Enter the serum sodium concentration and glucose concentration into the calculator to determine the effective osmolarity.

Effective Osmolarity Calculator

Enter serum sodium and glucose to calculate effective osmolarity

Disclaimer: For educational purposes only; not medical advice, diagnosis, or treatment. Results are estimates and may differ from measured labs due to other osmoles and clinical factors. Consult a qualified clinician for interpretation, and seek urgent care for severe symptoms (e.g., confusion, seizures, severe dehydration).

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Effective Osmolarity Formula

The following formulas are used to estimate effective osmolarity (often discussed clinically as “tonicity”) from serum sodium and glucose.

O \approx 2 * Na + (G / 18) \quad \text{(G in mg/dL)}
O \approx 2 * Na + G \quad \text{(G in mmol/L)}

Variables:

  • O is the effective osmolarity (typically reported here as mOsm/L)
  • Na is the serum sodium concentration (mEq/L or mmol/L)
  • G is the glucose concentration (mg/dL or mmol/L, depending on the formula used)

To calculate the effective osmolarity, multiply the serum sodium concentration by 2 and then add glucose in mmol/L (or add glucose in mg/dL divided by 18).

What is Effective Osmolarity?

Effective osmolality (often referred to clinically as tonicity) estimates the osmotic effect of largely non-penetrating solutes that drive water movement across cell membranes. In many clinical contexts it is approximated primarily from sodium (and accompanying anions) and glucose; other effective osmoles (for example, mannitol) may also contribute depending on the situation.

How to Calculate Effective Osmolarity?

The following steps outline how to calculate the Effective Osmolarity.

  1. First, determine the serum sodium concentration (Na).
  2. Next, determine the glucose concentration (G).
  3. Next, calculate the effective osmolarity using the appropriate formula based on glucose units.
  4. Finally, calculate the Effective Osmolarity.
  5. After inserting the values and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Serum sodium concentration (Na) = 140 mEq/L

Glucose concentration (G) = 90 mg/dL