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Clinical & Lab Medicine Established lab calculation Source-mapped educational formula Routine educational estimate

Serum Osmolality Calculator

Calculate estimated serum osmolality from sodium, glucose, and BUN.

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Interactive tool

Calculator

Enter values carefully. Results appear after calculation and should be interpreted with the safety notes and source method on this page.

Source-mapped educational formula

Step 1 — Enter inputs

4 fields required for this tool

Step 2 — Review the result

The result area updates below and keeps safety wording visible.

Result

Complete the form and select Calculate.

About this calculator

Calculated serum osmolality estimates the major measured osmoles from a basic metabolic panel. It is often compared with measured osmolality to evaluate an osmolal gap.

Source-backed
Common calculated osmolality = 2 × Na + glucose/18 + BUN/2.8. Dorwart-Chalmers variant = 1.86 × Na + glucose/18 + BUN/2.8 + 9.
Review status
Source-mapped educational formula
Limitations
Educational estimate only; not a diagnosis, prescription, or treatment plan.

Formula and method

Simple calculated osmolality = 2 × Na + glucose/18 + BUN/2.8. Dorwart-Chalmers variant = 1.86 × Na + glucose/18 + BUN/2.8 + 9.

Medical safety note: This page is for education only and should not replace professional medical advice. For emergencies, medication decisions, or severe symptoms, contact a qualified clinician or local emergency service.

Limitations and when not to rely on this result

  • Educational estimate only; not a diagnosis, prescription, or treatment plan.
  • Result depends on accurate inputs and may not apply to complex medical situations.
  • Use clinician judgment, local guidance, and urgent care pathways when symptoms are severe.

Frequently asked questions

Is calculated osmolality the same as measured osmolality? +

No. Measured osmolality is a lab measurement. Calculated osmolality is an estimate.

What is an osmolal gap? +

It is the difference between measured and calculated osmolality and can suggest unmeasured osmoles in the right context.

Does this include ethanol? +

This version does not add ethanol. Ethanol and toxic alcohols require measured osmolality and clinical evaluation.

Which formula should I choose? +

Use the formula preferred by your institution or teaching reference. The simple equation is common; Dorwart-Chalmers is another published option.

Can this diagnose poisoning? +

No. Suspected toxic alcohol exposure is an emergency and needs measured osmolality, labs, and clinician management.