GFR Calculator

What eGFR is

Glomerular filtration rate is the volume of blood your kidneys clear of waste each minute. It is the single most useful number for gauging kidney function, but it is hard to measure directly, so in practice it is estimated from a blood test. That estimate is the eGFR, reported in millilitres per minute per 1.73 m² of body surface area.

This calculator estimates it from serum creatinine and a few details. For adults it uses the CKD-EPI 2021 creatinine equation, the current standard, which deliberately contains no race term. Older equations, the MDRD and the 2009 CKD-EPI, applied a race coefficient that has since been removed from recommended practice, and this tool follows that newer guidance. For children it uses the bedside Schwartz equation instead. Below are the equations, how to read the result against the CKD stages, and, just as importantly, when the number should not be trusted.

The equations this calculator uses

For adults, the primary result is the CKD-EPI 2021 creatinine equation:

eGFR = 142 × min(Scr ÷ κ, 1)^α × max(Scr ÷ κ, 1)^−1.200 × 0.9938^age × 1.012 [if female]

where Scr is serum creatinine in mg/dL, κ is 0.7 for women and 0.9 for men, and α is −0.241 for women and −0.302 for men. Alongside it, the tool shows the Mayo Quadratic equation as a secondary estimate, since it also carries no race term. For children, it applies the bedside Schwartz equation, which is far simpler:

eGFR = (0.413 × height in cm) ÷ Scr

The adult equations are meant for ages 18 and over, and the tool will steer you to the children's option below that. Both report the result standardised to 1.73 m² of body surface area.

A worked example

Take a 55 year old man with a serum creatinine of 1.2 mg/dL. His creatinine over κ is 1.2 ÷ 0.9, which is 1.33, so the minimum term is 1 and the maximum term carries the exponent:

eGFR = 142 × 1 × 1.33^−1.200 × 0.9938⁵⁵
eGFR ≈ 142 × 0.708 × 0.710
eGFR ≈ 71.4 mL/min/1.73 m²

So his estimated GFR is about 71, which falls in stage G2. The tool would also display the Mayo Quadratic result beside it, roughly 85 here, as a secondary cross-check. Where the two diverge, that spread is a useful reminder that both are estimates, not measurements.

Reading the result: CKD stages

Chronic kidney disease is staged by eGFR using the KDIGO categories:

Two important qualifiers. An eGFR of 90 or above (G1) or 60 to 89 (G2) only indicates chronic kidney disease if there is other evidence of kidney damage, such as albuminuria, since a healthy person can sit in G2. And chronic kidney disease is defined by an eGFR under 60 that persists for at least three months, so a single low value is not the same as a diagnosis. Full staging pairs the GFR category with an albuminuria category, which this tool does not assess.

When eGFR is unreliable

This is the part that matters most, because the estimate rests on creatinine behaving predictably, and often it does not:

• Acute kidney injury. The equations assume a steady state. When creatinine is rising or falling quickly, the eGFR lags behind reality and can badly mislead.
• Unusual muscle mass. Creatinine comes from muscle, so very muscular people, amputees, those with low muscle from illness or age, and anyone with a very high or very low body size can all get a skewed estimate.
• Pregnancy. The equations are not validated in pregnancy.
• Diet and supplements. Large creatine intake, high meat intake, or certain drugs that affect creatinine can shift the number without a real change in filtration.

A closing point on how to use this. It is an estimate, and a reference and educational aid rather than a diagnostic tool. It supports clinical judgement; it does not replace it. Trend serial values rather than reacting to one, confirm anything unexpected, and interpret every result against the full clinical picture. If you are a patient who has landed here, a number from this page cannot tell you whether you have kidney disease; that is a conversation for your doctor, who can order the right tests and read them in context.

Questions people ask

Why is there no race adjustment anymore?

The 2009 equation multiplied results for Black patients by a fixed factor, which was increasingly seen as biologically unjustified and a driver of inequitable care. In 2021 a joint task force recommended refitting the equation without race, and that race-free CKD-EPI 2021 equation, which this tool uses, is now the standard.

What is a normal eGFR?

A healthy young adult typically has an eGFR above 90. It declines gently with age even in healthy kidneys, so a value in the 60s or high 80s in an older adult, without other signs of kidney damage, is often normal rather than disease. Context is everything, which is why the stages are read alongside albuminuria and clinical findings.

Why does it show two numbers?

The CKD-EPI 2021 result is the primary, standard estimate. The Mayo Quadratic figure beside it is a secondary equation shown as a cross-check, since it also avoids a race term. When they agree, that is reassuring; when they diverge, it is a useful prompt that these are estimates and worth confirming.

References

Where the figures come from. The adult equation is the CKD-EPI 2021 creatinine equation from Inker and colleagues, reproduced above as the tool applies it. The CKD stage boundaries follow the KDIGO clinical practice guideline, and the children's equation is the bedside Schwartz formula.

1. Inker LA, Eneanya ND, Coresh J, et al. New creatinine- and cystatin C-based equations to estimate GFR without race. New England Journal of Medicine. 2021;385(19):1737-1749. https://doi.org/10.1056/NEJMoa2102953
2. Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney International.
3. Schwartz GJ, Muñoz A, Schneider MF, et al. New equations to estimate GFR in children with CKD. Journal of the American Society of Nephrology. 2009;20(3):629-637.