BUN/Creatinine Ratio Calculator

Range	BUN/Cr Ratio	Typical meaning
Low	Below 10:1	Intrinsic renal disease, malnutrition, liver disease, or rhabdomyolysis
Normal	10:1 – 20:1	No strong signal toward a prerenal or intrinsic renal process
Elevated	Above 20:1	Prerenal azotemia, GI bleed, high-protein intake, or steroid use

Direction	Cause	Why it shifts the ratio
Elevated	Prerenal azotemia (dehydration / volume depletion)	Reduced renal perfusion increases passive urea reabsorption more than creatinine, which is not reabsorbed.
Elevated	GI bleed	Digested blood is a large protein load that raises BUN independent of kidney function.
Elevated	High-protein intake	More dietary protein increases urea production, raising BUN relative to creatinine.
Elevated	Corticosteroid use	Steroids increase protein catabolism, raising urea production.
Low	Intrinsic renal disease	Damaged tubules reabsorb urea less efficiently, so BUN rises less than creatinine.
Low	Malnutrition / low-protein diet	Less dietary protein means less urea is produced in the first place.
Low	Liver disease	Urea is synthesized in the liver; impaired synthesis lowers BUN independent of kidney function.
Low	Rhabdomyolysis	Muscle breakdown releases creatinine, raising it disproportionately relative to BUN.

📋

Walk-through

How to Use This Calculator

3 steps

1

Enter BUN

Enter the blood urea nitrogen (BUN) value in mg/dL, taken from a basic metabolic panel or comprehensive metabolic panel.

2

Enter creatinine

Enter the serum creatinine in mg/dL, drawn from the same panel as the BUN so the two values reflect the same point in time.

3

Read the ratio and interpretation

The calculator divides BUN by creatinine instantly and classifies the result — low (below 10:1), normal (10:1-20:1), or elevated (above 20:1). Check the Interpretation tab to see your value highlighted against every range, and the Common Causes tab for what typically drives the ratio up or down.

⚡

Reference

Formula & Methodology

1 formula

BUN/creatinine ratio

BUN/Cr = BUN ÷ Creatinine

Both BUN and creatinine are entered in mg/dL. The result is a unitless ratio, conventionally expressed as X:1. A normal ratio is roughly 10:1 to 20:1; values outside that range point toward a prerenal or intrinsic renal process, per standard nephrology teaching and MDCalc's BUN:Creatinine Ratio reference.

✓

High YMYL

Trust, Methodology & Sources

Reviewed by Calculover Clinical Team · Updated 2026-07-22 · 3 sources

Editorial accountability

Author: Calculover Clinical Team - Clinical content team

Owner: Calculover Editorial - Editorial oversight

Reviewer: Calculover Clinical Team - Clinical reviewer

Last reviewed: 2026-07-22

Last verified: 2026-07-22

Data effective: 2026-07-22

Methodology

Computes BUN/Cr = blood urea nitrogen ÷ serum creatinine (both mg/dL) and classifies the result against the standard nephrology reference range of roughly 10:1-20:1, per MDCalc's BUN:Creatinine Ratio reference and standard teaching on prerenal versus intrinsic renal azotemia.

Assumption: BUN and creatinine are assumed to be drawn from the same blood sample at the same point in time.

Assumption: The calculator does not adjust for diuretic use, dietary protein intake, corticosteroid use, muscle mass, or liver function, all of which can shift the ratio independent of renal perfusion or intrinsic kidney damage.

Assumption: A single ratio is treated as a screening data point, not a diagnosis — serial trending and additional labs (eGFR, FENa, urinalysis) are not computed by this calculator.

Limitations & guidance

The BUN/creatinine ratio alone does not identify a specific cause of an abnormal creatinine or diagnose acute kidney injury.

GI bleeding, high- or low-protein diets, corticosteroids, recent diuretic use, liver disease, and rhabdomyolysis can all distort the ratio independent of renal perfusion status.

Results are for clinical decision support and education only and do not replace a treating clinician's full assessment, including eGFR, creatinine trend, and urinalysis.

Professional guidance: This calculator is for clinical decision support and educational purposes only and is not a substitute for professional medical judgment, diagnosis, or treatment. Always consult a qualified clinician for patient care decisions.

Primary sources

MDCalc. BUN:Creatinine Ratio.

National Kidney Foundation. Blood and Urine Tests for Kidney Disease.

KDIGO Clinical Practice Guideline for Acute Kidney Injury — KDIGO Guidelines

📖

Glossary

Key Terms Explained

6 terms

BUN/creatinine ratio The ratio of blood urea nitrogen to serum creatinine (BUN ÷ Cr, both in mg/dL). A normal ratio is roughly 10:1-20:1; values above or below that range help distinguish prerenal from intrinsic renal causes of an elevated BUN or creatinine.

Prerenal azotemia An elevation in BUN and creatinine caused by reduced blood flow to the kidneys (dehydration, blood loss, heart failure) rather than direct kidney damage. Urea is reabsorbed more than creatinine during reduced perfusion, so the BUN/Cr ratio rises.

Intrinsic renal disease Kidney injury caused by damage within the kidney itself, such as acute tubular necrosis or glomerulonephritis. Damaged tubules reabsorb urea less efficiently, so BUN rises less than creatinine and the ratio tends to fall toward or below normal.

GI bleed Gastrointestinal bleeding. Digested blood in the gut is a large protein load that the body breaks down into urea, raising BUN independent of kidney function and elevating the BUN/creatinine ratio.

Malnutrition Inadequate protein or caloric intake. Because urea production depends on dietary protein, malnutrition or a low-protein diet lowers BUN relative to creatinine, pulling the ratio below normal.

Rhabdomyolysis Breakdown of skeletal muscle tissue that releases creatine and creatinine into the bloodstream. It raises creatinine disproportionately to BUN, lowering the BUN/creatinine ratio.

👥

Scenarios

Real-World Examples

3 worked examples

🟢

Normal ratio

BUN 20 mg/dL, creatinine 1.2 mg/dL

BUN 20 mg/dL Creatinine 1.2 mg/dL

20 ÷ 1.2 ≈ 16.7:1. A BUN/creatinine ratio of 16.7:1 falls within the normal range (10:1-20:1), with no strong signal toward a prerenal or intrinsic renal process from this ratio alone.

🔴

Elevated — possible prerenal azotemia

BUN 45 mg/dL, creatinine 1.0 mg/dL

BUN 45 mg/dL Creatinine 1.0 mg/dL

45 ÷ 1.0 = 45:1. A ratio of 45:1 is well above the normal range and suggests a prerenal cause such as dehydration, volume depletion, or a GI bleed — the near-normal creatinine argues against significant intrinsic renal damage.

🟣

Low — possible malnutrition

BUN 6 mg/dL, creatinine 0.9 mg/dL

BUN 6 mg/dL Creatinine 0.9 mg/dL

6 ÷ 0.9 ≈ 6.7:1. A ratio of 6.7:1 falls below the normal range, which can reflect malnutrition or a low-protein diet, liver disease with reduced urea synthesis, or intrinsic renal disease — correlate with clinical context and other renal labs.

📄

Deep Dive

Understanding the BUN/Creatinine Ratio

3 chapters · 3 min read

The BUN/creatinine ratio is a simple bedside calculation — blood urea nitrogen divided by serum creatinine — that helps clinicians distinguish prerenal azotemia from intrinsic renal disease when kidney function tests are abnormal. This calculator computes the ratio and classifies it against low, normal, and elevated reference ranges.

How the BUN/creatinine ratio works

The BUN/creatinine ratio is simply BUN divided by creatinine: BUN/Cr = BUN ÷ Cr, with both values entered in mg/dL. A normal ratio is roughly 10:1 to 20:1.

The ratio moves because BUN and creatinine are cleared differently by the kidney. Creatinine is filtered and not meaningfully reabsorbed, so it tracks glomerular filtration fairly directly. Urea, by contrast, is passively reabsorbed along with sodium and water — more so when renal perfusion is low. In prerenal states (dehydration, volume depletion, GI bleed, high protein load), urea reabsorption rises more than creatinine clearance falls, pushing the ratio above 20:1. In intrinsic renal disease, damaged tubules reabsorb urea less efficiently, so BUN rises less than creatinine and the ratio trends toward or below normal.

Inputs and what they mean

BUN (mg/dL) is blood urea nitrogen, a byproduct of protein metabolism cleared by the kidneys, typically measured on a basic or comprehensive metabolic panel.

Creatinine (mg/dL) is a muscle-metabolism byproduct filtered by the kidneys and commonly used as a marker of glomerular filtration rate. Both values should come from the same blood draw — mixing a BUN from one panel with a creatinine from another can produce a misleading ratio.

Limits and edge cases

The BUN/creatinine ratio is a screening aid, not a standalone diagnosis. Recent diuretic use, a high- or low-protein diet, corticosteroid use, GI bleeding, liver disease, and rhabdomyolysis can all shift the ratio independent of the underlying cause of an abnormal creatinine. It is most useful when interpreted alongside eGFR, the creatinine trend over time, urinalysis, and — where available — the fractional excretion of sodium (FENa), which is generally considered more specific for distinguishing prerenal azotemia from acute tubular necrosis, particularly after diuretics have been given.

❓

Questions

Frequently Asked Questions

6 questions

What is a normal BUN/creatinine ratio?+

A normal BUN/creatinine ratio (BUN divided by serum creatinine, both in mg/dL) falls roughly between 10:1 and 20:1. Values in this range indicate the two labs are rising and falling proportionately, with no strong signal toward a prerenal or intrinsic renal process.

What causes an elevated BUN/creatinine ratio?+

A ratio above roughly 20:1 most often reflects a prerenal state — dehydration or volume depletion, gastrointestinal bleeding (digested blood is a protein load that raises BUN disproportionately), a high-protein diet, or corticosteroid use, all of which raise BUN more than creatinine.

What causes a low BUN/creatinine ratio?+

A ratio below roughly 10:1 can reflect intrinsic renal disease (impaired tubular reabsorption of urea relative to creatinine), malnutrition or a low-protein diet, liver disease (reduced hepatic urea synthesis), or rhabdomyolysis, which raises creatinine disproportionately from muscle breakdown.

What units does the BUN/creatinine ratio calculator use?+

Enter both blood urea nitrogen (BUN) and serum creatinine in mg/dL, the standard US lab units. The BUN/creatinine ratio itself is a unitless number, conventionally expressed as X:1.

Should I use the BUN/creatinine ratio alongside FENa?+

Yes — the BUN/creatinine ratio and the fractional excretion of sodium (FENa) are complementary. FENa is generally considered more specific for distinguishing prerenal azotemia from acute tubular necrosis, especially in a patient who has already received diuretics, which can distort the BUN/Cr ratio.

What are the limitations of the BUN/creatinine ratio?+

The ratio alone does not diagnose a specific cause of kidney injury. It can be confounded by GI bleeding, corticosteroid use, diet, muscle mass and rhabdomyolysis, liver disease, and recent diuretic use. It should always be interpreted alongside eGFR, the creatinine trend, urinalysis, and the full clinical picture.

Download a one-page PDF of your numbers instantly. Add your email to also get our occasional calculator tips — no spam, unsubscribe anytime. Privacy.

Lab Values

BUN/Creatinine Ratio & Interpretation

BUN/Creatinine Ratio Reference Ranges

Common Causes by Direction

How to Use This Calculator

Enter BUN

Enter creatinine

Read the ratio and interpretation

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

Normal ratio

Elevated — possible prerenal azotemia

Low — possible malnutrition

Understanding the BUN/Creatinine Ratio

How the BUN/creatinine ratio works

Inputs and what they mean

Limits and edge cases

Frequently Asked Questions

Get a branded PDF of your results

BUN/Creatinine Ratio Calculator

Lab Values

BUN/Creatinine Ratio & Interpretation

BUN/Creatinine Ratio Reference Ranges

Common Causes by Direction

How to Use This Calculator

Enter BUN

Enter creatinine

Read the ratio and interpretation

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

Understanding the BUN/Creatinine Ratio

How the BUN/creatinine ratio works

Inputs and what they mean

Limits and edge cases

Frequently Asked Questions

Get a branded PDF of your results

Keep Exploring

Related calculators

Guides & articles