A creatinine GFR calculator is a calculator that calculates glomerular filtration rate, which is an index used to describe kidney function, notes the National Kidney Foundation (NKF). Normal GFR varies based on age, body size and sex, and the rate goes down with aging.
To calculate GFR using the National Kidney Foundation's calculator, the user must know his creatinine level in milligrams per deciliter or moles per liter along with his serum cystatin C level, notes the NKF. The user must also indicate his age, race and gender to see his GFR. Chronic kidney disease is indicated by a GFR of 60 or less.