How to Use FeNa Calculator
The FeNa Calculator estimates fractional excretion of sodium using four common lab values: urine sodium, serum sodium, urine creatinine, and serum creatinine. It is designed for health education and formula checking, especially when learning how sodium handling is discussed in acute kidney injury.
Enter urine sodium and serum sodium in mEq/L or mmol/L. For sodium, those units are numerically equivalent for this purpose, so the values can be used directly. Then enter urine creatinine and serum creatinine. Creatinine values may be entered as mg/dL, µmol/L, or mmol/L. The FeNa Calculator converts creatinine values to a common basis before calculating the percentage.
The result card shows the FeNa percentage rounded to two decimals. It also displays a short interpretation: below 1% is commonly associated with prerenal low-perfusion patterns, 1% to 2% is a gray zone, and above 2% is commonly associated with tubular or intrinsic kidney injury patterns. These descriptions are not diagnostic rules. Diuretic use, chronic kidney disease, very low urine output, recent fluid therapy, and the full clinical history can all change how the number should be interpreted.
Use the optional checkboxes to mark contextual factors such as diuretics, chronic kidney disease, oliguria or anuria, and recent IV fluids. These selections do not change the FeNa formula. They simply add reminders below the result so the output is easier to read responsibly.
Formula & Theory - FeNa Calculator
The FeNa Calculator uses the standard fractional excretion of sodium formula:
FeNa(%) = (Urine Sodium × Serum Creatinine) ÷ (Serum Sodium × Urine Creatinine) × 100Where:
| Term | Meaning |
|---|---|
| Urine Sodium | Sodium concentration in urine |
| Serum Sodium | Sodium concentration in blood |
| Urine Creatinine | Creatinine concentration in urine |
| Serum Creatinine | Creatinine concentration in blood |
The key unit rule is consistency. Sodium in mEq/L and mmol/L is treated as equivalent for sodium ions. Creatinine units must match because urine creatinine and serum creatinine are divided against each other. This tool converts creatinine using common clinical relationships before calculating:
Creatinine µmol/L ÷ 88.4 = creatinine mg/dL
Creatinine mmol/L × 11.312 = creatinine mg/dLThe FeNa Calculator also shows the substitution line, so users can see exactly how the values were placed into the formula. That transparency is useful when checking work, teaching the formula, or comparing the same labs after changing units.
Use Cases for FeNa Calculator
The FeNa Calculator is useful for students, clinicians in training, and health educators who want a quick way to understand the FeNa formula without sending data to a server.
- Formula learning: See how urine sodium, serum sodium, urine creatinine, and serum creatinine interact in the FeNa formula.
- Acute kidney injury education: Review why FeNa is often discussed when comparing prerenal physiology with intrinsic tubular injury.
- Unit conversion practice: Check how creatinine values in mg/dL, µmol/L, and mmol/L can be normalized before calculation.
- Clinical discussion preparation: Generate a clear, readable value and substitution line to bring into supervised learning or documentation review.
This FeNa Calculator should be used cautiously. A low or high result can be misleading in the presence of diuretics, chronic kidney disease, obstructive patterns, sepsis, contrast exposure, or mixed causes of acute kidney injury. The calculator is a local browser tool for education only, not a diagnostic or treatment system.