BUN Creatinine Ratio Calculator

How to use this BUN / creatinine ratio calculator

What this calculator does

This calculator takes a blood urea nitrogen result and a serum creatinine result, converts either value when a commonly reported alternate unit is selected, and then divides BUN in mg/dL by creatinine in mg/dL. The result is a unitless BUN / creatinine ratio. The tool also places that number beside a commonly used adult comparison band of 10 – 20. It is designed for checking arithmetic, converting units, reviewing a lab report, or preparing a clear record for a conversation with a clinician. It does not diagnose dehydration, kidney injury, liver disease, bleeding, nutritional problems, or any other condition. Laboratory reference intervals and clinical meaning depend on the person, the laboratory method, the surrounding results, and the clinical situation. For background on why the two tests are considered together, see the National Library of Medicine's Clinical Methods chapter on BUN and creatinine.

When to use it

• Check the ratio shown on a metabolic panel or laboratory report when both BUN and serum creatinine are available.
• Convert BUN reported in mmol/L or creatinine reported in µmol/L before comparing the two values.
• See how a changed laboratory value would change the arithmetic ratio without manually repeating the unit conversions.
• Export the current inputs and result to a structured Excel workbook for personal record keeping or discussion with a health professional.

How to calculate

1. The calculator opens with a complete demonstration: BUN is 18 mg/dL and serum creatinine is 1.0 mg/dL. The live result is already calculated, and the example Excel workbook is immediately available.
2. Replace BUN (blood urea nitrogen) with the value from the lab report, then choose its unit. If the report uses mmol/L, selecting that unit converts the entered value to the mg/dL basis needed by the ratio.
3. Replace Serum creatinine with the matching blood creatinine result, then choose mg/dL or µmol/L. Unit changes convert the value already in the field rather than merely changing the label.
4. Read BUN / creatinine ratio, the two normalized values, and the interpretation text. The detail table repeats the entered and normalized values so the conversion can be checked before the ratio is used.
5. Select Download Excel to create a workbook from the current validated values. Select Reset to clear the demonstration and current result. After Reset, Download Excel is disabled until both required lab values are entered again in a complete valid state.

Input guide

BUN (blood urea nitrogen) is required and must be a positive decimal number. Choose mg/dL when the laboratory reports BUN in the common U.S. mass-concentration form, or mmol/L when it reports urea nitrogen using the molar convention supported here. The demonstration value is 18 mg/dL. The field accepts ordinary decimal-point notation such as 18, 18.5, or 1,250.5 when grouping is unambiguous; scientific notation and decimal-comma forms are rejected rather than silently reinterpreted. Raising BUN while creatinine is unchanged raises the ratio. A common mistake is entering a whole-urea result under a BUN label without confirming what the laboratory actually measured.

BUN unit is required because it determines the conversion to mg/dL. Changing the unit converts the current valid field value so that the underlying concentration stays equivalent. The conversion used for mmol/L is approximately 2.80112 mg/dL BUN per mmol/L. Do not switch units to make a number look closer to a reference interval; select the unit printed on the report.

Serum creatinine is required and must also be a positive decimal number. The demonstration value is 1.0 mg/dL. Raising creatinine while BUN is unchanged lowers the ratio because creatinine is the denominator. Values near zero are rejected because division would become unstable or undefined. Creatinine varies with muscle mass and other factors, so the number should not be interpreted by the ratio alone. The National Library of Medicine's creatinine test overview explains why clinicians often interpret creatinine alongside other kidney measures.

Serum creatinine unit may be mg/dL or µmol/L. When µmol/L is selected, the calculator divides by 88.42 to obtain mg/dL before computing the ratio. For example, 88.42 µmol/L converts to about 1.00 mg/dL. Using the wrong unit can change the apparent ratio by almost two orders of magnitude, which is why the normalized value is shown explicitly.

Output guide

BUN / creatinine ratio is the primary result and is displayed to two decimal places. It is an arithmetic comparison, not a direct measurement and not a diagnosis. BUN normalized and Creatinine normalized show the exact mg/dL basis used for the division, making unit conversion transparent. Interpretation classifies the ratio as below 10, within 10 – 20, or above 20 for educational context. A value below 10 or above 20 can occur for multiple reasons, while a value inside the comparison band does not prove normal kidney function. The current-value table lists Analyte, Entered value, and Normalized value; those rows are also sourced from the same calculation model used by the result and workbook.

Worked example

The opening example uses BUN = 18 mg/dL and serum creatinine = 1.0 mg/dL. No unit conversion is needed, so the normalized values remain 18.00 mg/dL and 1.00 mg/dL. The calculation is 18 ÷ 1.0 = 18.00. The displayed ratio is therefore 18.00, which falls within the common 10 – 20 comparison band used by this calculator. If the same BUN were entered as approximately 6.426 mmol/L, the normalized BUN would still be 18.00 mg/dL and the ratio would remain 18.00. The example demonstrates why conversion happens before division and why equivalent units should produce the same result.