If you want to know how to count the patient’s serum osmolality and make a correct medical diagnosis easier using four parameters, check out our Serum Osmolality Calculator.

For more calculators and calculators related to medicine and health, check out the list below:

What is serum osmolality

A serum osmolality is a test that looks for a chemical imbalance in your blood. You may also find it called an osmolality serum test. The serum is the fluid in your veins and arteries minus the blood cells. Anytime you get a serum test, you will have blood taken.

Osmolality also means the concentration of dissolved particles of chemicals and minerals in your serum, such as sodium and other electrolytes. If you have more particles in your serum, the higher osmolality will be. On the other hand, the lower osmolality means that the particles in your serum are more dissolved.

Your blood is pretty much like a liquid chemistry set. Besides oxygen, it contains minerals, proteins, hormones, and a very long list of chemicals. Generally, your body does a good job balancing these things. Sometimes it can happen that you have too many minerals or chemicals or too little. This can lead to different trigger reactions in your body, which can cause some serious health issues.

A serum osmolality test effectively checks the balance of fluid and particles in your body. Additionally, it can help your doctor diagnose several possible medical conditions. If your doctor thinks you have a chemical imbalance in your blood, they may recommend getting this test.

Serum osmolality formula

The formula used for the serum osmolality test derives from the traditional serum osmolality formula with an addition of alcohol:

\text{Serum Osmolality} = 2 \times Na + \frac {BUN}{2.8} + \frac {Glucose}{18} + \frac {Alcohol}{3.7}


Na – concentration of sodium serum

BUN – Blood Urea Nitrogen

Glucose – glucose(sugar)

Alcohol – quantity of serum alcohol measured in the test

Note: In our Serum Osmolality Calculator, you can switch between the most convenient units for you.

Using the serum osmolality calculator

So, we’ve come to the part where we will show you how easy it is to calculate serum osmolality using our handy calculator. You need to provide four laboratory parameters to use the calculator, including one optional.


  1. Specify the serum sodium in mEq/L or mmol/L
  2. Specify the blood urea nitrogen and glucose
  3. Enter the alcohol concentration
  4. In case you have a laboratory measurement of osmolality, enter the result you have in the “measured serum osmolality” field.
  5. You’re done. The calculator would return the serum osmolality in mOsm/kg H₂O, including the osmotic gap value, if you provided the measured value.

Low, normal, and high serum osmolality

If you want to know by which osmolality levels are affected, refer to the information below.

Low levels of serum osmolality may be caused by:

  • Too much water.
  • Low level of salt found in your veins. Mostly, this is caused by medicines like diuretics and certain blood pressure medicines.
  • Syndrome of inappropriate antidiuretic hormone secretion. SIADH occurs with lung disease, cancer, diseases of the central nervous system, or the use of certain medicines.

What about normal osmolality? Well, each lab has a different range for what’s ordinary. Your lab report should show the range your lab uses for each test. Therefore, the range is just a guide. The doctor needs to look at your results based on your age, health, and other factors. So a value that isn’t in the medium range may still be okay for you.

High levels of serum osmolality may be caused by:

  • Too little water (dehydration).
  • Low level of salt found in your veins. In addition, poorly controlled diabetes could lead to raising the level of serum osmolality, too.
  • Damaged kidneys.
  • Poisoning with some substances such as ethanol (the alcohol in alcoholic drinks), wood alcohol (methanol), rubbing alcohol (isopropanol), and antifreeze (ethylene glycol).
  • Rare diseases, such as diabetes insipidus, that causes the kidneys to lose water and produce large amounts of urine.

Osmolality versus osmolarity

Osmolarity is a bit less than the osmolality of a given solution. In addition, the volume covers the volume of solutes, and the divisor is bigger, so the result is slightly lesser than for osmolality. However, the changes are quite negligible at very low concentrations (<500 mOsm).

In practice, both terms are frequently confused and incorrectly interchanged. For example, osmolality represents the number of solute particles in 1 kg of solvent, whereas osmolarity is the number of solute particles per 1 L of solvent. If we talk about dilute solutions, then the difference between osmolarity and osmolality is insignificant.

Measurements of osmolarity are temperature-dependent because the solvent volume varies with temperature – the volume is larger at higher temperatures. In contrast, osmolality is temperature independent because it’s based on the mass of the solvent.

For many reasons, osmolality is the preferred term for biological systems. Osmolalities are expressed as milliosmoles per kilogram of water because of the dilute nature of physiologic solutions, and water is the solvent.

We measure the osmolality of a patient using an osmometer. We then use osmolality to calculate the osmolarity. However, we never measure the osmolarity of our patients directly. So if you see something hinting at that on a test, it’s wrong. We usually use milliosmoles (mOsm) in humans because we work with pretty small numbers. For example, a milliosmole is just 1/1000 of an osmole. And for reference, a standard plasma osmolarity for humans is 280 – 300 mOsm/L.

In medicine, an osmolarity is a preferable option instead of osmolality. Why? Simply because our blood is liquid in the first place, and we prefer to measure the amount of something per liter of plasma. This option makes more sense than dealing with the unit of kilograms.

Osmotic gap

An osmotic gap is a number we get after subtracting the measured osmolality and the calculated one. It is calculated using the following formula:

\text{Osmotic gap = Measured osmolality} - \text{Counted osmolality}

When talking about the typical range of the osmotic gap, we consider it everything between -14 to +10 mOsm/kg.
If this number increases for some reason, it is a good indicator of the presence of the substances like salicylates, mannitol, or alcohol. Samely, it’s also an indicator of a state of hypertriglyceridemia or hypergammaglobulinemia.


What is normal serum osmolality?

The average serum osmolality ranges from 275 to 295 mOsm/kg (275 to 295 mmol/kg). However, those values should not be taken as the final indicators because they may vary slightly in different laboratories. In addition, each lab could use different measurements or test different samples. Therefore, it’s important to talk to the doctor to get the right interpretation of your test.

What does low serum osmolality mean?

Low serum osmolality could mean that there is:
– Too much water in the body
– A low level of salt in the blood
– A condition called syndrome of inappropriate antidiuretic hormone secretion (SIADH)