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Fractional Excretion of Sodium Calculator

Free fractional excretion of sodium calculator for AKI — same FeNa formula as major nephrology tools, with clear prerenal vs intrinsic interpretation and urine lytes visuals.

FeNa nephrology calculator

Plasma creatinine
Urine creatinine
same unit

Plasma & urine Cr must share the same unit.

Serum sodium
mmol/L

mmol/L = mEq/L for Na⁺.

Urine sodium
mmol/L

What is FeNa?

FeNa — fractional excretion of sodium — is the percentage of filtered sodium that ends up in the urine. This sodium excretion calculator turns paired serum and urine electrolytes into that percent so you can support urine lytes interpretation in acute kidney injury. It is a classic fena nephrology calculator endpoint: low FeNa suggests the tubules are still reclaiming sodium (often prerenal); higher FeNa suggests tubular injury (often intrinsic / ATN) — with important caveats below.

Written by Dr. Emily Carter, PharmD, BCPS Reviewed August 2026

Sodium at the nephron

Why FeNa rises or falls in AKI.

Glomerulus Afferent Na⁺ + Cr filtered Efferent blood out Tubule reabsorbs Na⁺ prerenal → FeNa falls Urine Na⁺ out ● Na⁺ / blood ● Creatinine marker

FeNa normal range bands

AKI bedside guide (not a single “lab normal”).

Euvolemic adults often sit near ~1% — the normal fractional excretion of sodium context differs from AKI cut-offs.

FeNa formula

The standard fractional excretion of sodium formula (identical structure to MDCalc / MSD-style tools):

Fractional excretion of sodium FeNa %
FeNa = (PCr × UNa) (PNa × UCr) × 100

Keep plasma & urine creatinine in the same unit (mg/dL or µmol/L). Na⁺ in mmol/L = mEq/L.

How to calculate FeNa

  1. Draw simultaneous serum Na, serum Cr, urine Na, urine Cr.
  2. Match creatinine units on both specimens.
  3. Plug into the FeNa formula above (or use this calculator).
  4. Apply FeNa interpretation bands and check FeNa and diuretics.

FeNa calculation example

PNa 140 · PCr 2.4 · UNa 12 · UCr 120

  1. Numerator: 2.4 × 12 = 28.8
  2. Denominator: 140 × 120 = 16,800
  3. FeNa: (28.8 ÷ 16,800) × 100 = 0.17%
  4. Read: FeNa <1% → suggests prerenal pattern if not on diuretics.
FeNa0.17%

FeNa less than 1 meaning

FeNa less than 1 meaning: tubules are reclaiming almost all filtered sodium — the classic signal for prerenal physiology. Remember low FeNa is not exclusive to hypovolemia: hepatorenal syndrome, cardiorenal syndrome, contrast nephropathy, and acute glomerulonephritis can also sit <1%.

FeNa prerenal vs intrinsic renal

Side-by-side map

Prerenal FeNa < 1% UNa often < 20 Intact tubules Intrinsic FeNa > 2% UNa often > 40 Tubular injury (ATN)

What is FeNa in AKI?

In oliguric AKI without confounders, FeNa helps separate low-perfusion states from ATN. Best performance is in patients without CKD, obstruction, or acute glomerular disease — and without recent diuretics.

FeNa vs FeUrea (related index)

FeNa vs FeUrea: this page is a dedicated fractional excretion of sodium calculator. When FeNa and diuretics conflict, clinicians often prefer FeUrea because loop/thiazide agents raise urine sodium. A separate FeUrea tool can compute that later — here the focus stays on FeNa for what is FeNa in AKI and FeNa prerenal vs intrinsic renal interpretation.

FeUrea (%) = (PCr × Uurea) ÷ (Purea × UCr) × 100 · commonly <35% suggests prerenal and >50% suggests intrinsic when FeNa is unreliable.

FeNa and diuretics

If the patient recently received a loop or thiazide diuretic, treat an elevated FeNa as unreliable for ATN. Prefer FeUrea in that setting, review the medication list, and lean on exam, weight, ultrasound, and sediment.

Urine sodium creatinine ratio

The urine sodium creatinine ratio is a quick companion metric shown in the results card for urine lytes interpretation. FeNa remains the preferred sodium excretion calculator endpoint because it also accounts for plasma sodium and the creatinine concentration effect.

Fractional excretion of potassium (related)

Fractional excretion of potassium (FEK) uses the same ratio structure as FeNa — (UK × PCr) ÷ (PK × UCr) × 100 — for K wasting vs conservation questions. It is a related nephrology index, not a substitute for this FeNa calculator in AKI typing.

FAQ

What is FeNa?

FeNa is the percent of filtered sodium excreted in urine. It helps differentiate prerenal from intrinsic AKI when read with clinical context.

What is the FeNa formula?

FeNa (%) = (PCr × UNa) ÷ (PNa × UCr) × 100 with paired labs in consistent units.

What does FeNa less than 1 mean?

It suggests avid sodium reabsorption — classic for prerenal AKI off diuretics, but also possible in HRS, cardiorenal, contrast nephropathy, and some GN.

FeNa prerenal vs intrinsic renal — how do I read it?

Rough guide: <1% prerenal, >2% intrinsic, 1–2% indeterminate. Always integrate volume status, sediment, and drugs.

When should I use FeUrea?

When diuretics may falsely raise FeNa. FeUrea <35% suggests prerenal; >50% suggests intrinsic. This page calculates FeNa only.

What is FeNa in AKI?

It estimates tubular sodium handling during acute kidney injury to support prerenal vs ATN-type reasoning.

What is a normal fractional excretion of sodium?

Euvolemic adults often near ~1%. AKI uses <1% and >2% bands rather than a single universal normal range.

How do you calculate FeNa step by step?

Pair serum Na/Cr with urine Na/Cr, keep Cr units matched, compute (PCr×UNa)/(PNa×UCr)×100, then interpret.

What is the urine sodium creatinine ratio for?

A simpler urine lytes clue; FeNa is still preferred because it includes plasma sodium and creatinine effects.

What about fractional excretion of potassium?

FEK = (UK×PCr)/(PK×UCr)×100 for renal K handling. Related to FeNa mathematically, but not used as the AKI FeNa substitute on this page.

Dr. Emily Carter, PharmD, BCPS
Dr. Emily Carter, PharmD, BCPS
Board-Certified Clinical Pharmacist · Nephrology & Internal Medicine

Dr. Emily Carter, PharmD, BCPS is a board-certified clinical pharmacist specializing in nephrology and internal medicine. She has over 12 years of experience optimizing medication therapy for patients with chronic kidney disease, acute kidney injury, and critically ill adults. Her work focuses on evidence-based dosing, renal pharmacotherapy, and improving medication safety. She reviews all clinical content to ensure accuracy, clarity, and alignment with current medical guidelines.

This FeNa calculator is educational only. FeNa/FeUrea never replace clinical judgment, imaging, or nephrology consultation.