Osmolar

Normal osmolality range

Serum and urine reference ranges, in mOsm/kg, with laboratory caveats.

Short answer: a commonly used adult reference interval for serum (blood) osmolality is 275–295 mOsm/kg (275–295 mmol/kg) [1, 2]. A random urine osmolality can be anywhere from 50 to 1,200 mOsm/kg water. After 12–14 hours without fluids, urine is normally above 850 mOsm/kg water [3]. Normal ranges vary between laboratories, so a result should be read against the range printed on the lab report [1, 3].

Units. mOsm/kg (milliosmoles per kilogram of water) equals mmol/kg in SI units [1]. Laboratories measure osmolality per kg. A value calculated from a formula is strictly an osmolarity estimate, although it is usually reported in mOsm/kg so that the two can be compared. See osmolality vs osmolarity.

Reference ranges at a glance

MeasurementRangeSource
Serum osmolality (adult)275–295 mOsm/kgMedlinePlus [1]; StatPearls [2]
Serum osmolality, variant275–290 mOsm/kgMerck Manual [4]
Serum osmolality, by age (one laboratory)Newborns may be as low as 266; 0–60 years 275–295; 61 years and older 280–301 mOsm/kgLabcorp [5]
Urine osmolality, random sample50–1,200 mOsm/kg waterMedlinePlus [3]
Urine osmolality after 12–14 h fluid restriction> 850 mOsm/kg waterMedlinePlus [3]
Serum sodium (for context)135–145 mEq/L (mmol/L)MedlinePlus [6]

Ranges vary by laboratory. MedlinePlus notes that “some labs use different measurements or test different samples” [1]. The variants above are other published intervals, not errors. This site uses 275–295 mOsm/kg as its default and shows the variation alongside it.

Urine values span a wide range because the kidneys concentrate or dilute urine to keep blood osmolality steady [1]. A single random urine value has little meaning without the matching blood result and the clinical context.

Calculated vs measured osmolality

  • Measured osmolality comes from an osmometer. For gap work this should be freezing-point depression, because vapor-pressure osmometers miss volatile alcohols [8].
  • Calculated osmolality is estimated from routine chemistry [9, 10]:
    • US units: 2 × Na + Glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8
    • SI units: 2 × Na + Glucose (mmol/L) + Urea (mmol/L)
    • BUN and urea are different measurements. Urea in mg/dL is divided by 6.0, not 2.8.

Label every number as measured or calculated. Both can be compared with the 275–295 interval, but only the measured value includes substances the formula leaves out. The serum osmolality calculator shows the formula step by step.

Effective osmolality (tonicity)

Effective osmolality = 2 × Na + Glucose (mg/dL) ÷ 18 (SI: 2 × Na + glucose in mmol/L). Urea is left out. Merck explains that solutes such as urea “freely diffuse across cell membranes and have little or no effect on water shifts” [4]. StatPearls puts it as “tonicity excludes urea because it freely crosses cell membranes” [11].

Total and effective osmolality can tell different stories. When BUN is high, the total osmolality can look high while the effective value, which determines water movement into or out of cells, is normal or low. Tonicity of IV fluids and body fluids is covered in isotonic, hypotonic, hypertonic.

Some thresholds are quoted against effective osmolality rather than the reference interval. The 2024 consensus on hyperglycemic crises lists effective osmolality > 300 or total osmolality > 320 mOsm/kg among its criteria for hyperosmolar hyperglycemic state, alongside glucose, ketone and acid–base criteria [12]. These are criteria applied by clinicians, not a self-check. For how high glucose changes the sodium value, see corrected sodium for hyperglycemia.

The osmolar gap

The osmolar (osmolal) gap is measured minus calculated osmolality. A gap of 10 or more (Merck ≥ 10) is the commonly used threshold, a convention rather than a sharp boundary [4, 9]. Gaps up to about 15–20 occur in non-toxic states such as ketoacidosis, lactic acidosis and renal failure [13]. A gap above 20 strongly suggests an unmeasured osmole such as a toxic alcohol, including ethanol if it was not counted [13]. Every threshold depends on the equation used. A normal osmolar gap does not exclude toxic alcohol ingestion [7]. Full explanation: osmolar gap.

Worked example

A lab reports Na 140 mmol/L, glucose 90 mg/dL and BUN 14 mg/dL. The measured serum osmolality (freezing-point depression) is 300 mOsm/kg.

  1. Glucose: 90 ÷ 18 = 5.0 mmol/L
  2. BUN → urea: 14 ÷ 2.8 = 5.0 mmol/L
  3. Calculated osmolality: 2 × 140 + 5.0 + 5.0 = 290 mOsm/kg, inside 275–295
  4. Effective osmolality: 2 × 140 + 5.0 = 285 mOsm/kg
  5. Measured osmolality: 300 mOsm/kg, just above the 275–295 interval (and further above Merck’s 275–290)
  6. Osmolar gap: 300 − 290 = 10 mOsm/kg: 10 or more (Merck ≥ 10), so it is flagged as elevated

The calculated value looks “normal”. Only the measured value, and therefore the gap, shows that something the formula does not count may be present. This does not identify what it is. Interpretation needs the clinical picture and a clinician.

Practice questions

Q1. A serum osmolality is reported as 288 mmol/kg. Is that the same as 288 mOsm/kg, and is it within the usual interval?

Answer

Yes. mOsm/kg and mmol/kg are the same quantity in different unit systems [1]. 288 falls within 275–295 mOsm/kg, but compare it with the reporting lab’s own range [1].

Q2. A random urine osmolality is 90 mOsm/kg water. Is that outside the normal range?

Answer

No. Random urine osmolality normally ranges from 50 to 1,200 mOsm/kg water [3]. After a person drinks more water than the body needs, the kidneys pass dilute urine to get rid of the excess [1, 3]. It is interpreted together with the blood osmolality and the clinical context.

Q3. Na 130 mmol/L, glucose 90 mg/dL, BUN 84 mg/dL. Calculate the total and effective osmolality.

Answer

Glucose 90 ÷ 18 = 5.0. BUN 84 ÷ 2.8 = 30.0. Total: 2 × 130 + 5.0 + 30.0 = 295 mOsm/kg, at the top of the interval. Effective: 2 × 130 + 5.0 = 265 mOsm/kg, low. Urea raises the total but not the tonicity [4, 11].

Q4. A lab reports “urea 30 mg/dL” (not BUN). What value goes into the SI urea term?

Answer

30 ÷ 6.0 = 5.0 mmol/L urea. Dividing by 2.8 would be correct only for BUN and would overstate the urea term (30 ÷ 2.8 = 10.7).

Q5. A patient’s osmolar gap is 4 mOsm/kg. Does that exclude methanol poisoning?

Answer

No. A small or normal gap does not exclude toxic alcohol ingestion [7]. As methanol is metabolized, the osmolar gap falls while the anion gap rises, so a late presentation can have a normal gap [14].

If you are looking at your own results

This page explains what the numbers mean. It cannot interpret your result. Talk to your doctor or the clinician who ordered the test about your specific results [1]. Your lab’s printed range is the one that applies to you.

Get urgent medical help. Call your local emergency number if someone collapses, has a seizure, has trouble breathing, can’t be woken, or has new confusion [15]. If you think someone has swallowed antifreeze, windshield-washer fluid or another chemical, call your local emergency number or poison centre straight away, even if they seem well. (US only: Poison Help 1-800-222-1222, free and 24/7 [15, 16].)

Do not try to correct a result yourself by drinking large amounts of water or taking salt.

Educational content, not medical advice. Reference ranges vary by laboratory and method, and a value outside a range is not a diagnosis. Verify results against the reporting laboratory’s reference interval and discuss them with a clinician. In a suspected poisoning, contact poison control or emergency services. See our disclaimer and how we check content.

Related: Serum osmolality calculator · Osmolar gap · Osmolality vs osmolarity · Isotonic, hypotonic, hypertonic · Corrected sodium for hyperglycemia · Free water deficit calculator

References

  1. MedlinePlus Medical Encyclopedia. Osmolality blood test. US National Library of Medicine. Review date 2025-07-13. https://medlineplus.gov/ency/article/003463.htm
  2. Najem O, Shah MM, Zubair M, De Jesus O. Serum Osmolality. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK567764/
  3. MedlinePlus Medical Encyclopedia. Osmolality urine test. US National Library of Medicine. Review date 2025-07-01. https://medlineplus.gov/ency/article/003609.htm
  4. Merck Manual Professional Edition. Water and Sodium Balance. https://www.merckmanuals.com/professional/nephrology/fluid-metabolism/water-and-sodium-balance
  5. Labcorp. Osmolality (test 002071). https://www.labcorp.com/tests/002071/osmolality (accessed 2026-10-18).
  6. MedlinePlus Medical Encyclopedia. Sodium blood test. US National Library of Medicine. https://medlineplus.gov/ency/article/003481.htm
  7. Hoffman RS, Smilkstein MJ, Howland MA, Goldfrank LR. Osmol gaps revisited: normal values and limitations. J Toxicol Clin Toxicol. 1993;31(1):81–93. doi:10.3109/15563659309000375. https://pubmed.ncbi.nlm.nih.gov/8433417/
  8. Walker JA, Schwartzbard A, Krauss EA, Sherman RA, Eisinger RP. The missing gap: a pitfall in the diagnosis of alcohol intoxication by osmometry. Arch Intern Med. 1986;146(9):1843–1844. doi:10.1001/archinte.146.9.1843. https://pubmed.ncbi.nlm.nih.gov/3753127/
  9. Lynd LD, Richardson KJ, Purssell RA, et al. An evaluation of the osmole gap as a screening test for toxic alcohol poisoning. BMC Emerg Med. 2008;8:5. doi:10.1186/1471-227X-8-5. https://pmc.ncbi.nlm.nih.gov/articles/PMC2390580/
  10. Smithline N, Gardner KD Jr. Gaps—anionic and osmolal. JAMA. 1976;236(14):1594–1597. doi:10.1001/jama.236.14.1594. https://pubmed.ncbi.nlm.nih.gov/989132/
  11. Rout P, Afzal M. Hyponatremia. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK470386/ (PMID 29262111)
  12. Umpierrez GE, Davis GM, ElSayed NA, et al. Hyperglycemic crises in adults with diabetes: a consensus report. Diabetes Care. 2024;47(8):1257–1275. doi:10.2337/dci24-0032. PMID 39052901. PMC11272983. https://pmc.ncbi.nlm.nih.gov/articles/PMC11272983/ Simultaneously published in Diabetologia 2024;67(8):1455–1479, doi:10.1007/s00125-024-06183-8, PMID 38907161 (open full text PMC11343900).
  13. Kraut JA, Kurtz I. Toxic alcohol ingestions: clinical features, diagnosis, and management. Clin J Am Soc Nephrol. 2008;3(1):208–225. doi:10.2215/CJN.03220807. PMID 18045860. https://pubmed.ncbi.nlm.nih.gov/18045860/
  14. Ashurst JV, Schaffer DH, Nappe TM. Methanol Toxicity. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK482121/ (PMID 29489213)
  15. Poison Control (National Capital Poison Center). https://www.poison.org/
  16. HRSA Poison Help. https://poisonhelp.hrsa.gov/

Sources

Numbered references are listed in the article above.