Osmolarity Calculator
2×Na + glu/18 + BUN/2.8 (+ EtOH/4.6). Click any i for detail.
Chem 7 (US units)
Sodium (mmol/L)
i
Na
Serum sodium. |
|
|---|---|
Glucose (mg/dL)
i
Glucose
Serum glucose in mg/dL. |
|
BUN (mg/dL)
i
BUN
Blood urea nitrogen, not urea in mmol/L. |
|
Ethanol (mg/dL)
i
EtOH
Optional. 0 if none. |
How to use this calculator
- Enter sodium, glucose, and BUN in US units shown.
- Add ethanol mg/dL if relevant, else leave 0.
- Read calculated osmolarity in mOsm/L.
- Compare to lab osmolality separately for gap math.
Results explained
Textbook calculated osm from Chem 7 values. SI glucose/urea units need conversion first. Osmolal gap needs measured osmolality.
Quick reference: calculated osm
Na is doubled; glucose and BUN use US divisors.
| Item | Detail |
|---|---|
| Sodium | 2 × Na mmol/L |
| Glucose | mg/dL ÷ 18 |
| BUN | mg/dL ÷ 2.8 |
| Ethanol | mg/dL ÷ 4.6 when present |
Toxic alcohols need measured osmolality, not this sum alone.
How the estimate is built
Osm ≈ 2Na + glu/18 + BUN/2.8 + EtOH/4.6.
Example scenario
Na 140, glu 100, BUN 14 → about 290 mOsm/L.
FAQ
What is an osmolal gap?
Measured osmolality minus calculated osm. This page only calculates the second term.
Can I paste mmol/L glucose?
Not directly. Convert units to the US fields this page expects.
Does this diagnose diabetes?
No. It is arithmetic on labs you enter.