Isotonicity calculator (sodium chloride equivalent)

The sodium chloride equivalent method multiplies the drug quantity by its E-value to find how much sodium chloride it behaves like, then subtracts that from the amount needed to make the volume isotonic. An isotonic solution needs 0.9 g of sodium chloride per 100 mL in total.

When to use the isotonicity (e-value) calculator

Use when compounding an injection, an eye drop or a nasal preparation that must not damage tissue. An E-value states how many grams of sodium chloride produce the same osmotic effect as one gram of the drug.

The isotonicity (e-value) formula

Sodium chloride required = (0.009 × volume in mL) − (drug quantity in g × its E-value). The 0.009 comes from 0.9% w/v, the concentration of sodium chloride that is isotonic with blood.

Worked example

0.3 g of a drug with an E-value of 0.18 in 30 mL. NaCl needed for isotonicity = 0.009 × 30 = 0.27 g. The drug already contributes 0.3 × 0.18 = 0.054 g. So add 0.27 − 0.054 = 0.216 g of sodium chloride.

The mistake that costs marks

A negative result means the drug alone already exceeds isotonicity, so no sodium chloride is added and the solution must be diluted instead. Adding salt anyway is the classic error. E-values are also specific to each drug and must be looked up rather than assumed.

Study aid, not a clinical tool

This calculator exists so you can learn the method and check your working for an examination. It is not medical advice and not clinical decision support, and it must not be used to make a decision about a real patient. Verify any result against your local formulary, the product literature and a qualified pharmacist. Our medical disclaimer sets out the full position.

Calculators you will usually need alongside this

Most questions that reach for one of these reach for another a step later, so these are the three that most often pair with it.

• Osmolarity calculator — Osmolarity
• Dilution calculator (C1V1 = C2V2) — Dilution
• Ratio strength and percentage converter — Ratio strength

Keep going

Calculations are only half of what examinations test. The other half is knowing why a drug does what it does, which is what decides whether a number you calculated is even the right number to be calculating. Start with Best pharmacy calculators. The drug class pages cover the mechanism behind the number, or browse the rest of the free calculators and try the practice questions.