GFR Calculator

This estimates glomerular filtration rate from a serum creatinine result. The adult calculator runs four published equations side by side; the children's calculator uses the bedside Schwartz formula. Creatinine can be entered in mg/dL or micromoles per litre.

What GFR is

Glomerular filtration rate describes how fast fluid is filtered through the kidneys, and it is the standard measure of kidney function. Higher is better. Each kidney holds about a million filtering units called nephrons, and GFR is the combined output of all of them, roughly 180 litres of fluid filtered a day, almost all of it reabsorbed before it becomes urine.

The figure is always quoted as mL/min per 1.73 m², normalised to an average adult body surface area so results are comparable between people of different sizes. That normalisation matters in one specific place: when dosing drugs cleared by the kidney, the absolute GFR for the patient's actual body size is what counts, not the indexed number. Our Body Surface Area Calculator covers that conversion.

Typical values by age

Kidney function declines with age in almost everyone, so an eGFR has to be read against the person's age rather than a single cut-off:

Age	Mean estimated GFR (mL/min/1.73 m²)
20–29	116
30–39	107
40–49	99
50–59	93
60–69	85
70+	75

Roughly a point a year from the age of 30. That gentle slope is why a GFR in the 70s means something very different at 35 than at 75, and it is what the age chart above shows for a fixed creatinine.

Reading the stages

A GFR above 60 is usually adequate and does not by itself indicate chronic kidney disease. This is the single most misread part of a kidney result. A CKD diagnosis needs either a marker of kidney damage (protein in the urine being the usual one) or a reduced GFR that has persisted for at least three months. One low reading is not a diagnosis.

That is why the table above separates "Normal" at 90+ with no proteinuria from CKD1 at 90+ with evidence of damage. The number is identical; the diagnosis is not.

Modern KDIGO staging goes further and grades albuminuria alongside GFR, from A1 to A3, because someone at GFR 50 with no protein in the urine has a very different outlook from someone at GFR 50 with heavy proteinuria. GFR alone tells you half the story.

Measuring against estimating

The accurate way to determine GFR is to measure how fast the body clears an external marker such as inulin. Inulin is freely filtered by the glomerulus and neither reabsorbed nor secreted by the tubules, so its clearance is the filtration rate. It is the reference standard and it is also slow, expensive and impractical, which is why it is confined to research. Iohexol and radioisotope methods are the practical alternatives when a real measurement is genuinely needed.

Everything this calculator produces is an estimate, hence eGFR. The equations were fitted by regression against measured GFR in specific study populations, and they carry that population's characteristics with them.

The four equations

MDRD 4-variable, from the Modification of Diet in Renal Disease study:

GFR = 175 × SCr^−1.154 × age^−0.203 × 0.742 (female) × 1.212 (Black)

It was developed in people who already had kidney disease, and it is known to underestimate at higher filtration rates. Many labs stopped reporting values above 60 with it for that reason.

CKD-EPI (2009) was built to fix exactly that. It uses the same inputs but switches exponent depending on whether creatinine sits above or below a sex-specific threshold, which straightens out the high end:

GFR = 141 × min(SCr/κ, 1)^α × max(SCr/κ, 1)^−1.209 × 0.993^age × 1.018 (female) × 1.159 (Black)

κ is 0.7 for women and 0.9 for men; α is −0.329 and −0.411 respectively.

The Mayo quadratic was derived including healthy people with normal kidneys, so it reads highest of the three at normal creatinine levels. It floors creatinine at 0.8 mg/dL:

GFR = exp(1.911 + 5.249/SCr − 2.114/SCr² − 0.00686 × age − 0.205 if female)

CKD-EPI (2021) is the current US standard and the one this calculator leads with. It is the 2009 equation refitted without the race coefficient:

GFR = 142 × min(SCr/κ, 1)^α × max(SCr/κ, 1)^−1.200 × 0.9938^age × 1.012 (female)

with κ as before and α of −0.241 for women, −0.302 for men.

Why race was removed

The two older equations multiply the result upward for Black patients. That coefficient came from an observed average difference in creatinine between groups in the original study cohorts, usually attributed to differences in muscle mass, but race is a social category, not a biological measurement, and applying a population average to an individual is not sound.

The practical harm was concrete: a higher reported eGFR makes kidney disease look less severe, which delayed specialist referral and transplant waiting-list eligibility for Black patients. A joint National Kidney Foundation and American Society of Nephrology task force recommended removing it, and the race-free 2021 equation was published that year. US laboratories have largely adopted it.

The race toggle is kept here because the two older equations are defined with it and you may be reconciling an older lab report. For a current estimate, use the 2021 figure.

The spread between formulas

Run one blood sample through all four and they disagree by around 30 units at normal creatinine: on the defaults, from 89.3 to 120.3 mL/min/1.73 m². The first chart shows that directly.

The practical lesson is that an eGFR number means nothing without the equation that produced it. Comparing a result from one lab against another that uses a different equation is not a comparison at all, and a "drop" between two reports can be entirely an artefact of the lab changing its method. Trend one equation over time; do not mix them.

Where creatinine misleads

Every equation here rests on serum creatinine, which is a waste product of muscle metabolism. That makes it a proxy for filtration only when muscle mass is typical.

• Muscular people produce more creatinine and get an eGFR that reads too low. Bodybuilders and some athletes are routinely flagged for kidney disease they do not have.
• People with low muscle mass produce less. The frail elderly, amputees and those with cirrhosis or long-term illness are the usual cases, so eGFR reads too high and real impairment is missed.
• Diet shifts it. A large meal of cooked meat raises creatinine for hours; long-term vegetarian diets lower it. Creatine supplements raise it too.
• Acute illness. The equations assume creatinine is in a steady state. During acute kidney injury it lags well behind the true filtration rate, so eGFR is misleading exactly when decisions are most urgent.
• Pregnancy changes renal physiology substantially, and these equations are not validated for it.
• Some drugs, including trimethoprim and cimetidine, block creatinine secretion in the tubules. Creatinine rises, filtration has not changed.

Where creatinine is unreliable, cystatin C is the usual alternative. It is produced by all nucleated cells rather than muscle, so it sidesteps the body composition problem, and a combined creatinine-and-cystatin-C equation is more accurate than either input alone. It is the recommended confirmatory test when an eGFR does not fit the clinical picture.

Children

The adult equations do not apply under 18. The bedside Schwartz formula is used instead:

GFR = 0.413 × height (cm) ÷ SCr (mg/dL)

Height stands in for muscle mass, which is why it replaces age and sex. The 0.413 constant assumes creatinine measured by an IDMS-traceable enzymatic method. The older Jaffe assays used a different constant, so an old result and a new one are not comparable.

One thing to know when reading a child's result: newborn GFR is genuinely low, around 20 to 40 mL/min/1.73 m², and climbs to adult levels by about age two. A low number in an infant is not necessarily abnormal.

The second children's chart shows why small changes matter at the low end. The relationship is a reciprocal, so a creatinine shift from 0.3 to 0.4 moves the estimate far more than a shift from 1.3 to 1.4.

Reading your result

Take the 2021 CKD-EPI figure as the headline, note which stage band it falls in, and compare it against the age table rather than a flat cut-off. Then treat it as one input among several. A single eGFR, without a urine albumin result, without a repeat after three months, and without knowing the person's muscle mass, is not enough to conclude anything.

This is a reference tool. Any decision about diagnosis, medication dosing or referral belongs with a clinician who has the full picture.