Blood urea nitrogen
Serum alcohol concetration
Measured serum osmolality
mOsm/kg H₂O
Serum osmolality
mOsm/kg H₂O

This serum osmolality calculator, using four parameters, counts the patients serum osmolality for you and makes a correct diagnosis easier. Thanks to the improved serum osmolality formula, which includes alcohol, it is even more accurate than the traditional equation. If you provide us with your measured serum osmolality, we will compare it with the result and give you an answer - your osmotic gap. Take a look at the clinical states section to remind yourself what are the causes of low serum osmolality and high serum osmolality.

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment.

What is serum osmolality?

Osmolality is a special variation of molality, and is a measure of the number of dissolved, osmotically active, particles in a fluid, and is an indicator of your water-electrolytes balance. In the case of serum osmolality, we measure substances such as sodium, urea nitrogen, glucose, alcohol, and potassium. Sodium is the main component of serum osmolality, whereas potassium can be omitted, as it has a minuscule influence.

Patients who have altered levels of osmotically active substances will have an altered serum osmolality and an increased osmolar gap. These patients include the diabetic (raised glucose levels), the kidney insufficient (raised urea nitrogen levels), and those with sodium disorders - many elderly people and people suffering from hypothalamus disorders.

Clinicians measure the serum osmolality quite often when it comes to fluid and electrolyte disorders. Still, there are clinical situations in which this measurement is crucial - in patients where poisoning or overdose is suspect. Substances such as salicylates, ethyl alcohol (food alcohol), methyl alcohol, or propylene glycol affect the serum osmolality and osmotic gap.

Serum osmolality formula

The equation that we use in this osmolar gap calculator derives from the traditional serum osmolality formula with an addition of alcohol, based on the study by doctor Roy Pursell:

serum osmolality = 2*Na + BUN/2.8 + glucose/18 + alcohol/3.7


Na - sodium serum concentration [mEq/L]

BUN - Blood Urea Nitrogen [mg/dL]

glucose - glucose [mg/dL]

alcohol - quantity of measured serum alcohol [mg/dL]

Remember, that in our serum osmolality calculator, you can switch between the units that are most convenient for you. Just click on the unit to see the possible options, and if we've missed something - don't hesitate to contact us.

Using the serum osmolality calculator

To use the osmolar gap calculator, you have to provide four laboratory parameters - and one optional parameter:

  1. Input the serum sodium concentration in mEq/L or mmol/L - in this case, they are equivalent.
  2. Fill in the BUN (blood urea nitrogen) and glucose fields.
  3. Fill in the alcohol concentration, if applicable. It can be ethanol, methanol, or ethylene glycol.
  4. If you have a laboratory measurement of osmolality, you can put the result into the fifth field.
  5. Your result is ready - as well as providing the serum osmolality, the calculator will also give you the osmotic gap value if you provided the measured value.

Osmolality versus osmolarity

By definition, we measure serum osmolality in the laboratory (e.g., using osmometer or freezing point depression). Its units are mOSm/kg H₂O. ** Osmolarity** is the one that we calculate from other parameters and is given in mOsm/L H₂O.

Osmolarity is a little less than the osmolality of a given solution. In osmolality, the weight of the soluble substances doesn't include the solvent weight. In the case of osmolarity, the volume covers the volume of solutes. The divisor in osmolarity is bigger, so the result is slightly lesser than for osmolality. But at very low concentrations (<500mOsm), the changes are negligible.

Thus, in practice, both terms are used interchangeably. But technically, what we calculate here, despite the name of serum osmolality calculator, is osmolarity.

Osmotic gap

An osmotic gap is a difference between the measured osmolality and the calculated one (osmolarity to be precise 😉).

osmotic gap = measured osmolality - counted osmolality

The normal range of the osmotic gap is -14 to +10 mOsm/kg.

An increase in the serum osmolar gap indicates the presence of other osmotically active substances, like mannitol, salicylates, or alcohol. It's also an indicator of a state of hypertriglyceridemia or hypergammaglobulinemia.

High serum osmolality and low serum osmolality - clinical states

As mentioned above, many diseases and clinical conditions cause low serum osmolality, and even more diseases cause high serum osmolality. The table below sums up the most common causes that should always be taken into consideration:

high serum osmolality low serum osmolality
dehydration fluid overload
diabetes insipidus (inappropriate ADH secretion) hyponatremia
hyperglycemia SIADH (syndrome of inappropriate ADH secretion)
hypernatremia paraneoplastic syndrome
alcohol intake (incl. methanol)
kidney insufficiency
Aleksandra Zając, MD