BUN Creatinine Ratio Calculator (BUN:Cr, prerenal vs renal pattern)

Understanding the BUN creatinine ratio calculator — concept and principle

This calculator works out the ratio of blood urea nitrogen (BUN) to serum creatinine, converting from urea in mmol/L or creatinine in µmol/L if needed, and shows whether the ratio is low, in the usual range or in a prerenal pattern. Patients reading kidney tests and students use it.

Urea vs creatinine — with the default inputs: 25.5.

How it is calculated

BUN (mg/dL) = urea (mmol/L) × 2.8 when urea is reported
Creatinine (mg/dL) = µmol/L ÷ 88.4 when reported in µmol/L
Ratio = BUN (mg/dL) ÷ creatinine (mg/dL)
Usual range about 10–20; above 20 prerenal pattern; below 10 low ratio

The principle

Both urea and creatinine are waste products cleared by the kidneys, but they are handled differently. Creatinine is filtered and hardly reabsorbed, while urea is partly reabsorbed, more so when blood flow to the kidneys falls, for example with dehydration, bleeding or heart failure. Then BUN rises more than creatinine and the ratio goes above 20. Bleeding in the stomach or bowel, a high-protein diet, steroids and catabolic illness also raise urea production. A low ratio is seen with low protein intake, liver disease, pregnancy and after dialysis.

Worked example

Defaults: BUN 28 mg/dL; creatinine 1.1 mg/dL.

BUN = 28 mg/dL; creatinine = 1.1 mg/dL.
Ratio = 28 ÷ 1.1 = 25.5.
Above 20: Above 20 – prerenal pattern.

Each point is the calculator re-run with only “Serum creatinine” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Serum creatinine	BUN/creatinine ratio
0.275	101.8
0.55	50.9
0.825	33.9
1.1	25.5
1.38	20.4
1.65	17
2.2	12.7

How to read the result

Where does your result fall?

The ratio is only a clue and must be read with the absolute values, the history and other tests. A high ratio with normal BUN and creatinine means little on its own, while a high ratio with rising values may suggest reduced kidney perfusion. Your doctor will interpret the pattern.

Common mistakes

Using urea in mg/dL as if it were BUN; urea mass is about 2.14 times BUN.
Mixing units without converting.
Reading the ratio without the absolute values.
Ignoring recent diet, bleeding or medicines.

Keep in mind: Not a diagnosis. Abnormal kidney tests should be discussed with your doctor.

Frequently asked questions

What is a normal BUN to creatinine ratio?

About 10 to 20 in common clinical references.

What does a ratio above 20 mean?

Often reduced blood flow to the kidneys, such as dehydration, but other causes exist.

How do I convert urea mmol/L to BUN?

Multiply by 2.8 to get BUN in mg/dL.

Sources: Standard clinical reference values for the BUN:creatinine ratio; conversion factors urea mmol/L × 2.8 = BUN mg/dL and 88.4 µmol/L creatinine = 1 mg/dL

Checked 9 Nov 2026. Standard clinical reference values for the BUN:creatinine ratio (about 10–20:1; above 20:1 suggests reduced renal perfusion); conversion factors urea mmol/L × 2.8 = BUN mg/dL and creatinine 88.4 µmol/L = 1 mg/dL.

Your numbers