Kidney Function Calculator — BUN/Cr, FENa, Osmolality

Calculate BUN/creatinine ratio, FENa (fractional excretion of sodium), and calculated serum osmolality with osmolar gap — clinical AKI interpretation aids.

July 7, 2026

BUN (mg/dL)

Serum Creatinine (mg/dL)

BUN/Creatinine Ratio

16.0:1

Within the normal 10:1–20:1 range — no ratio-based signal of pre-renal or intrinsic renal pathology.

BUN/Cr Ratio = BUN (mg/dL) ÷ Serum Creatinine (mg/dL). Normal is roughly 10:1 to 20:1. A ratio above 20:1 points toward a pre-renal cause; a ratio below 10:1 points toward intrinsic renal disease or low creatinine production (malnutrition, low muscle mass).

Clinical decision-support tool. These three calculations are lab-interpretation aids for healthcare providers and require correlation with the full clinical picture — they are not diagnostic on their own and are not intended for patient self-diagnosis or self-treatment.

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Reference Values

Last verified: Jul 6, 2026

Category	Range	What It Means	Status
BUN/Cr ratio — normal ★	10:1 to 20:1	Typical ratio of BUN to serum creatinine (both in mg/dL) in a patient without significant renal or pre-renal pathology.	★ Best
BUN/Cr ratio — elevated (>20:1)	greater than 20:1	Suggests pre-renal azotemia — dehydration, reduced renal perfusion (e.g. heart failure, GI bleed), or a high-protein state — where the kidney reabsorbs urea disproportionately to creatinine.	Poor
BUN/Cr ratio — low (<10:1)	less than 10:1	Suggests intrinsic renal disease, malnutrition, or low muscle mass (low creatinine production), rather than a pre-renal cause.	Poor
FENa — pre-renal ★	less than 1%	Kidneys are appropriately conserving sodium in response to reduced perfusion — classic pre-renal azotemia pattern.	★ Best
FENa — indeterminate zone	1% to 2%	Overlap zone between pre-renal and intrinsic causes; interpret alongside other clinical findings rather than in isolation.	Okay
FENa — intrinsic renal damage	greater than 2%	Sodium is being wasted rather than conserved, consistent with acute tubular necrosis (ATN) or other intrinsic renal injury.	Poor
FENa validity caveat	oliguric AKI only	FENa is only validated in oliguric acute kidney injury and is NOT reliable if the patient has received diuretics recently — diuretics force natriuresis and invalidate the result regardless of the underlying cause.	Okay
Serum osmolality — normal ★	275 to 295 mOsm/kg	Typical calculated serum osmolality range using sodium, glucose, and BUN.	★ Best
Osmolar gap — normal	under 10 mOsm/kg	Difference between a lab-measured osmolality and the calculated value; a normal gap means no significant unmeasured osmoles are present.	Good
Osmolar gap — elevated	greater than 10 mOsm/kg	Suggests unmeasured osmotically active substances in the blood — classically toxic alcohols (methanol, ethylene glycol, isopropyl alcohol) — and warrants urgent clinical correlation.	Poor

Source: MDCalc FENa (Fractional Excretion of Sodium) calculator methodology; "The meaning of the BUN/creatinine ratio in acute kidney injury," PMC; LITFL Osmolar Gap CCC review. Reference ranges reflect commonly cited clinical cutoffs — local lab reference ranges and clinical context always take precedence.

Worked Examples

BUN/Cr Ratio — Pre-Renal Azotemia

BUN: 45 mg/dL
Serum Creatinine: 1.2 mg/dL

37.5:1 ratio

45 ÷ 1.2 = 37.5, well above the 20:1 upper limit — a pattern consistent with dehydration or reduced renal perfusion (pre-renal azotemia) rather than intrinsic kidney damage.

BUN/Cr Ratio — Low Ratio

BUN: 8 mg/dL
Serum Creatinine: 1.2 mg/dL

6.67:1 ratio

8 ÷ 1.2 = 6.67, below the 10:1 lower limit — suggests intrinsic renal disease, malnutrition, or low muscle mass rather than a pre-renal cause.

FENa — Pre-Renal Pattern

Urine Na: 20 mEq/L
Serum Creatinine: 1.5 mg/dL
Serum Na: 140 mEq/L
Urine Creatinine: 60 mg/dL

0.36% FENa

(20 × 1.5) ÷ (140 × 60) × 100 = 30 ÷ 8,400 × 100 = 0.36%, well under 1% — the kidneys are appropriately conserving sodium, consistent with pre-renal azotemia (assuming no recent diuretic use).

FENa — Intrinsic Renal Damage (ATN)

Urine Na: 60 mEq/L
Serum Creatinine: 2.0 mg/dL
Serum Na: 138 mEq/L
Urine Creatinine: 40 mg/dL

2.17% FENa

(60 × 2.0) ÷ (138 × 40) × 100 = 120 ÷ 5,520 × 100 = 2.17%, above 2% — sodium is being wasted rather than conserved, a pattern consistent with acute tubular necrosis (ATN) or other intrinsic renal injury.

Serum Osmolality With an Elevated Osmolar Gap

Serum Na: 140 mEq/L
Glucose: 100 mg/dL
BUN: 14 mg/dL
Measured Osmolality: 310 mOsm/kg

290.6 mOsm/kg calculated, gap 19.4

Calculated = 2×140 + (100÷18) + (14÷2.8) = 280 + 5.56 + 5.00 = 290.6 mOsm/kg. Gap = 310 − 290.6 = 19.4, above the 10 mOsm/kg threshold — suggesting unmeasured osmoles such as a toxic alcohol and warranting urgent clinical correlation.

How to Use This Calculator

1

Choose a tab

BUN/Cr Ratio, FENa, or Serum Osmolality — each sub-tool uses its own set of lab inputs.
2

Enter the lab values

BUN/Cr needs BUN and serum creatinine; FENa needs urine sodium, urine creatinine, serum sodium, and serum creatinine; Osmolality needs serum sodium, glucose, and BUN (plus an optional measured osmolality for the gap).
3

Flag diuretic use on the FENa tab

Checking this box surfaces a warning that FENa is not a valid pre-renal/intrinsic discriminator once a patient has received diuretics.
4

Read the result and interpretation banner

Each tab shows the calculated value alongside a color-coded interpretation based on the standard clinical thresholds.
5

Correlate clinically before acting

Use the result alongside volume status, medication history, and trend over time — none of these three numbers are diagnostic in isolation.

What Each Value Means

BUN/Creatinine Ratio (ratio (e.g. 15:1)): The ratio of blood urea nitrogen to serum creatinine, both in mg/dL. Used as a quick screen for pre-renal versus intrinsic causes of kidney injury.
FENa (Fractional Excretion of Sodium) (%): The percentage of filtered sodium that ends up excreted in urine, calculated from paired serum and urine sodium and creatinine values. Distinguishes pre-renal azotemia from intrinsic renal damage in oliguric acute kidney injury — invalid if the patient is on diuretics.
Calculated Serum Osmolality (mOsm/kg): An estimate of blood osmolality derived from sodium, glucose, and BUN, used to screen for unmeasured osmotically active substances when compared against a lab-measured osmolality.

Frequently Asked Questions

What is the BUN/creatinine ratio and what does it mean?

BUN/Cr Ratio = BUN (mg/dL) ÷ Serum Creatinine (mg/dL). A normal ratio is roughly 10:1 to 20:1. A ratio above 20:1 suggests pre-renal azotemia — the kidneys are reabsorbing urea disproportionately to creatinine because of dehydration, reduced renal blood flow, GI bleeding, or a high-protein state. A ratio below 10:1 suggests intrinsic renal disease, malnutrition, or low muscle mass (since creatinine production depends on muscle mass). It's a quick screening ratio, not a standalone diagnosis.

How is FENa calculated, and why doesn't it work if the patient is on diuretics?

FENa (%) = [(Urine Sodium × Serum Creatinine) ÷ (Serum Sodium × Urine Creatinine)] × 100. A FENa under 1% suggests the kidneys are appropriately conserving sodium in response to reduced perfusion (pre-renal azotemia); a FENa over 2% suggests intrinsic renal damage such as acute tubular necrosis, where the kidney has lost its ability to conserve sodium. Diuretics force sodium excretion directly, regardless of the underlying cause of kidney injury — so a recently diuresed patient can show a falsely elevated FENa even with a purely pre-renal problem. FENa is also only validated in oliguric acute kidney injury, not in normal or high urine output states. If a patient is on diuretics, Fractional Excretion of Urea (FEUrea) is the preferred alternative since urea handling is less affected by diuretic therapy.

What is calculated serum osmolality, and what does the osmolar gap tell you?

Calculated Osmolality (mOsm/kg) = 2 × Serum Sodium + (Glucose ÷ 18) + (BUN ÷ 2.8), with sodium in mEq/L and glucose/BUN in mg/dL. The normal range is roughly 275–295 mOsm/kg. If a lab also reports a directly measured osmolality (via freezing-point depression), the Osmolar Gap = Measured − Calculated. A gap under 10 mOsm/kg is normal. A gap over 10 mOsm/kg means something osmotically active is in the blood that this formula doesn't account for — classically toxic alcohols like methanol, ethylene glycol, or isopropyl alcohol, though severe renal failure, mannitol infusion, and lab error can also raise it. An elevated gap in a symptomatic patient is a time-sensitive finding.

When would a clinician use BUN/Cr ratio vs FENa to evaluate kidney injury?

BUN/Cr ratio is the faster first-pass screen — it only needs two blood values that are typically already drawn together, so it's often the first clue that a kidney injury looks pre-renal. FENa requires a paired urine sample (urine sodium and urine creatinine) collected around the same time as the serum labs, so it's usually reserved for cases where the picture is unclear, the patient is oliguric, or the BUN/Cr ratio is ambiguous. Neither one replaces a full clinical assessment — volume status, medication history (especially diuretics and NSAIDs), and trend over time all factor into distinguishing pre-renal, intrinsic, and post-renal causes of acute kidney injury.

Is this calculator safe for patients to use to diagnose kidney problems themselves?

No. This tool is built as a lab-interpretation aid for healthcare providers working through the differential diagnosis of acute kidney injury, not for patients to self-diagnose or self-treat. Correctly interpreting any of these three values requires an accurate, contemporaneous set of paired labs, knowledge of recent medications (especially diuretics), and clinical context that a calculator cannot capture. If you're a patient concerned about kidney function, discuss your lab results directly with your physician rather than relying on any online calculator.