GFR Calculator

Your kidneys filter roughly 180 litres of blood every day, and the glomerular filtration rate — GFR — is the number used to describe how well they are doing it. Because true GFR is awkward to measure directly, clinics estimate it (eGFR) from a routine blood test. This calculator produces that estimate from your creatinine level, age and sex, the same inputs your laboratory uses.

Quick Answer

eGFR is calculated from serum creatinine, age and sex using the CKD-EPI equation, and is reported in mL/min/1.73 m². As a rough guide, 90 or above is in the normal range, while a value persistently below 60 is a warning sign that needs medical follow-up.

What the Number Actually Represents

Each kidney contains about a million tiny filters called glomeruli. The filtration rate is the volume of fluid those filters clear per minute, standardised to a body surface area of 1.73 m² so people of different sizes can be compared. A young, healthy adult typically filters at 90–120 mL/min/1.73 m², and the rate declines gradually with age even in healthy kidneys.

The estimate works because of creatinine, a waste product of muscle metabolism that the kidneys remove at a fairly steady rate. When filtration falls, creatinine accumulates in the blood, so a rising creatinine implies a falling GFR. Modern practice uses the CKD-EPI equation — the 2021 version dropped the race coefficient used in older formulas — because it is more accurate than the older MDRD equation, particularly at higher filtration rates. Kidney function is then described in stages: G1 (90 and above), G2 (60–89), G3a (45–59), G3b (30–44), G4 (15–29) and G5 (below 15). The stage alone does not diagnose disease; doctors combine it with urine tests, blood pressure and how long the reduction has persisted.

eGFR, Measured GFR and the Cystatin C Alternative

The "e" in eGFR is an honest admission: this is an estimate, calculated from a marker in the blood rather than a direct measurement. A true measured GFR involves injecting or infusing a substance such as inulin or iohexol and timing how fast the kidneys clear it — accurate, but far too slow and expensive for routine care, so it is reserved for situations like kidney-donor assessment and some drug-dosing decisions where the estimate is not trusted enough.

Between the two sits cystatin C, a second blood marker that depends much less on muscle mass than creatinine does. Laboratories increasingly report an eGFR calculated from cystatin C, or from both markers combined, for patients whose creatinine is likely to mislead — the very muscular, the frail elderly, amputees, and people with unusual diets. Kidney assessment also pairs the filtration estimate with a urine albumin result, staged A1 to A3 (from under 30 mg/g of albumin-to-creatinine ratio up to above 300), because the two together predict risk far better than either alone. A calculator like this one handles the creatinine-based estimate; the fuller picture is assembled by the clinic.

How to Use This GFR Calculator

1. Find your serum creatinine result on a recent blood test.
2. Check the unit on the report — mg/dL in the US system, µmol/L in most other countries.
3. Enter the creatinine value, selecting the matching unit.
4. Enter your age and sex, both of which the equation requires.
5. Read the estimated GFR in mL/min/1.73 m².
6. Compare the result with any previous values you have — the trend over time matters more than one reading.

Worked Example: Two Patients, One Creatinine

Take a serum creatinine of 1.1 mg/dL in a 55-year-old. For a man, the CKD-EPI 2021 equation gives an eGFR of about 79 mL/min/1.73 m², inside the G2 band. Run the identical blood result for a woman of the same age and the estimate is about 59 — right at the top of the G3a band. The twenty-point gap from the same laboratory value is not an error: women on average carry less muscle mass, so the same creatinine concentration implies less filtration behind it. This is precisely why age and sex are not optional details in the formula.

What Typically Happens at Each Stage

For orientation only: stages G1 and G2, where filtration is preserved or mildly reduced, usually mean monitoring — the result is noted, blood pressure and urine are checked, and the value is rechecked periodically, especially if diabetes or hypertension is present. Stage G3, the 30–59 band, is where kidney care becomes active and regular: kidney-related blood tests join the routine, medications are reviewed for kidney effects, and cardiovascular risk gets close attention because it rises as filtration falls. By G4, specialist nephrology care is standard and conversations about future treatment options begin; G5 is the stage where dialysis or transplantation enter the plan if symptoms and overall health point that way. Many people, particularly older adults with a stable value in the low 60s or high 50s, remain in their stage for years — progression is common but never automatic, which is why clinicians watch the trend.

Why a Single eGFR Should Be Read Carefully

Creatinine depends on muscle, so the equation misleads at the extremes: it can overestimate kidney function in people with very low muscle mass and underestimate it in heavily muscled athletes or people taking creatine supplements. A large cooked-meat meal, dehydration, a urinary blockage or certain antibiotics can also move creatinine temporarily without any lasting change in the kidneys themselves.

Units are the most common source of calculator errors. A result reported as 97 µmol/L is the same concentration as 1.1 mg/dL; entering "97" into a field expecting mg/dL produces a meaningless, alarming number. Finally, chronic kidney disease is defined by abnormality lasting at least three months — one low estimate is a prompt to recheck and investigate, not a diagnosis, and eGFR is less precise at the top of the range, where laboratories may report it simply as "above 60" or "above 90".

This calculator provides a general-information estimate only and is not a diagnosis. Kidney health decisions require proper laboratory testing and interpretation by a qualified clinician.

Related Kidney and Health Estimates

GFR sits alongside other size- and blood-based estimates in the health calculators collection, including the body surface area calculator — the 1.73 m² in the eGFR unit comes from exactly that measurement. For the staging system and testing guidance in depth, see the glomerular filtration rate reference.