eGFR calculator: CKD-EPI 2021, with or without cystatin C

eGFR is not measured. The lab calculates it from your creatinine, age, and sex, and prints the result on almost every metabolic panel. That means you can recalculate it yourself, for an old result that has no eGFR, for a lab that used an older equation, or to see what a cystatin C result adds. Enter your creatinine below. The calculator uses the 2021 equation that U.S. labs have been switching to since 2021.

How the calculation works

The calculator uses the CKD-EPI 2021 equations from Inker and colleagues, which a joint task force of the National Kidney Foundation and the American Society of Nephrology recommended for immediate use in the United States. Unlike the equations before it, it has no term for race.

The creatinine equation, as the National Kidney Foundation publishes it:

eGFR = 142 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^-1.200 × 0.9938^age × 1.012 (if female)

Scr is your creatinine in mg/dL. A result in µmol/L is divided by 88.4 first.

The threshold is there because creatinine and filtration don’t have a straight-line relationship. Below κ, small changes in creatinine move eGFR only a little. Above it, they move it a lot. The result is in mL/min/1.73m², which scales it to an average body size so people of different sizes can be compared.

Adding cystatin C

Cystatin C is a protein made by nearly every cell, not just muscle, so it isn’t thrown off by muscle mass the way creatinine is. If you enter a cystatin C result in mg/L, the calculator also uses the 2021 creatinine–cystatin C equation:

eGFR = 135 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^-0.544
           × min(Scys/0.8, 1)^-0.323 × max(Scys/0.8, 1)^-0.778
           × 0.9961^age × 0.963 (if female)

Here κ is the same, and α is −0.219 for females and −0.144 for males. The KDIGO 2024 guideline recommends this combined estimate when the creatinine estimate is likely to be less accurate, such as in very muscular or very frail people, or when the number will drive a treatment decision. When you enter both, the category shown uses the combined result.

What the result means

The categories come from the KDIGO guideline and are the same at every lab:

A category is not a diagnosis. Chronic kidney disease means abnormal kidney structure or function for at least three months, so one low eGFR isn’t enough to call it. An eGFR of 60 to 89 counts as kidney disease only with other evidence of damage, most often albumin in the urine. KDIGO stages the urine albumin-to-creatinine ratio alongside eGFR, and the two together say more than either one.

Why your lab’s number may differ

If the calculator doesn’t match your report, it’s usually one of these:

• An older equation. Before about 2022, many labs used MDRD or CKD-EPI 2009. Both included a multiplier for patients identified as Black, and both give a different eGFR from the same creatinine. If your report says “eGFR non-African American”, “eGFR if Black”, or “MDRD”, it was calculated with one of them. The 2021 equation usually reads a little higher than the 2009 one for the same creatinine.
• A capped value. Some labs print “>60” or “>90” instead of a number. The calculator gives the exact value.
• Rounding. Labs report eGFR as a whole number, and some round creatinine before calculating. A difference of 1 is rounding.
• A different age. The equation uses your age on the day of the draw. For an old result, enter the age you were then.

Recalculating old results with one equation is a fair way to compare them. As long as you have the creatinine, you can put a history that spans an equation switch on one scale.

When the creatinine estimate is off

Anything that changes creatinine without changing your kidneys also changes eGFR. More muscle, creatine supplements, a high-protein diet, a cooked meat meal in the hours before the draw, intense exercise, and some medications, such as trimethoprim, can all raise creatinine and lower eGFR. Low muscle mass does the opposite. In those cases a cystatin C result is the better check. How to track your eGFR over time covers these in more detail, along with how much eGFR varies from draw to draw and what counts as a real change.

eGFR in HealthViewer

HealthViewer is a lab results tracker that runs on your own computer. It imports eGFR and creatinine from lab report PDFs, MyChart downloads, and direct connections to your health system, and files the MDRD, CKD-EPI, and CKD-EPI 2021 results under one test. The race-specific “African American” and “Black” lines are kept separate.

In Trends, the overlay view puts eGFR and creatinine on one chart. If creatinine is flat and eGFR stepped, it was the equation. A journal note on the date your lab switched equations keeps that step from looking like a decline.

Everything stays in an encrypted file on your computer, with no account and no cloud service. Getting Started walks through the first import.

This calculator is for understanding your results, not for diagnosis or drug dosing. Talk to your clinician about what your eGFR means for you and before changing any treatment.