Blood Alcohol Unit Converter
Blood Alcohol Unit Converter
Convert blood ethanol between mg/dL, mg/L, g/L, g/dL and mmol/L, and see why a clinical serum result is not the same figure as a forensic whole-blood one.
Blood alcohol converter
Mass ⇄ molarBlood ethanol 80 mg/dL
Formula and conversion factors
g/dL = mg/dL ÷ 1000 = the percentage BAC figure
Osmolal gap contribution (mOsm/kg) = mg/dL ÷ 4.6
- 0.217
- derived from the molecular weight of ethanol, 46.07 — 10 ÷ 46.07 = 0.217
- g/dL
- numerically identical to the percentage BAC used in law enforcement: 0.080 g/dL is 0.08% BAC
- ÷ 4.6
- one tenth of the molecular weight, which gives ethanol's contribution to the osmolal gap in mOsm/kg
- serum vs whole blood
- serum or plasma ethanol runs roughly 10–20% higher than whole blood because of differing water content
Worked example
Blood ethanol 80 mg/dL
80 × 0.217 = 17.4 mmol/L
= 800 mg/L = 0.800 g/L = 0.080 g/dL, which is 0.08% BAC
Osmolal gap contribution = 80 ÷ 4.6 = 17.4 mOsm/kg — numerically almost the same as the mmol/L figure, because both derive from one tenth of the molecular weight
Common concentrations across units
| mg/dL | g/dL (% BAC) | g/L | mmol/L |
|---|---|---|---|
| 50 | 0.050 | 0.50 | 10.9 |
| 80 | 0.080 | 0.80 | 17.4 |
| 150 | 0.150 | 1.50 | 32.6 |
| 300 | 0.300 | 3.00 | 65.1 |
| 400 | 0.400 | 4.00 | 86.8 |
Which ethanol number you are actually holding
Blood ethanol is reported in at least five different units depending on the country and the purpose of the test, and the same patient can generate numbers that look nothing alike. The molar conversion comes from ethanol’s molecular weight of 46.07: dividing 10 by 46.07 gives 0.217, so a concentration in mg/dL is multiplied by 0.217 to give mmol/L. The g/dL figure is simply mg/dL divided by a thousand, and it is numerically identical to the percentage BAC used in law enforcement — 0.080 g/dL is 0.08% BAC.
The distinction that causes the most trouble is not between units but between specimens. Clinical laboratories measure ethanol in serum or plasma, whereas forensic testing uses whole blood, and serum runs roughly 10 to 20 per cent higher than whole blood because serum contains more water and ethanol distributes into body water. A hospital serum result is therefore not interchangeable with a forensic whole-blood result, and converting one to the other requires an explicit correction rather than a change of unit.
Ethanol also contributes to the osmolal gap, and its contribution in mOsm/kg is the concentration in mg/dL divided by 4.6 — one tenth of the molecular weight again. That is why the mmol/L figure and the osmolal contribution come out almost identical. In a patient being screened for a toxic alcohol, measured ethanol must be subtracted in this way before the residual gap is attributed to methanol or ethylene glycol, or a drinker will appear to have a large unexplained gap that is entirely accounted for by alcohol.
Finally, no concentration maps reliably onto clinical state. Tolerance in a chronic drinker can be extreme, and a level that would render a naive person unconscious may leave a tolerant one conversant, while a modest level in an unwell patient never explains a reduced conscious state on its own. Alcohol intoxication frequently accompanies other poisoning, and poisoning management is time-critical and should be guided by a poisons centre or clinical toxicology service; this converter supports that assessment rather than replacing it.
Frequently asked questions
How do I convert blood alcohol from mg/dL to mmol/L?
Multiply by 0.217, derived from ethanol’s molecular weight of 46.07. A blood ethanol of 80 mg/dL is 17.4 mmol/L.
Is g/dL the same as percentage BAC?
Yes, numerically. A blood ethanol of 0.080 g/dL is 0.08% BAC, and 80 mg/dL is the same concentration expressed differently. This converter is a unit conversion, not a legal determination.
Is a hospital serum ethanol the same as a forensic blood alcohol?
No. Serum or plasma ethanol runs roughly 10 to 20 per cent higher than whole blood because of the higher water content of serum. The two are not interchangeable without an explicit correction.
How much does ethanol add to the osmolal gap?
Divide the concentration in mg/dL by 4.6 to get the contribution in mOsm/kg. Measured ethanol must be subtracted before a residual osmolal gap is attributed to methanol or ethylene glycol.
Does the level tell me how impaired someone is?
Only loosely. Tolerance varies enormously, and a level cannot explain reduced consciousness on its own. Poisoning management is time-critical and should be guided by a poisons centre or clinical toxicology service, which this converter supports rather than replaces.
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References
- Rainey PM. Relation between serum and whole-blood ethanol concentrations. Clin Chem. 1993;39(11):2288–2292.
- Purssell RA, Pudek M, Brubacher J, Abu-Laban RB. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med. 2001;38(6):653–659.
- Vonghia L, Leggio L, Ferrulli A, et al. Acute alcohol intoxication. Eur J Intern Med. 2008;19(8):561–567.