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Plasma Osmolality Calculator

Table of contents

What is plasma osmolality?Plasma osmolality vs osmolarityWhat is normal plasma osmolality?How do I use this plasma osmolality calculator?Plasma osmolality formulaFAQs

This plasma osmolality calculator not only calculates plasma osmolality for you but also provides thorough information about the topic of plasma osmolality, including:

  • The definition of plasma osmolality;
  • Plasma osmolarity vs osmolarity;
  • What normal plasma osmolality is;
  • How to calculate plasma osmolality; and
  • The according plasma osmolality formula.

We try our best to make Omni's calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment. All information on this website is for informational purposes only and is not intended as a substitute for medical consultation. Always consult your healthcare provider if you are unwell.

What is plasma osmolality?

Before diving into the technicalities, let's first get a handle on plasma osmolality and why it's relevant🤔.

First, we need to settle the whole principle of osmolality: osmolality shows the direction of fluid movement within a system — our body constantly strives to have a state of homeostasis, which means that fluid from a compartment with high osmolality will flow to a compartment with lower osmolality.

This might sound abstract, so let's look at an example we can all relate to — drinking water. Drinking enough water is of utmost importance, as our cells need this element for a variety of essential biochemical reactions.💧

  • If we drink too little, the water inside the cell will move, due to osmotic pressure, to a compartment with lower water concentration, which is the extracellular space. This leads to dehydration.
  • Drinking too much water, however, causes electrolytes within the cells to flow to the compartment of lower osmolality out of the cell. Electrolytes, especially sodium, are essential for proper nerve and muscle function, and a low concentration of electrolytes is termed hyponatremia.

Now you can see that the right balance of water and electrolytes is essential for a healthy body.

If you are unsure about how much water you should drink, you can always use our water intake calculator to be on the safe side. Now, let's move on to plasma osmolality.

Plasma osmolality or serum osmolality, is a measure of several plasma solutes expressed in milliosmoles of solute per kilogram of water (mOsm/kg H2O), which shows the electrolyte-water balance of the organism. Thus, it indicates conditions affecting osmolality, such as dehydration and alcohol ingestion and diseases related to hydrolytic imbalance and kidney function.

Plasma osmolality is assessed by the concentration of plasma solutes — sodium, glucose, and blood urea nitrogen (BUN).

If you want to know more about calculations connected to these plasma solutes, you may check out our sodium correction rate calculator, the estimated average glucose calculator, and the bun creatinine ratio calculator.

Plasma osmolality vs osmolarity

Though osmolality and osmolarity are similar concepts, they provide different information.
As mentioned, osmolality quantifies solute particles per kilogram of fluid, whereas osmolarity measures solute particles per liter.
You might still wonder why we are specifically using osmolality in this calculator. The explanation, though not immediately apparent, is quite straightforward. Unlike osmolality, osmolarity is susceptible to temperature fluctuations because water density varies with temperature. Furthermore, osmolality is the preferred measurement in medicine. 🩺

What is normal plasma osmolality?

Plasma osmolality may differ between individuals and is influenced by the person's state at the moment of blood draw 💉🩸. However, if you are within a healthy range, there is usually no need to worry.

  • Normal plasma osmolality is considered between 275 and 295 mOsm/kg H2O.
  • Abnormal and potentially health-threatening plasma osmolality levels concern
    • Low (<265 mOsm/kg H2O); and
    • High (>320 mOsm/kg H2O) osmolality.

By measuring plasma osmolality, physicians derive relevant indications about the following conditions:

  • Diabetes insipidus;
  • Congestive heart failure;
  • Dehydration;
  • Liver cirrhosis;
  • Psychogenic polydipsia; and
  • Nephrotic syndrome.

How do I use this plasma osmolality calculator?

This calculator is very intuitive. You need to enter three variables:

  1. Sodium, expressed in milliequivalents per liter (mEq/L);
  2. Glucose, expressed in milligrams per deciliter (mg/dL) or millimole per liter (mmol/L); and
  3. Plasma BUN level, also given in mg/dL or mmol/L.

There! You have your result.

Plasma osmolality formula

If you are determined to calculate plasma osmolality by yourself or are just curious about the hows of the calculation, this plasma osmolality calculator does not disappoint. Although there are different though similar formulas to calculate plasma osmolality, the Smithline-Gardner formula, which is used here, is the most popular:

For values in mg/dL:
Serum osmolality = (2 × Na) + (glucose / 18) + (BUN / 2.8)

For values in mmol/L:
Serum osmolality = (2 × Na) + glucose + BUN


Na – Sodium, expressed in mEq/L;
Glucose – Plasma glucose levels, measured in mg/dL or mmol/L; and
BUN – Blood urea nitrogen, measured in mg/dL or mmol/L.


What is plasma osmolality?

Plasma osmolality measures several solutes within the plasma to assess the water-electrolyte balance in the body. Physicians often use this information to find indications for underlying conditions linked to hyponatremia or hypernatremia.

How to calculate plasma osmolality?

To calculate plasma osmolality, follow these steps:

  1. Find your sodium levels (mEq/L);

  2. Find your glucose levels (mg/dL or mmol/L);

  3. Determine your BUN levels (mg/dL or mmol/L);

  4. Enter these variables into the Smithline-Gardner formula

    For values in mg/dL:
    Serum osmolality = (2 × Na) + (glucose / 18) + (BUN / 2.8)

    For values in mmol/L:
    Serum osmolality = (2 × Na) + glucose + BUN

  5. Calculate.

There! You have your results.

What is normal plasma osmolality?

Normal plasma osmolality is 275-295 mOsm/kg H2O. These values indicate a balance between water and solute concentrations in the body, including sodium.

Is a plasma osmolality of 300 high?

High plasma osmolality concerns values above the normal range of 275-295 mOsm/kg H2O. Critically high plasma osmolality is above 320 mOsm/kg H2O and may be caused by hyperglycemia, diabetes insipidus or hypernatremia.

Note that a medical professional should interpret these results in the context of a patient's clinical background and physical examination results.

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