Urine Osmolality Calculator (Bogart Formula)

The Bogart formula is a clinical estimation method for urine osmolality from specific gravity. The simple form is the last two digits of SG multiplied by 35, and the linear form is 40,000 multiplied by (SG - 1.000). For example, SG 1.020 gives a simple estimate of 700 mOsm/kg and a linear estimate of 800 mOsm/kg, with an average of 750 mOsm/kg. This is a different tool from the Bogart Calculator used by US tax preparers for education credits.

The Bogart estimates are reasonably accurate when the urine contains only typical solutes (urea, sodium, creatinine). They become unreliable when glucose, protein, or radiographic contrast is present, because these heavy molecules raise specific gravity without raising osmolality proportionally. Direct osmometry remains the reference method when precision is critical.

Normal urine specific gravity for healthy adults typically falls between 1.005 and 1.030, with 1.010 to 1.020 considered the usual mid-range. Values below 1.005 suggest very dilute urine (overhydration or diabetes insipidus), and values above 1.030 suggest concentrated urine (dehydration, glycosuria, or proteinuria).

It is useful for rapid bedside or outpatient screening when an osmometer is not immediately available, for hydration assessment in ICU and inpatient rounds, and for screening for diabetes insipidus (very low estimates) or SIADH and dehydration (very high estimates). It does not replace measured osmolality for diagnostic decisions.

SG 1.025 corresponds to roughly 875 mOsm/kg by the simple Bogart estimate and 1,000 mOsm/kg by the linear estimate, averaging around 938 mOsm/kg. This indicates concentrated urine, typically suggesting mild to moderate dehydration or reduced fluid intake.

The simple form uses a multiplier of 35 per SG unit (last two digits), while the linear form uses 40 per SG unit. The two were derived from different reference data sets and bedside heuristics. Reporting both and averaging gives a more balanced estimate than either alone, and the gap between them is usually small.

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