🧠 The theory: moles count particles, not grams
Doses of electrolytes matter as a number of particles (ions), not as a weight, because it is the ions that have the physiological effect. A mole is a fixed number of particles, and one mole of a substance weighs its molecular weight in grams. So dividing a mass by the molecular weight converts grams into a count of particles. Working in millimoles (thousandths of a mole) just keeps the numbers a convenient size for clinical use.
millimoles = mass in mg / molecular weight
from a percentage: mmol per litre = (grams per litre / molecular weight) x 1000
Worked example
How many millimoles are in 585 mg of sodium chloride? (molecular weight 58.5)
- Step 1 — One millimole weighs the molecular weight in milligrams, so divide the mass by the molecular weight.
- Step 2 — 585 / 58.5 = 10.
Answer: 10 mmol
Worked example
How many millimoles of sodium are in 1 litre of 0.9% w/v sodium chloride? (molecular weight 58.5)
- Step 1 — Find the mass per litre: 0.9% w/v is 0.9 g per 100 mL, which is 9 g per litre.
- Step 2 — Convert grams to milligrams: 9 g = 9000 mg.
- Step 3 — Divide by the molecular weight: 9000 / 58.5 = 153.8.
Answer: about 154 mmol of sodium
Worked example
How many millimoles of potassium are in 1 litre of 0.3% w/v potassium chloride? (molecular weight 74.5)
- Step 1 — Mass per litre: 0.3% is 3 g per litre = 3000 mg.
- Step 2 — Divide by the molecular weight: 3000 / 74.5 = 40.3.
Answer: about 40 mmol of potassium
Worked example
How many millimoles of potassium are in 10 mL of 10% w/v potassium chloride? (molecular weight 74.5)
- Step 1 — Find the mass in 10 mL: 10% is 10 g per 100 mL, so 10 mL holds 1 g = 1000 mg.
- Step 2 — Divide by the molecular weight: 1000 / 74.5 = 13.4.
Answer: about 13.4 mmol
💡 Useful reference value
Sodium chloride 0.9% contains about 150 mmol of sodium and 150 mmol of chloride per litre, which is why it is described as physiological or normal saline. Knowing this lets you sense-check your answer.
Test yourself
How many millimoles of sodium are in 1 litre of 0.45% sodium chloride? (MW 58.5)
Answer: 0.45% = 4.5 g/L = 4500 mg; 4500 / 58.5 = about 77 mmol
How many millimoles are in 1.49 g of potassium chloride? (MW 74.5)
Answer: 1.49 g = 1490 mg; 1490 / 74.5 = 20 mmol
How many millimoles of potassium are in 500 mL of 0.3% potassium chloride? (MW 74.5)
Answer: 0.3% = 3 g/L, so 500 mL has 1.5 g = 1500 mg; 1500 / 74.5 = about 20 mmol