What is the osmolarity of common IV fluids?

From label composition: 0.9% NaCl (normal saline) 308 mOsm/L; 0.45% NaCl 154; 3% NaCl 1,027; lactated Ringer's 273; Plasma-Lyte A 294; D5W 252; D10W 505; D5NS 560; D5 ½NS 406; D5 ¼NS 321; 8.4% sodium bicarbonate 2,000. The table below shows each one's composition.

Why is D5W 252 and not 278 mOsm/L?

US labels state dextrose as the hydrous form (dextrose monohydrate, MW 198.17). 50 g/L ÷ 198.17 g/mol = 252 mOsm/L. The figure 278 comes from anhydrous glucose (MW 180.16). Select anhydrous only if the product is labelled as anhydrous dextrose.

How is IV fluid osmolarity calculated?

Multiply each solute's millimolar concentration by the number of particles it forms and add them up. 0.9% NaCl contains 154 mmol/L of NaCl, which forms Na⁺ and Cl⁻, so 2 × 154 = 308 mOsm/L. Dextrose does not dissociate, so 5% hydrous dextrose is 252 mOsm/L. D5NS is 252 + 308 = 560 mOsm/L. Labels use this ideal calculation. The measured osmolality of NS is about 286 mOsm/kg, because ions do not act fully independently (osmotic coefficient about 0.93).

Why is D5 ½NS called hypertonic and hypotonic?

In the bag, D5 ½NS is 406 mOsm/L, which is hypertonic. Once infused, the dextrose is taken up and metabolised, leaving the effect of 0.45% NaCl (154 mOsm/L), which is hypotonic. D5W likewise starts near isotonic (252) and ends up as free water. The calculator shows both values.

What osmolarity is safe for a peripheral IV?

ASPEN guidance limits peripheral parenteral nutrition to 900 mOsm/L, and the Infusion Nurses Society advises against peripheral catheters for solutions above 900 mOsm/L. Many units are cautious from about 600 mOsm/L because phlebitis risk rises with osmolarity, pH and infusion time. Above 900 mOsm/L (for example 3% NaCl at 1,027 or D50W) a central line is preferred, although local protocols allow short peripheral use of 3% saline in emergencies.

How do I calculate serum osmolality?

Calculated serum osmolality = 2 × Na + glucose/18 + BUN/2.8 in US units (mg/dL), or 2 × Na + glucose + urea in SI units (mmol/L). Example: Na 140, glucose 90 mg/dL, BUN 14 mg/dL gives 280 + 5 + 5 = 290 mOsm/kg. The normal range is about 275–295 mOsm/kg. The osmolal gap is measured minus calculated osmolality and is normally under 10. A raised gap suggests unmeasured osmoles such as ethanol, methanol or ethylene glycol.

What is the difference between osmolarity and osmolality?

Osmolarity is osmoles per litre of solution (mOsm/L), which is what IV fluid labels give. Osmolality is osmoles per kilogram of water (mOsm/kg), which is what the laboratory measures in serum by freezing-point depression. For dilute solutions the two are close.

Is this calculator medical advice?

No. It is an educational arithmetic aid. Fluid choice, rate, additives and access route are clinical decisions for the prescriber and pharmacist, following local protocols and product labels.

Osmolarity of IV Fluid Calculator

Build any IV fluid from its components and get osmolarity in mOsm/L (NS 308, LR 273, D5W 252), tonicity in the bag and after dextrose is metabolised, and the peripheral vs central line check. Educational use only — not medical advice.

Not medical advice. Educational calculation from label compositions. Fluid selection, additives and IV access are decisions for the prescriber and pharmacist under local protocols.

IV Solution Components

Start from a fluid

Dextrose (%)

5 = D5W

NaCl (mEq/L)

154 = 0.9%

KCl (mEq/L)

20–40 typical

Na bicarbonate / lactate / acetate / gluconate (mEq/L)

LR lactate 28

CaCl₂ or MgCl₂ (mEq/L)

×1.5

MgSO₄ (mEq/L)

×1

Amino acids (%)

≈100 per 1%

Other additives (mOsm/L)

e.g. drug

Dextrose form

Total Osmolarity

—

mOsm/L (calculated)

Tonicity in the bag

—

In the body

—

Administration Route

—

Component Breakdown

Component	mOsm/L	%
Total		100%

Osmolarity of common IV fluids

Calculated from label composition (mEq/L) with hydrous dextrose. "In body" subtracts dextrose, which is metabolised.

Fluid	Composition (mEq/L)	mOsm/L	Label	In bag	In body
0.9% NaCl (normal saline)	Na 154, Cl 154	308	308	Isotonic	—
0.45% NaCl (½NS)	Na 77, Cl 77	154	154	Hypotonic	—
3% NaCl	Na 513, Cl 513	1,027	1,027	Hypertonic	—
Lactated Ringer's	Na 130, K 4, Ca 2.7, Cl 109, lactate 28	273	273	Isotonic	—
Plasma-Lyte A	Na 140, K 5, Mg 3, Cl 98, acetate 27, gluconate 23	294	294	Isotonic	—
D5W (5% dextrose)	Dextrose 50 g/L	252	252	Isotonic	Hypotonic
D10W	Dextrose 100 g/L	505	505	Hypertonic	Hypotonic
D5NS (D5 0.9% NaCl)	Dextrose 50 g/L, Na 154, Cl 154	560	560	Hypertonic	Isotonic
D5 ½NS (D5 0.45% NaCl)	Dextrose 50 g/L, Na 77, Cl 77	406	406	Hypertonic	Hypotonic
D5 ¼NS (D5 0.2% NaCl)	Dextrose 50 g/L, Na 34, Cl 34	321	321	Isotonic	Hypotonic
D5 ½NS + 20 mEq KCl	Dextrose 50 g/L, Na 77, K 20, Cl 97	446	—	Hypertonic	Hypotonic
8.4% sodium bicarbonate	Na 1000, HCO₃ 1000	2,000	2,000	Hypertonic	—
D50W (50% dextrose)	Dextrose 500 g/L	2,523	2,525	Hypertonic	Hypotonic

Calculated serum osmolality and osmolal gap

2 × Na + glucose/18 + BUN/2.8 (mg/dL) or 2 × Na + glucose + urea (mmol/L). Normal about 275–295 mOsm/kg.

Units

Na (mEq/L)

Glucose (mg/dL)

BUN (mg/dL)

Ethanol (mg/dL)

Measured (mOsm/kg)

Calculated osmolality

—

Osmolal gap

—

Summary

This calculator adds up the osmotic contribution of each solute in an IV bag. Dextrose counts once per molecule: 1% hydrous dextrose (the USP form used on US labels, MW 198.17) adds 50.5 mOsm/L. Fully dissociating salts count for every ion: NaCl, KCl and sodium bicarbonate, lactate, acetate or gluconate add 2 mOsm per mEq, calcium or magnesium chloride 1.5 per mEq, magnesium sulfate 1 per mEq, and amino acids about 100 mOsm/L per 1%. The total is classified as hypotonic, isotonic or hypertonic in the bag, and again after dextrose is metabolised, which is why D5W and D5 ½NS behave as hypotonic fluids in the body. The tool flags solutions above 900 mOsm/L as central-line only. A table lists 13 common IV fluids, calculated from their label composition, and a second calculator gives calculated serum osmolality (2 Na + glucose/18 + BUN/2.8) and the osmolal gap. Educational use only — not medical advice.

How it works

Pick a preset (NS, LR, D5W, Plasma-Lyte A, 3% saline…) or enter each component.
Dextrose: % × 10 g/L ÷ 198.17 g/mol × 1000 (hydrous, as labelled in the US). Choose anhydrous (180.16) only if your product is labelled that way.
Salts: NaCl, KCl and sodium bicarbonate, lactate, acetate or gluconate add 2 mOsm per mEq. CaCl₂ and MgCl₂ add 1.5 mOsm per mEq (3 ions per 2 mEq). MgSO₄ adds 1 mOsm per mEq. Amino acids add about 100 mOsm/L per 1%.
Tonicity in the bag uses the usual nursing bands: under 250 mOsm/L hypotonic, 250–375 isotonic, over 375 hypertonic. Tonicity in the body subtracts dextrose, because it is metabolised to water and CO₂.
Route: up to 600 mOsm/L carries a low phlebitis risk peripherally, 600–900 needs caution, and over 900 mOsm/L should be given through a central venous catheter.

Use cases

Checking whether a compounded bag or peripheral parenteral nutrition stays under 900 mOsm/L.
Showing why D5 ½NS is hypertonic in the bag but hypotonic once infused.
Comparing NS, LR and Plasma-Lyte A when choosing a balanced crystalloid.
Adding KCl to maintenance fluids and seeing the new osmolarity.
Studying for nursing and pharmacy exams on tonicity and IV fluid classification.
Estimating calculated serum osmolality and the osmolal gap from Na, glucose and BUN.

Frequently Asked Questions

Last updated: 2026-10-10 · Reviewed by Nham Vu