The Glomerular Filtration Calculator

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Your personal and physiological data (including serum creatinine and cystatin) is never transmitted, stored, or tracked.

The Glomerular Filtration Audit

eGFR vs. Absolute GFR (CKD-EPI & Mosteller)

Calculated Body Surface Area (BSA): 2.06 m²
(Standard Lab Assumption: 1.73 m²)

Creatinine Pathway (Flawed by Muscle Mass)

Standard eGFR (Normalized to 1.73 m²): --
Absolute GFR (Your True Filtration): --

A Note on Outdated Lab Reports

A Note on Outdated Lab Reports: If the “Standard eGFR” in this calculator comes out slightly higher than the eGFR printed on your recent lab report, do not panic. This calculator uses the modern CKD-EPI 2021 formula, which is the current gold standard endorsed by the National Kidney Foundation. Many commercial laboratories are slow to update their software and are still printing out results using the outdated 2009 formula or the ancient MDRD formula. This tool brings your math up to the modern standard.

Where the Math Comes From

This calculator applies the medical establishment’s gold-standard equations while stripping away their “average human” assumption.

Standard lab reports artificially shrink or expand your kidney marker values to fit a theoretical 1.73 m² person. This tool calculates your actual Body Surface Area to reveal your true, individualized filtration rate (Absolute GFR). Furthermore, it provides the mathematical framework to upgrade your diagnosis from muscle-dependent creatinine to the superior, muscle-blind Cystatin C.

If your physician questions why your Absolute GFR differs from the automated number on their lab printout, direct them to the following foundational texts:

  • The Baseline Engine (Inker et al. 2021):

    This paper presented the initial development and validation of the revised race-free equation.

    • Reference: Inker LA, Eneanya ND, Coresh J, Tighiouart H, Wang D, Sang Y, et al. New Creatinine- and Cystatin C–Based Equations to Estimate GFR without Race. N Engl J Med 2021;385:1737–49

  • The Creatinin plus Cystatin Strategy (Schaeffner et al., 2026): This paper describes the scenario in which each of the two biomarkers may misrepresent true GFR and argues for both to be measured simultaneously.

    • Reference: Schaeffner E, Ebert N, White CA. Direct Measurement of GFR: Who, When, and How?: A Practical Approach Emphasizing Iohexol Plasma Clearance. J Am Soc Nephrol 2026;37

  • The Body Surface Area Correction (Nagayama et al., 2025): This paper presents the rationale and clinical significance of adjusting eGFR to actual Body Surface Area (BSA) .

    • Reference: Nagayama D, et al.. Clinical Significance of Adjusting the Estimated Glomerular Filtration Rate by an Individual’s Body Surface Area from the Perspective of the Cardio-ankle Vascular Index: A Cross-sectional Study. J Atheroscler Thromb 2025:292–301

  • The Body Surface Area Calculation (Mosteller et al., 1987): .

    • Reference: Mosteller RD. Simplified calculation of body-surface area. N Engl J Med 1987;317:1098

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Disclaimer: This calculator is an educational and research tool. It is not a medical device. The information, analysis, and outputs provided by this software are not intended to be a substitute for professional medical advice, diagnosis, or treatment. Do not disregard professional medical advice or delay in seeking it because of something you have read here