GFR Calculator

The kidneys filter waste products and excess fluid out of the blood through roughly a million tiny filtering units in each kidney, called nephrons. The glomerular filtration rate is the volume of blood the kidneys filter per minute, and it is the single most widely used summary number for how well the kidneys are performing that core function — the higher the rate, generally, the more effectively the kidneys are filtering.

Measuring true GFR directly requires injecting a marker substance — such as inulin or iohexol — into the bloodstream and precisely tracking how quickly it is cleared, a process accurate enough for research but too involved for routine use. In everyday clinical practice, GFR is instead estimated ("eGFR") from a routine blood test measuring creatinine, a waste product generated at a fairly steady rate by muscle tissue and normally cleared by the kidneys almost entirely. As kidney filtering capacity declines, creatinine builds up in the blood, and equations like the one this calculator uses convert that single creatinine value into an estimated filtration rate.

Creatinine production is loosely tied to muscle mass, which differs on average by sex and tends to decline somewhat with age. Two people can have identical creatinine readings and quite different actual kidney function if their expected creatinine production differs — the equation’s age and sex terms are a population-level correction for that, not an assumption about any individual’s health. Estimating equations have been revised over time as better data on these relationships becomes available; the equation used here is the CKD-EPI 2021 version, the current standard recommended by the major US kidney-health professional bodies.

A single eGFR result is a screening number, not a diagnosis on its own. Creatinine — and therefore the eGFR calculated from it — can shift temporarily with dehydration, acute illness, recent heavy exercise, or diet, which is why standard clinical practice for identifying chronic kidney disease relies on a reduced eGFR (or other evidence of kidney damage) confirmed across at least two tests separated by three or more months, rather than a single reading. eGFR is also typically interpreted alongside other markers, such as the amount of protein in urine (albuminuria), particularly in the normal-to-mildly-reduced range where eGFR alone is a less complete picture.