it clears, which nobody does for a routine check-up. Labs estimate it from a blood test instead, which is why the result is written eGFR rather than GFR.
The estimate is scaled to a standard body surface area of 1.73 m2 so that results are comparable between a 55 kg person and a 95 kg one. It is a filtration rate for a standard body, not a rate for your body specifically.
Creatinine is a proxy, and proxies drift
Creatinine is a waste product of muscle turnover. Healthy kidneys clear it, so more creatinine left in the blood implies less filtering going on. The weak point is the first half of that sentence: more muscle means more creatinine produced, whatever the kidneys are doing. A heavily built weightlifter can sit at 1.3 mg/dL with normal kidneys, while a frail 80 year old with little muscle can sit at 0.9 mg/dL and have reduced function.
That is why the equations correct for age and sex. Feed the same creatinine of 1.1 mg/dL into the calculator twice, once as a 40 year old man and once as a 70 year old woman, and the two results differ by 25 points or more. Same blood, different arithmetic.
Run your own numbers
It is free, there is no sign-up, and it works on your phone.
Open the GFR CalculatorThe stages, and the line that matters
The standard bands run G1 at 90 and above, G2 at 60 to 89, G3a at 45 to 59, G3b at 30 to 44, G4 at 15 to 29, and G5 below 15. The meaningful line is 60. Above it, a lower reading on its own is not a diagnosis. Below it, sustained for three months, it is.
Rate of change matters more than any single value. Take a creatinine that drifts from 0.9 to 1.2 mg/dL over two years, a rise of 33%. For a 60 year old man that moves calculated eGFR from about 98 down to about 69, close to 30% of estimated function gone. Losing five points from 95 to 90 is a very different event from losing four from 62 to 58.
Using the GFR calculator
Getting the inputs off your blood test
Find the line labelled creatinine, serum creatinine or Cr. US labs report it in mg/dL, typically between 0.6 and 1.3. UK labs report micromoles per litre, typically 60 to 110. To convert, divide the UK figure by 88.4: 97 micromol/L is about 1.10 mg/dL. Then enter age in years and sex as recorded on the test. The 2021 CKD-EPI equation removed the old race coefficient, so most calculators no longer ask.
Height and weight are not needed for the standard result, because it is already normalised to 1.73 m2. They are only needed to de-normalise the figure for drug dosing, which pairs with a body surface area calculation. If you are working through the rest of the panel, how to calculate BMI covers the one people ask about most.
Common questions
Does one low reading mean I have kidney disease? No. Creatinine moves with dehydration, a heavy protein meal, hard exercise in the previous day or two, and several common medicines. A single reading below 60 is a reason for a repeat test, not a conclusion.
Can I raise my eGFR? Function already lost usually does not come back, but the slope can change. Blood pressure control, blood sugar control and avoiding regular high dose anti-inflammatories are the usual levers, and staying hydrated keeps readings honest rather than artificially low. Working out a daily water target is a reasonable start. Beyond that it is a conversation with your doctor, not a calculator.
Why did my eGFR drop right after starting a new medicine? A few common drugs block the tubular secretion of creatinine. Blood creatinine rises, calculated eGFR falls, and actual filtration has not changed. Your prescriber will know which ones do it.
Does eGFR fall with age anyway? Yes. Filtration declines gradually from about the fourth decade on, which is why 75 means something different at 35 than it does at 75. The calculator already builds age into the equation, so do not subtract for it twice.