Fractional Excretion of Sodium (FENa) Calculator
The fractional excretion of sodium (FENa) is the percentage of filtered sodium that is excreted in the urine. It is used as one aid in distinguishing prerenal acute kidney injury (typically FENa below 1%) from intrinsic renal causes such as acute tubular necrosis (typically above 2%), although the cut-offs are not absolute and are affected by diuretics and other factors. Enter serum and urine sodium and creatinine to compute FENa. This is an arithmetic tool and is not a substitute for clinical judgement.
FENa formula
FENa (%) = 100 * (urine Na * serum creatinine) / (serum Na * urine creatinine)
Sodium values are in mmol/L and creatinine values in mg/dL. Because creatinine appears as a ratio, its units cancel as long as serum and urine creatinine use the same unit. The result is a percentage.
Worked example
For urine sodium 40, serum sodium 140, urine creatinine 60 mg/dL, and serum creatinine 2 mg/dL: FENa = 100 * (40 * 2) / (140 * 60) = 100 * 80 / 8400 = 0.95%. A value below 1% is consistent with a prerenal pattern, but must be interpreted in context.
Frequently asked questions
What does FENa mean?
FENa is the fraction of sodium filtered by the glomerulus that ends up in the urine, expressed as a percentage. A low value suggests the kidneys are avidly reabsorbing sodium, as happens in prerenal states such as volume depletion.
What FENa values suggest prerenal versus intrinsic injury?
A commonly cited rule is that FENa below 1% suggests a prerenal cause and above 2% suggests intrinsic renal injury such as acute tubular necrosis. These cut-offs are imperfect: diuretics, chronic kidney disease, and other conditions can raise FENa in prerenal states. Interpret alongside the full clinical picture.
Why does this use a ratio of creatinines?
FENa compares sodium clearance to creatinine clearance. Using the urine-to-serum creatinine ratio cancels the need to know urine flow rate, so FENa can be computed from a single paired spot serum and urine sample.
Is this a diagnostic tool?
No. It performs the arithmetic of the published FENa formula on values you enter. It does not diagnose and does not replace clinical assessment by a qualified clinician.
Sources
- U.S. National Library of Medicine, StatPearls: Fractional Excretion of Sodium.
- U.S. National Institute of Diabetes and Digestive and Kidney Diseases: Kidney Disease information.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. Educational tool, not medical advice. See our methodology.