BUN Creatinine Ratio Calculator
Divide blood urea nitrogen by serum creatinine to get the BUN-to-creatinine ratio. Enter both in mg/dL (or switch BUN to mmol/L urea and creatinine to µmol/L) to see where your ratio falls against the usual 10:1–20:1 range.
Example: with BUN (blood urea nitrogen) 15 · BUN unit mg/dL (US) · Serum creatinine 1 · Creatinine unit mg/dL (US) → BUN : creatinine ratio: 15.0 : 1.
- Interpretation10:1 to 20:1 — within the usual reference range
- Values used (converted to mg/dL)15 mg/dL BUN / 1.00 mg/dL creatinine
Computed by the calculator below using its default values. Change any input to see your own numbers.
Ratio = BUN (mg/dL) ÷ creatinine (mg/dL). The usual reference band is 10:1 to 20:1; the ratio is a clue, never a diagnosis on its own.
What the BUN-to-creatinine ratio adds
BUN and creatinine are both waste products the kidneys clear, but they behave differently. Creatinine is filtered and largely stays in the urine, so it tracks filtration fairly cleanly. Urea nitrogen is partly reabsorbed, and that reabsorption climbs when the kidneys are holding onto water. Dividing one by the other cancels much of the day-to-day noise and highlights the pattern.
A ratio near 10:1–20:1 is typical. A high ratio with normal creatinine points upstream of the kidney, most often dehydration or a gastrointestinal bleed, where urea rises faster than creatinine. A low ratio can reflect liver disease making less urea, a low-protein diet, or dilution.
Why a single number is not the whole story
The ratio is only meaningful alongside the absolute BUN and creatinine and the clinical picture. A perfectly normal ratio can sit on top of two abnormal values, and a mildly odd ratio in a healthy, well person usually means nothing. Trends over days matter more than one draw, and hydration, muscle mass, diet, and medications all shift the inputs.
How it’s calculated
Ratio = BUN (mg/dL) ÷ serum creatinine (mg/dL). SI inputs are converted first: urea 1 mmol/L = 2.8 mg/dL BUN; creatinine µmol/L ÷ 88.42 = mg/dL. The commonly cited reference band is 10:1 to 20:1, with >20:1 often prerenal and <10:1 often intrinsic-renal, hepatic, or dilutional.
Reference bands vary slightly by lab and source (some cite 6–22). This is an educational estimate, not a diagnosis — interpret with the absolute lab values and your clinician.
BUN/creatinine ratio — common interpretation bands
| Ratio | Band | Frequently associated with |
|---|---|---|
| Under 10:1 | Low | Intrinsic renal disease, low-protein diet, liver disease, overhydration, rhabdomyolysis |
| 10:1 to 20:1 | Typical range | No specific pattern flagged |
| Over 20:1 | High | Prerenal: dehydration, GI bleed, high protein load, heart failure |
Bands per widely published clinical chemistry references; ratio computed as BUN ÷ creatinine in mg/dL.
Common mistakes
- Reading the ratio without the absolute BUN and creatinine — both can be abnormal while the ratio looks normal.
- Mixing units: SI urea (mmol/L) and creatinine (µmol/L) must be converted to mg/dL before dividing, or the ratio is meaningless.
- Treating a mildly high ratio as proof of kidney failure — dehydration alone commonly pushes it above 20:1.
- Comparing one snapshot to a friend's instead of watching your own trend over time.
Frequently asked questions
What is the BUN-to-creatinine ratio formula?
It is simply BUN divided by serum creatinine, both in mg/dL. A BUN of 15 and creatinine of 1.0 gives a ratio of 15 to 1, which sits inside the usual 10:1 to 20:1 range.
What does a high BUN-to-creatinine ratio mean?
A ratio above about 20:1 is often prerenal, meaning something before the kidney is the driver — most commonly dehydration, a gastrointestinal bleed, a high-protein load, or heart failure. It is a clue that prompts a closer look, not a diagnosis.
What does a low ratio suggest?
A ratio under 10:1 can go with intrinsic kidney disease, a low-protein diet, liver disease making less urea, or overhydration. As with a high ratio, the absolute values and your history decide what it means.
Is a normal ratio proof my kidneys are fine?
No. The ratio can look normal while both BUN and creatinine are high, so filtration can still be impaired. Always read it together with the individual values and an eGFR.
Should I act on this number myself?
No. Use it to understand your report, then review any abnormal or trending result with a doctor. Do not change fluids, protein, or medications based on a ratio alone.