Kidney lab calculator
BUN to Creatinine Ratio Calculator
Your BUN to creatinine ratio is BUN (mg/dL) divided by creatinine (mg/dL). A normal adult ratio is about 10 to 20. A ratio above 20 may suggest dehydration or low blood flow to the kidneys. A ratio of 30 or more may suggest bleeding in the upper gut. Using urea in mmol/L? Multiply it by 2.8 first, and divide creatinine in µmol/L by 88.4.
Free calculator
What is your BUN to creatinine ratio?
Enter two numbers from your blood test. You get your ratio, its band and what it may mean.
Required field
Enter your values and press Calculate.
Your BUN to creatinine ratio
0: 1
Scale 0 to 40. Bands: under 10, 10 to 20, over 20, 30 or more.
BUN:
Creatinine:
Formula used:
Critical value: your BUN is over 65 mg/dL or your creatinine is over 5.0 mg/dL. Many labs report these values urgently. Call your doctor today. If you feel very unwell, are confused, are passing little urine, have black stools or are vomiting blood, go to an emergency department now.
This tool is for education only. It does not diagnose any condition. Your lab’s reference range and your clinician’s judgment come first.
What does your ratio band mean?
Your band shows how BUN compares with creatinine. It points to possible causes. It does not give a diagnosis.
What can a low ratio mean?
A low ratio may suggest low protein intake, advanced liver disease or rhabdomyolysis (fast muscle breakdown). Source: Hosten 1990.
What is the normal range?
Most adults sit at 10 to 20. Source: MDCalc.
What can a high ratio mean?
A high ratio may suggest a prerenal cause, meaning reduced blood flow to the kidneys, such as dehydration. Source: MDCalc.
When does GI bleeding come up?
Above 30, the ratio flags upper GI bleeding with 95 to 99% specificity. Source: Higgins.
Which reference ranges apply to you?
Use the ranges printed on your own report first. The tables below show common adult values.
| Ratio | Band | May suggest | Source |
|---|---|---|---|
| Under 10 | Low | Low protein intake, liver disease, rhabdomyolysis, a cause inside the kidney | Hosten 1990 |
| 10 to 20 | Normal | Usual adult range | MDCalc |
| Over 20 to under 30 | High | Dehydration, heart failure, high protein intake, steroids | MDCalc, Higgins |
| 30 or more | Very high | Upper GI bleeding, marked dehydration | Higgins |
| Test | US units | SI units | Source |
|---|---|---|---|
| BUN | 6 to 20 mg/dL | 2.1 to 7.1 mmol/L urea (calculated, ÷ 2.8) | MedlinePlus |
| Creatinine, men | 0.7 to 1.3 mg/dL | 62 to 115 µmol/L | MedlinePlus |
| Creatinine, women | 0.5 to 0.95 mg/dL | 44 to 84 µmol/L | MedlinePlus |
| Critical lab alert | BUN over 65 mg/dL; creatinine over 5.0 mg/dL | Over 23.2 mmol/L; over 442 µmol/L (calculated) | Brown University Health |
How do you calculate the BUN to creatinine ratio?
Divide BUN by creatinine, with both in mg/dL. If your lab uses SI units, convert first.
Ratio = BUN (mg/dL) ÷ creatinine (mg/dL)
BUN (mg/dL) = urea (mmol/L) × 2.8
Creatinine (mg/dL) = creatinine (µmol/L) ÷ 88.4
SI ratio = urea (mmol/L) ÷ (creatinine µmol/L ÷ 1000)
What do the variables mean?
BUN (blood urea nitrogen) is the nitrogen part of urea, a waste product your liver makes from protein. Creatinine is a waste product your muscles make. Your kidneys filter both.
What does a worked example look like?
Take urea 5.0 mmol/L and creatinine 75 µmol/L. BUN = 5.0 × 2.8 = 14.0 mg/dL. Creatinine = 75 ÷ 88.4 = 0.85 mg/dL. Ratio = 14.0 ÷ 0.85 = 16.5, which is normal. The SI ratio is 5.0 ÷ 0.075 = 66.7.
What should you know before you trust one result?
The ratio is a clue, not a verdict. These four numbers show why.
Kidney filtration can fall by about half before BUN or creatinine rises above the reference range. Source: Higgins.
Share of filtered urea your kidneys reabsorb. Creatinine is not reabsorbed. Source: Hosten 1990.
Share of 3,641 patients with acute kidney injury who had a ratio over 20. Source: Uchino 2012.
Odds ratio for hospital death when the ratio was over 40. Source: Uchino 2012.
Which other kidney tools should you use?
Pair your ratio with a kidney function estimate. Try the Creatinine Clearance Calculator and the Urine Albumin to Creatinine Ratio Calculator. Dehydration is a common cause of a high ratio, so check your fluid needs with the Daily Water Requirement Calculator.
What do people ask about the BUN to creatinine ratio?
What is a normal BUN to creatinine ratio?
A normal adult BUN to creatinine ratio is about 10 to 20. Some labs use 10 to 24 or 12 to 20, so check the range printed on your report.
What does a BUN to creatinine ratio over 20 mean?
A ratio over 20 may suggest reduced blood flow to the kidneys, such as from dehydration or heart failure. High protein intake, steroids or bleeding in the gut can also raise it. A 2012 study of 3,641 patients with acute kidney injury found the ratio alone could not reliably identify the cause.
What does a BUN to creatinine ratio under 10 mean?
A ratio under 10 may suggest low protein intake, advanced liver disease, rhabdomyolysis or damage inside the kidney. Pregnancy can also lower it.
How do I calculate the ratio from urea in mmol/L?
Multiply urea in mmol/L by 2.8 to get BUN in mg/dL, and divide creatinine in µmol/L by 88.4 to get mg/dL. Then divide BUN by creatinine. Urea 5.0 mmol/L and creatinine 75 µmol/L give a ratio of 16.5.
Can a high ratio point to bleeding in the stomach?
Yes, it can. A ratio above 30 identifies upper gastrointestinal bleeding with about 95 to 99 percent specificity but only about 50 percent sensitivity, so a lower ratio does not rule bleeding out. Black stools or vomiting blood need urgent care.
Does dehydration raise the BUN to creatinine ratio?
Yes. When you are short of fluid, your kidneys reabsorb more urea, so BUN rises faster than creatinine. Creatinine is not reabsorbed, so the ratio climbs.
Which sources support this page?
Every figure on this page comes from the peer-reviewed papers, clinical references and laboratory sources below.
References
Hosten AO. BUN and creatinine. In: Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Boston: Butterworths; 1990. Chapter 193. ncbi.nlm.nih.gov/books/NBK305
Higgins C. Urea and creatinine concentration, the urea:creatinine ratio. acutecaretesting.org. Radiometer. acutecaretesting.org
Uchino S, Bellomo R, Goldsmith D. The meaning of the blood urea nitrogen/creatinine ratio in acute kidney injury. Clin Kidney J. 2012;5(2):187-191. PMC5783213
Morgan DB, Carver ME, Payne RB. Plasma creatinine and urea:creatinine ratio in patients with raised plasma urea. Br Med J. 1977;2(6092):929-932. PMC1631607
MDCalc. BUN Creatinine Ratio. MDCalc. mdcalc.com/calc/10501
National Library of Medicine. BUN (blood urea nitrogen). MedlinePlus Medical Encyclopedia. medlineplus.gov/ency/article/003474
National Library of Medicine. Creatinine blood test. MedlinePlus Medical Encyclopedia. medlineplus.gov/ency/article/003475
Brown University Health Laboratories. Critical values list. Brown University Health. brownhealth.org
Disclaimer: this page is for education only. It does not replace advice from your doctor. Lab ranges vary, so use the range on your report.
Ready to check another result?
Run new numbers in seconds, or estimate your kidney function next.