Fractional Excretion of Sodium (FENa) Calculator

About this calculator

The fractional excretion of sodium (FENa) is a clinical calculation used to help distinguish the cause of acute kidney injury: it measures the percentage of filtered sodium that ends up excreted in the urine rather than reabsorbed by the kidneys. The formula is FENa = (urine sodium × plasma creatinine) ÷ (plasma sodium × urine creatinine) × 100, using creatinine as a reference marker because it is filtered but not reabsorbed under normal conditions.

A FENa below 1% generally points toward prerenal azotemia — the kidneys are functioning normally but under-perfused, so they aggressively conserve sodium — while a FENa above 2% suggests intrinsic kidney damage such as acute tubular necrosis, where damaged tubules can no longer reabsorb sodium properly. This threshold is a standard part of the bedside workup for AKI, though it is less reliable in patients who have already received diuretics. This calculator takes urine and plasma sodium and creatinine values to compute the FENa percentage.