Fractional Excretion of Sodium Calculator
Calculate FENa to differentiate prerenal from intrinsic renal failure. Enter values for instant results with step-by-step formulas.
Calculator
Adjust values & calculatePrerenal azotemia (volume depletion, heart failure, hepatorenal syndrome). The kidneys are avidly reabsorbing sodium in response to decreased renal perfusion.
FENa Reference Ranges
Formula
Where UNa = urine sodium (mEq/L), SCr = serum creatinine (mg/dL), SNa = serum sodium (mEq/L), UCr = urine creatinine (mg/dL). FENa < 1% suggests prerenal, > 2% suggests intrinsic renal disease.
Last reviewed: January 2026
Worked Examples
Example 1: Prerenal Azotemia Evaluation
Example 2: Acute Tubular Necrosis Assessment
Background & Theory
The Fractional Excretion of Sodium Calculator applies the following established principles and formulas. Health and medicine calculators are grounded in validated physiological measurement methods established through decades of clinical research. Body Mass Index, or BMI, is calculated by dividing weight in kilograms by height in meters squared (kg/m²), a formula originating from Adolphe Quetelet's 19th-century statistical work and later codified by the WHO into standard classifications: underweight below 18.5, normal weight 18.5 to 24.9, overweight 25 to 29.9, and obese at 30 and above. Basal Metabolic Rate quantifies the minimum energy required to sustain life at rest. The Mifflin-St Jeor equation, published in 1990 and widely regarded as the most accurate for most adults, calculates BMR as (10 × weight in kg) + (6.25 × height in cm) − (5 × age) ± sex adjustment. The older Harris-Benedict equations, revised in 1984 by Roza and Shizgal, remain in common use. Total Daily Energy Expenditure is derived by multiplying BMR by a physical activity factor ranging from 1.2 for sedentary individuals to 1.9 for extremely active ones, following the methodology validated by doubly labeled water studies. Body fat percentage can be estimated without laboratory equipment using the U.S. Navy circumference method, which uses neck, waist, and hip measurements, or via BMI-derived equations adjusted for age and sex. The Jackson-Pollock skinfold method offers higher precision with calipers. Blood pressure classification, according to the American College of Cardiology and the 2017 ACC/AHA guidelines, defines normal as below 120/80 mmHg, elevated as 120 to 129 systolic, and hypertension stage 1 as 130 to 139 systolic or 80 to 89 diastolic. Target heart rate zones for aerobic exercise are derived from maximum heart rate estimates, most commonly using the formula 220 minus age in years, with moderate-intensity training typically defined as 50 to 70 percent of maximum heart rate and vigorous intensity at 70 to 85 percent, consistent with CDC and American Heart Association guidelines. These thresholds guide safe and effective cardiovascular conditioning.
History
The history behind the Fractional Excretion of Sodium Calculator traces back through the following developments. The history of health measurement stretches back to ancient Greece, where Hippocrates around 400 BCE laid the foundation for observational medicine by systematically recording patient symptoms, diet, and environment. His humoral theory, though scientifically superseded, established the principle that the body operates as an interconnected system subject to measurable imbalance. The transformation toward modern medicine accelerated in the 19th century. Louis Pasteur and Robert Koch developed germ theory in the 1860s and 1870s, identifying microorganisms as disease agents and enabling targeted interventions. Florence Nightingale, working during the Crimean War in the 1850s, introduced statistical analysis to nursing practice, demonstrating through data visualization that sanitation reduced mortality. Her work is foundational to evidence-based health measurement. The discovery of vitamins in the early 20th century, beginning with Casimir Funk's coinage of the term in 1912 and culminating in the isolation of vitamins A through K, created the field of nutritional science and gave rise to dietary reference intake frameworks. The World Health Organization, founded in 1948, subsequently established global standards for health metrics, disease classification through the International Classification of Diseases, and recommended daily allowances. The BMI as a clinical screening tool gained traction in the 1970s through Ancel Keys' large-scale epidemiological work, which validated Quetelet's index as a population-level obesity indicator. Through the 1980s and 1990s, the Framingham Heart Study produced landmark data linking cholesterol, blood pressure, and lifestyle factors to cardiovascular disease risk, directly shaping the numeric thresholds still used in health calculators. The evidence-based medicine movement, formalized by Gordon Guyatt and colleagues at McMaster University in the early 1990s, demanded that all health recommendations derive from systematically graded clinical evidence. The digital health era beginning in the 2000s brought these formulas to consumer devices, wearable sensors, and smartphone applications, expanding access to health self-monitoring on a global scale and enabling population-level data collection that continues to refine clinical reference ranges.
Frequently Asked Questions
Formula
FENa = (UNa x SCr) / (SNa x UCr) x 100
Where UNa = urine sodium (mEq/L), SCr = serum creatinine (mg/dL), SNa = serum sodium (mEq/L), UCr = urine creatinine (mg/dL). FENa < 1% suggests prerenal, > 2% suggests intrinsic renal disease.
Worked Examples
Example 1: Prerenal Azotemia Evaluation
Problem: A dehydrated patient presents with AKI. Labs show: Urine Na = 8 mEq/L, Serum Na = 140 mEq/L, Urine Creatinine = 120 mg/dL, Serum Creatinine = 2.5 mg/dL.
Solution: FENa = (UNa x SCr) / (SNa x UCr) x 100\nFENa = (8 x 2.5) / (140 x 120) x 100\nFENa = 20 / 16800 x 100\nFENa = 0.119%\nFENa < 1% indicates prerenal azotemia.
Result: FENa = 0.12% | Diagnosis: Prerenal azotemia | Treatment: Volume resuscitation
Example 2: Acute Tubular Necrosis Assessment
Problem: A patient with aminoglycoside exposure develops AKI. Labs: Urine Na = 45 mEq/L, Serum Na = 138 mEq/L, Urine Creatinine = 30 mg/dL, Serum Creatinine = 3.2 mg/dL.
Solution: FENa = (UNa x SCr) / (SNa x UCr) x 100\nFENa = (45 x 3.2) / (138 x 30) x 100\nFENa = 144 / 4140 x 100\nFENa = 3.48%\nFENa > 2% indicates intrinsic renal disease (ATN).
Result: FENa = 3.48% | Diagnosis: Intrinsic renal disease (ATN) | Stop nephrotoxins
Frequently Asked Questions
What is Fractional Excretion of Sodium (FENa) and what does it measure?
Fractional Excretion of Sodium (FENa) is a clinical laboratory calculation that measures the percentage of filtered sodium that is ultimately excreted in the urine. It serves as a key diagnostic tool to differentiate between prerenal azotemia and intrinsic renal disease in patients presenting with acute kidney injury. The test works because functioning kidneys respond to decreased perfusion by aggressively reabsorbing sodium to conserve volume, resulting in a very low FENa below 1 percent. When the renal tubules are damaged, as in acute tubular necrosis, they lose the ability to reabsorb sodium efficiently, and FENa rises above 2 percent. The calculation requires simultaneous measurement of sodium and creatinine in both urine and serum samples.
Why might my result differ from another tool or reference?
Differences typically arise from rounding conventions, the specific version of a formula (for example, simple vs compound interest), or unit inconsistencies between inputs. Check that both tools are using the same formula variant and the same units. The References section links to the authoritative source behind the formula used here.
How do I verify Fractional Excretion of Sodium Calculator's result independently?
The Formula section on this page shows the equation used. You can reproduce the calculation manually or in a spreadsheet using those steps. Compare your answer against the worked examples in the Examples section, which use known reference values so you can confirm the calculator is behaving as expected.
How do I interpret the result?
Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.
Does Fractional Excretion of Sodium Calculator work offline?
Once the page is loaded, the calculation logic runs entirely in your browser. If you have already opened the page, most calculators will continue to work even if your internet connection is lost, since no server requests are needed for computation.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.
References
Reviewed by Rahul Singh, Health & Wellness Specialist · Editorial policy