The normal GFR of an average adult is considered to be around 120 milliliters per minute per 1.73 square meters. This number declines with age but can also fall as a result of kidney disease, according to the National Kidney Foundation.
GFR is considered to be the best way to measure kidney function, and represents the total rate of filtration of all of the functional nephrons in the kidney. It is often used to assess the level of kidney damage in patients with chronic kidney disease. The complications of chronic kidney disease begin to increase once GFR falls below 60 milliliters per minute per 1.73 square meters, explains the National Kidney Foundation.