Fractional excretion of urea (FEUrea) is the percentage of filtered urea that the kidneys actually excrete, calculated as (Urine Urea × Serum Creatinine) / (Serum Urea × Urine Creatinine) × 100, and it is most often used to help separate prerenal azotemia from intrinsic acute kidney injury. The formula is a ratio of two paired concentrations, so the arithmetic is identical to dividing one fraction by another — exactly the operation a fraction calculator is designed to handle cleanly. Enter the urine and serum values as two fractions, choose the division operation, and the tool returns the result in lowest terms, as a decimal, and as a mixed number when the numerator is larger than the denominator. All four numbers must come from the same lab draw, paired serum and urine, because the ratio only makes sense when the kidneys were filtering the same blood at the same moment. The decimal the tool displays is the raw fraction; multiplying it by 100 gives the percentage clinicians report. None of the calculation requires rounding the input values, because every step runs on whole-number arithmetic and only simplifies at the end, so the answer stays exact from the moment you finish typing.
What Fractional Excretion of Urea Tells the Care Team
The four values in the FEUrea formula describe how hard the kidneys are working to hold on to urea. When the kidneys sense that blood flow to them is dropping — dehydration, heart failure, blood loss, or redistribution of fluid — they reabsorb urea aggressively to preserve nitrogen. A small share of the filtered urea reaches the urine, and the FEUrea comes out low. When the kidney tubules themselves are damaged, as in acute tubular necrosis, the reabsorption machinery stops working. A much larger share of filtered urea leaks through, and the FEUrea comes out high.
Clinicians order FEUrea when acute kidney injury is suspected and they want to know whether the cause is a problem before the kidney (prerenal) or inside the kidney (intrinsic). It is especially useful when the patient is already on diuretics, because loop diuretics distort the more familiar fractional excretion of sodium (FENa) and make FENa hard to trust. Urea reabsorption is less affected by diuretic pharmacology, which is why FEUrea is the go-to alternative. The fraction the formula produces is not a diagnosis on its own — it is one input the clinician weighs alongside urine output, imaging, medication history, and the trend in serum creatinine.
The Four Lab Values You Need
Every FEUrea calculation starts from the same four inputs, drawn as a paired serum and urine sample. Each one describes a concentration in a different fluid, and the values are typically reported in milligrams per deciliter (mg/dL). The calculator only accepts whole-number numerators and denominators, so any decimal lab value such as 1.5 mg/dL must be converted to a fraction (3/2) before it is typed in. Mixed units in a single calculation will also produce a meaningless answer, so keep both urea values and both creatinine values in the same unit system.
| Input | Typical unit | What it represents |
|---|---|---|
| Urine urea | mg/dL | Concentration of urea in the urine sample |
| Serum urea (BUN) | mg/dL | Concentration of urea nitrogen circulating in the blood |
| Urine creatinine | mg/dL | Concentration of creatinine in the urine sample |
| Serum creatinine | mg/dL | Concentration of creatinine circulating in the blood |
The pair on each row matters more than the absolute number, because the calculation is a ratio of urine concentration to serum concentration for two different solutes. The fractional excretion of any filtered solute follows the same shape — urine concentration times the reciprocal serum concentration for that solute, then compared with the same ratio for creatinine to cancel out the effect of water reabsorption. The Fraction Calculator applies this kind of ratio arithmetic without telling you which solutes are involved.
Entering the Values Into the Fraction Calculator
Because FEUrea can be rewritten as (Urine Urea ÷ Serum Urea) ÷ (Urine Creatinine ÷ Serum Creatinine), it maps directly onto the divide operation of the free Fraction Calculator. The tool combines two fractions with a single operation and shows the simplified fraction, the decimal, and the worked-out steps as soon as you finish typing — there is no calculate button to press. To run a FEUrea problem:
- In the first fraction box, type the urine urea value as the numerator and the serum urea (BUN) value as the denominator.
- In the second fraction box, type the urine creatinine value as the numerator and the serum creatinine value as the denominator.
- Open the operation dropdown and select ÷ (divide).
- Read the simplified fraction, the decimal value, and the step-by-step working that appear below the inputs.
- Multiply the displayed decimal by 100 to express the result as the percentage clinicians usually quote.
If you prefer to enter the formula in its multiplied-out form, type (Urine Urea × Serum Creatinine) as the top of a single fraction and (Serum Urea × Urine Creatinine) as the bottom — do the multiplications first, then put the products into one fraction box and let the calculator reduce the result. Both routes give the same answer, and the tool uses whole-number arithmetic internally, so there is no rounding drift between the two approaches.
Worked Example With One Set of Paired Labs
Suppose a paired lab draw returns urine urea of 150 mg/dL, serum BUN of 30 mg/dL, urine creatinine of 150 mg/dL, and serum creatinine of 3 mg/dL. Run the calculation as a division of two fractions in the calculator:
- First fraction: 150 / 30, which simplifies to 5
- Second fraction: 150 / 3, which simplifies to 50
- Operation: 5 ÷ 50 = 5 × (1/50) = 5/50
- Reduce 5/50 by the GCD of 5: 1/10
As a single multiplied-out fraction, the same numbers produce 450 / 4500, and the Euclidean reduction step finds a GCD of 450 and divides both numerator and denominator by it to reach 1/10. The decimal column of the Fraction Calculator reads 0.1 exactly, the simplified fraction reads 1/10. Multiplying 0.1 by 100 gives an FEUrea of 10 percent.
Reading the Simplified Fraction, Decimal, and Mixed Number
The Fraction Calculator returns three views of the same answer at once. The simplified fraction is the form most useful on paper — for this example, 1/10 — and it is the form that lets a clinician or student communicate the ratio at a glance. The decimal column gives a quick sense of size without further arithmetic; the calculator marks the value as exact when the decimal terminates (such as 1/2 = 0.5) and as approximate when it repeats (such as 1/3 ≈ 0.3333333333), so it is always clear which values can be trusted to full precision. The mixed-number column appears only when the numerator is larger than the denominator, which usually does not happen with a typical FEUrea — most results sit below one — but the column is there if a future calculation produces an improper fraction.
To turn the decimal into the percentage clinicians report, multiply by 100 in your head, on paper, or with a basic calculator. If you would rather go straight from the simplified fraction to a percentage without an intermediate decimal, the guide on how to calculate fraction to percentage in one step walks through the short conversion.
How Clinicians Tend to Interpret the Result
FEUrea is reported as a percentage, and the size of that percentage tells the clinician how aggressively the tubules are reabsorbing urea. A low value points toward a prerenal pattern, in which the kidneys themselves are healthy but the blood supply reaching them is reduced. A high value points toward intrinsic tubular injury, such as acute tubular necrosis, where the reabsorption machinery has stopped working and a much larger share of filtered urea is escaping into the urine.
The exact threshold used to draw the line between the two patterns depends on the reference and the clinical setting, and the percentage should be confirmed against the cutoff your lab or institution publishes rather than memorized from a single number. The percentage is also only one input among several. Urine output, blood pressure trends, the patient's medication list, the trajectory of serum creatinine, and any imaging findings all weigh into the final call. FEUrea is not a static measurement either — it can shift as the clinical picture evolves, and a single value on a single day is rarely the whole story.
Why Browser-Only Calculation Matters for Lab Values
Lab values are protected health information, and pasting them into a tool that uploads to a remote server is rarely appropriate. The Fraction Calculator runs every multiplication, division, and simplification step locally in the browser, and the product contract confirms that nothing leaves the page while the calculation runs. There is no account to create, no submit button that posts data, and no analytics ping attached to your numbers — the result appears the moment the last digit is typed.
For a clinician working through several paired lab sets at once, this also means the tool behaves like scratch paper that does the reduction for you. You can enter values, read the simplified fraction and decimal, and discard the page without a trace. The arithmetic is performed with integers and simplified only at the end, so a stack of fractions that produces rounding drift on a standard decimal calculator — say, 1/3 + 1/3 + 1/3 — stays exact, and the same precision carries through to your FEUrea calculation.