The formula
How it works
Estimate your glomerular filtration rate (GFR) — a measure of how well your kidneys filter waste from the blood — using the MDRD equation. It is the main number doctors use to check kidney health and stage kidney disease.
FAQ
What is a normal GFR?
A GFR of 90 or above is generally considered normal for healthy kidneys. Values decline naturally with age, and a persistently low GFR can indicate chronic kidney disease, which is staged from G1 down to G5.
Is this a diagnosis?
No. This is an estimate from a formula and should never replace a doctor’s assessment. GFR can vary with hydration, diet and lab methods, so any concern about kidney health needs proper medical testing and advice.
Why does the formula use age and sex?
Muscle mass affects how much creatinine the body produces, and muscle mass tends to differ with age and sex, so the MDRD equation adjusts for them to give a more accurate estimate than creatinine alone.
Why was the race adjustment used, and is it still recommended?
Older equations added a factor for Black patients because average measured GFR differed by race in the original study populations. Many labs and guidelines have since moved to race-free equations, such as the CKD-EPI 2021 formula, because race is not a reliable biological marker.
How often should GFR be checked?
That depends on your health status — people with diabetes, high blood pressure or known kidney disease are typically tested more often, sometimes annually or more, while healthy people may only have it checked incidentally. Your doctor sets the right schedule.
Can GFR improve over time?
It can rise somewhat if a reversible cause — such as dehydration or a medication effect — is corrected, but chronic kidney disease-related declines are usually permanent, and treatment focuses on slowing further loss rather than reversing it.
Is MDRD the only GFR equation?
No. The CKD-EPI equation is now often preferred because it is more accurate at higher GFR values, and cystatin C-based equations offer an alternative that does not depend on muscle mass. This calculator uses the widely known MDRD formula for simplicity.
About the GFR calculator
This calculator estimates your glomerular filtration rate (GFR), the standard measure of kidney function. The kidneys filter waste and excess fluid from the blood, and the GFR estimates how much blood they clean each minute. Because a blood test for creatinine alone is hard to interpret, doctors convert it into an estimated GFR using an equation that adjusts for age, sex and other factors — which is what this tool does.
How to use it
Enter your serum creatinine level from a blood test in mg/dL, your age, and your sex. The calculator applies the MDRD equation and returns your estimated GFR along with the corresponding chronic kidney disease stage. For example, a creatinine of 1.0 mg/dL in a 50-year-old man gives a GFR of about 79, which sits in the mildly reduced range. Only a doctor can interpret what a result means for you.
The formula
This uses the four-variable MDRD equation, , where is serum creatinine in mg/dL, age is in years, and is a correction factor — multiply by 0.742 for women. Some older versions also multiply by 1.212 for Black patients, a race adjustment now being phased out in favour of race-free equations. The result is expressed per 1.73 m² of body surface area.
Where it is used
GFR is the key number in nephrology, used to diagnose and stage chronic kidney disease, monitor its progression, and adjust the doses of drugs that the kidneys clear. Doctors track it over time rather than relying on a single reading. For patients, understanding their GFR helps them follow their kidney health, though the number should always be interpreted alongside other tests by a clinician.