FENa Calculator — Fractional Excretion of Sodium
FENa = (U_Na × P_Cr) / (P_Na × U_Cr) × 100. Enter urine and plasma sodium and creatinine values to calculate the Fractional Excretion of Sodium and distinguish pre-renal azotemia from intrinsic acute kidney injury.
mEq/L
mEq/L
mg/dL
mg/dL
Pre-renal (FENa < 1%)
- 1
Numerator (U_Na × P_Cr)
20 × 1.5 = 30 - 2
Denominator (P_Na × U_Cr)
140 × 100 = 14,000 - 3
FENa (%)
(30 ÷ 14,000) × 100 = 0.21Fraction of filtered sodium that is excreted in the urine, expressed as a percentage.
How does this calculator work?
FENa (%) = (U_Na × P_Cr) / (P_Na × U_Cr) × 100. FENa < 1% indicates pre-renal azotemia (intact tubules conserving Na); FENa > 2% indicates intrinsic renal failure (tubular damage). The 1–2% range is indeterminate. Not valid in diuretic users — use FEUrea instead.
Formula
How this is calculated
The Fractional Excretion of Sodium (FENa) quantifies what percentage of the sodium filtered by the glomerulus is ultimately excreted in the urine. In a healthy kidney that is sodium-depleted, almost all filtered sodium is reabsorbed (FENa < 1%). When kidney tubules are damaged — as in acute tubular necrosis (ATN) — sodium reabsorption is impaired and FENa rises above 2%. This makes FENa useful in separating pre-renal azotemia (reduced perfusion, intact tubules) from intrinsic renal failure (tubular damage).
The formula is derived from the sodium clearance divided by creatinine clearance. Sodium clearance = (U_Na × urine volume) / P_Na; creatinine clearance = (U_Cr × urine volume) / P_Cr. Their ratio simplifies to (U_Na × P_Cr) / (P_Na × U_Cr), multiplied by 100 to express as a percentage. The urine volume cancels out, so only a spot urine sample is needed — no 24-hour collection required.
Important limitation: FENa is unreliable when a patient is taking diuretics, because diuretics force sodium excretion regardless of volume status. In that setting, the Fractional Excretion of Urea (FEUrea) is preferred. FENa should always be interpreted alongside clinical history, urine microscopy, and other biochemistry.
Frequently asked questions
A FENa below 1% suggests pre-renal azotemia — the kidneys are intact but receiving reduced blood flow (for example from dehydration, heart failure, or hepatorenal syndrome) and are conserving sodium avidly. Treatment targets the underlying cause of hypoperfusion.
FENa is not reliable in patients on diuretics, because diuretics artificially raise urinary sodium excretion regardless of kidney perfusion. It is also less reliable in contrast nephropathy, myoglobinuria, and chronic kidney disease. In diuretic users, Fractional Excretion of Urea (FEUrea < 35% suggests pre-renal) is preferred.
Sodium should be in the same units for urine and plasma (typically mEq/L or mmol/L — they are numerically identical). Creatinine should be in the same units for urine and plasma (typically mg/dL). As long as units match within each pair, the calculation is unit-consistent.
Also known as
TG we-Calculate Editorial Team. (2026). FENa Calculator — Fractional Excretion of Sodium [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/fena-calculator
TG we-Calculate Editorial Team. "FENa Calculator — Fractional Excretion of Sodium." TG we-Calculate. 2026. https://we-calculate.com/calculator/fena-calculator.
TG we-Calculate Editorial Team, "FENa Calculator — Fractional Excretion of Sodium," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/fena-calculator
@misc{wecalculate_fena_calculator, title = {FENa Calculator — Fractional Excretion of Sodium}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/fena-calculator}}, year = {2026}, note = {TG we-Calculate} }
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