Abstract: A device for calculating the Glomerular Filtration Rate (GFR) of a patient as a marker or renal function, which is user-friendly, portable, and provides a virtually instantaneous patient-specific calculation of the GFR. The device comprises a hand-held precision slide ruler instrument from which calculation of the GFR is obtained from a measured creatinine level in combination with body weight, sex, and age. In the preferred embodiment the device can give results for both males and females by providing gender-specific scales on either face of the instrument. An electronic version may be implemented in a portable or other processing device.