FENa Calculator (Fractional Excretion of Sodium)
Distinguish prerenal from intrinsic renal azotemia in seconds.
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ⓘ Live results
- FENa
- 0.286 %
Below 1% — consistent with a prerenal cause (hypovolaemia, heart failure, cirrhosis).
Working
- 1Numerator: UNa × SCr = 20 × 2 = 40
- 2Denominator: SNa × UCr = 140 × 100 = 14000
- 3FENa = 40 / 14000 × 100 = 0.286%
Quick answer
FENa is the percentage of filtered sodium excreted in the urine: FENa = (UNa × SCr) / (SNa × UCr) × 100. Below 1% suggests a prerenal cause, above 2% suggests intrinsic renal injury such as acute tubular necrosis.
FENa (%) = (UNa × SCr) / (SNa × UCr) × 100
How to use the FENa Calculator
- 1Collect paired urine and serum sodium and creatinine values.
- 2Enter all four numbers in the matching units.
- 3Read FENa and the prerenal / intrinsic interpretation. Add urea values for FEUrea if the patient is on diuretics.
Frequently asked questions
What is a normal FENa?
In a patient with normal kidneys and normal volume status FENa is usually under 1%. In acute kidney injury, FENa under 1% points to a prerenal cause and over 2% to intrinsic renal disease.
When should you use FEUrea instead of FENa?
When the patient has received diuretics, which artificially raise urinary sodium. FEUrea under 35% still suggests a prerenal state.
Is this a substitute for clinical judgement?
No. FENa is one data point and is unreliable in chronic kidney disease, contrast nephropathy and glomerulonephritis. Always interpret it alongside the full clinical picture.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
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Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.