FENa Calculator (Fractional Excretion of Sodium)
Distinguish prerenal from intrinsic renal azotemia in seconds.
Enter your values
Enter the urine sodium (una) in mEq/L.
Enter the serum sodium (sna) in mEq/L.
Enter the urine creatinine (ucr) in mg/dL.
Enter the serum creatinine (scr) in mg/dL.
Enter the urine urea (optional, for feurea) in mg/dL.
Enter the serum bun (optional, for feurea) in mg/dL.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Distinguish prerenal from intrinsic renal azotemia in seconds. |
|---|---|
| Category | Health |
| Inputs needed | Urine sodium (UNa), Serum sodium (SNa), Urine creatinine (UCr), Serum creatinine (SCr), Urine urea (optional, for FEUrea), Serum BUN (optional, for FEUrea) |
| Formula | FENa (%) = (UNa × SCr) / (SNa × UCr) × 100 |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Urine sodium (UNa)(mEq/L)
- Enter the urine sodium (una) in mEq/L.
- Serum sodium (SNa)(mEq/L)
- Enter the serum sodium (sna) in mEq/L.
- Urine creatinine (UCr)(mg/dL)
- Enter the urine creatinine (ucr) in mg/dL.
- Serum creatinine (SCr)(mg/dL)
- Enter the serum creatinine (scr) in mg/dL.
- Urine urea (optional, for FEUrea)(mg/dL)
- Enter the urine urea (optional, for feurea) in mg/dL.
- Serum BUN (optional, for FEUrea)(mg/dL)
- Enter the serum bun (optional, for feurea) in mg/dL.
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.
What do I need to enter into the FENa Calculator?
Just 6 values: urine sodium (una), serum sodium (sna), urine creatinine (ucr), serum creatinine (scr), urine urea (optional, for feurea), serum bun (optional, for feurea). Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the FENa Calculator work out the 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. It applies the formula FENa (%) = (UNa × SCr) / (SNa × UCr) × 100 and shows the working so you can check each step by hand.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.