FEUrea formula: how to calculate fractional excretion of urea
Fractional excretion of urea (FEUrea, also written FEUN) is the share of filtered urea nitrogen that ends up in the urine, expressed as a percent. With a BUN of 20 mg/dL, urine urea nitrogen of 200 mg/dL, plasma creatinine of 2 mg/dL and urine creatinine of 100 mg/dL, FEUrea = 100 × (200 × 2) ÷ (20 × 100) = 20.00%.
Not a medical device. Not a diagnosis or dosing recommendation.
What is FEUrea?
FEUrea compares how much urea nitrogen the kidneys clear with how much creatinine they clear, using one blood sample and one urine sample taken at about the same time. Creatinine acts as the yardstick for filtration. If urea nitrogen and creatinine were handled the same way, the result would be 100%. Because the tubules reabsorb part of the filtered urea, the number is lower than that.
The method on this page follows Carvounis, Nisar and Guro-Razuman, who described it in Kidney International in 2002. Other names for it are FEUN, fractional urea excretion and Carvounis FEUN.
This page explains the arithmetic. It doesn't give a cutoff, and it doesn't say what a value means for a patient.
The FEUrea formula
FEUrea_percent = 100 * (urine_urea_mg_dl * plasma_creatinine_mg_dl) / (bun_mg_dl * urine_creatinine_mg_dl)In words: multiply urine urea nitrogen by plasma creatinine, divide by BUN times urine creatinine, then multiply by 100. Magent's calculator reports the result to 2 decimal places.
What each part means
| Term | Plain-English meaning | Unit | Accepted range |
|---|---|---|---|
bun_mg_dl | Blood urea nitrogen | mg/dL | 1–200 |
urine_urea_mg_dl | Urine urea nitrogen, same units as BUN | mg/dL | 1–2000 |
creatinine_mg_dl | Plasma (serum) creatinine | mg/dL | Give this or creatinine_umol_l |
creatinine_umol_l | Plasma creatinine in SI units | µmol/L | Converted ÷ 88.42 |
urine_creatinine_mg_dl | Urine creatinine | mg/dL | 1–500 |
urine_creatinine_umol_l | Urine creatinine in SI units | µmol/L | 88–44210, converted ÷ 88.42 |
feurea_percent | Fractional excretion of urea | % | 2 decimal places |
How the formula works
The same result comes out if you think of it as two ratios:
- Urine-to-blood ratio for urea nitrogen: urine urea nitrogen ÷ BUN.
- Urine-to-plasma ratio for creatinine: urine creatinine ÷ plasma creatinine.
- Divide the first ratio by the second.
- Multiply by 100 and round to 2 decimals.
For the opening example, the urea ratio is 200 ÷ 20 = 10 and the creatinine ratio is 100 ÷ 2 = 50. Then 10 ÷ 50 = 0.2, and 0.2 × 100 = 20.00%.
Written this way, the formula shows why urine concentration on its own doesn't decide the answer. A more concentrated urine raises both urine values together, and the two ratios move together.
Units and conversions
The urea pair and the creatinine pair each need matching units. BUN and urine urea nitrogen are both entered in mg/dL. For creatinine, you can enter either mg/dL or µmol/L for each sample; the calculator converts µmol/L to mg/dL by dividing by 88.42. Because creatinine appears on the top and the bottom of the fraction, the conversion factor cancels out when both creatinine values use the same unit.
A plasma creatinine of 176.84 µmol/L converts to 176.84 ÷ 88.42 = 2.0 mg/dL.
Worked examples
These use Magent's own expected values for the calculator.
Example 1: BUN 20, urine urea nitrogen 200, plasma creatinine 2, urine creatinine 100
- Top: 200 × 2 = 400
- Bottom: 20 × 100 = 2,000
- 400 ÷ 2,000 = 0.2
- FEUrea: 0.2 × 100 = 20.00%
Example 2: the same patient with plasma creatinine in µmol/L
- Plasma creatinine: 176.84 ÷ 88.42 = 2.0 mg/dL
- The rest is unchanged: FEUrea = 20.00%
Example 3: urine urea nitrogen 350
- 100 × (350 × 2) ÷ (20 × 100) = 70,000 ÷ 2,000 = 35.00%
Example 4: urine urea nitrogen 600
- 100 × (600 × 2) ÷ (20 × 100) = 120,000 ÷ 2,000 = 60.00%
Examples 3 and 4 change one input. FEUrea rises in direct proportion to urine urea nitrogen when the other three values stay fixed.
FEUrea reference table
These values are computed from the formula and rounded to 2 decimals, as the calculator reports them. Rows are the urine-to-blood urea nitrogen ratio; columns are the urine-to-plasma creatinine ratio. They show how the result moves; they aren't targets, reference ranges or categories.
| Urea N ratio (urine ÷ BUN) \ creatinine ratio (urine ÷ plasma) | 25 | 50 | 75 | 100 |
|---|---|---|---|---|
| 5 | 20.00 | 10.00 | 6.67 | 5.00 |
| 10 | 40.00 | 20.00 | 13.33 | 10.00 |
| 15 | 60.00 | 30.00 | 20.00 | 15.00 |
| 20 | 80.00 | 40.00 | 26.67 | 20.00 |
| 30 | 120.00 | 60.00 | 40.00 | 30.00 |
Example 1 sits in the row for 10 and the column for 50.
History of the formula
The method comes from C. P. Carvounis, S. Nisar and S. Guro-Razuman, writing in Kidney International in 2002 (volume 62, issue 6, pages 2223–2229). The full reference is under Source below.
The abstract sets out the reasoning. Fractional excretion of sodium (FENa) had long been used in acute renal failure, but many patients received diuretics, which decrease sodium reabsorption and so increase FENa. Urea handling, the authors wrote, is primarily dependent on passive forces and is less influenced by diuretic therapy. They compared the two measures prospectively across 102 episodes of acute renal failure.
When it's used
A 2026 meta-analysis describes FEUN as a proposed alternative to fractional excretion of sodium for separating causes of acute kidney injury in hospitalised adults.[1] It notes that diuretics are commonly given in hospital and can raise FENa, and that FEUN relies mainly on passive forces, which makes it less susceptible to that effect.[1]
Where it's used
The studies pooled in that meta-analysis were observational studies of hospitalised patients with acute kidney injury.[1] The inputs are routine laboratory values: BUN and plasma creatinine from a blood sample, and urea nitrogen and creatinine from a urine sample.
Who uses it
The same meta-analysis frames FEUN as a test used alongside clinical assessment at the bedside, not as a stand-alone answer.[1]
Limits of the calculation
- It's arithmetic on four laboratory values. It doesn't classify kidney injury or name a cause.
- FENa is a separate calculation. This calculator doesn't compute it.
- The studies behind FEUN used variable definitions of acute kidney injury and varying FEUN criteria, which limits certainty.[1]
- Diuretics and chronic kidney disease can change how the number is read, as the record notes. The calculator doesn't adjust for either.
- Inputs outside the accepted ranges are rejected, not estimated.
Clinical decisions belong to a qualified clinician who knows the patient.
How it differs from related calculators
- Fractional excretion of sodium (FENa) uses urine and plasma sodium in place of urea nitrogen. It's a separate tool.
- Creatinine clearance estimates creatinine clearance (Cockcroft–Gault) from age, weight, sex and serum creatinine. It isn't a urine-to-plasma ratio.
- eGFR estimates filtration rate from serum creatinine, age and sex (CKD-EPI).
Glossary
- BUN: blood urea nitrogen, in mg/dL.
- Urine urea nitrogen (UUN): urea nitrogen measured in a urine sample.
- Fractional excretion: the share of a filtered substance that is excreted, worked out against creatinine.
- FENa: fractional excretion of sodium, the older measure FEUrea is compared with.
- µmol/L: the SI unit for creatinine; divide by 88.42 for mg/dL.
Common mistakes
- Mixing units within a pair. BUN and urine urea nitrogen must both be in mg/dL. If one creatinine value is in µmol/L, enter it in the µmol/L field so it's converted.
- Swapping plasma and urine creatinine. Plasma creatinine goes on top with urine urea nitrogen. Swapping them changes the answer by a large factor.
- Using urine urea instead of urine urea nitrogen. The formula pairs BUN with urine urea nitrogen; both are nitrogen measures.
- Using samples taken far apart. The method assumes the blood and urine values describe the same period.
Source
Carvounis CP, Nisar S, Guro-Razuman S. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int. 2002;62(6):2223-2229. DOI: 10.1046/j.1523-1755.2002.00683.x
Further reading
- Pan HC, Jiang ZH, Chen HY, et al. Assessing the utility of fractional excretion of urea in distinguishing intrinsic and prerenal acute kidney injury in hospitalised patients: a systematic review and meta-analysis. BMJ Open. 2026;16:e100875.
Frequently asked questions
How do you calculate FEUrea?
Multiply urine urea nitrogen by plasma creatinine, divide by BUN times urine creatinine, and multiply by 100. For 200, 2, 20 and 100, that's 400 ÷ 2,000 × 100 = 20.00%.
What is the difference between FEUrea and FEUN?
None. Both names mean fractional excretion of urea nitrogen; the calculation is the same.
What is a normal FEUrea?
This page doesn't give a normal range or a cutoff. It calculates the percent; what a value means depends on the clinical context.
Why use FEUrea instead of FENa?
Carvounis and colleagues proposed it because diuretics raise FENa, while urea handling is less affected by them. The two are separate calculations.
Can I enter creatinine in µmol/L?
Yes. Each creatinine value can be entered in mg/dL or µmol/L; µmol/L is divided by 88.42.
Who developed FEUrea?
The method on this page is from C. P. Carvounis, S. Nisar and S. Guro-Razuman, published in Kidney International in 2002.