Isotonicity Calculator (Sodium Chloride Equivalent E-Value Method)
Calculates grams of Sodium Chloride (NaCl) required to render an ophthalmic, nasal, or parenteral drug solution isotonic with blood/tears (0.9% w/v NaCl).
⚠️ Educational Calculation & Clinical Safety Safeguard: If (Drug_g × E) exceeds (0.009 × V), the drug solution is already hypertonic and no NaCl should be added. Educational calculations do not replace clinician assessment, patient-specific laboratory monitoring, or institutional prescribing protocols.
Interactive Calculator
Allowed physiological/compounding range: 1 to 5000 mL
Allowed physiological/compounding range: 0.001 to 500 g
Allowed physiological/compounding range: 0.01 to 2.0 g NaCl / g drug
Calculated Output (g NaCl (and mg NaCl))
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Step-by-Step Mathematical Derivation & Explanation:
Validated Formula & Conversion Rules
Total NaCl for Isotonicity = 0.009 × Volume (mL); NaCl Equivalent of Drug = Drug (g) × E; NaCl to Add (g) = (0.009 × V) − (Drug_g × E)Worked Clinical Example
Prepare 30 mL of 1% w/v Pilocarpine HCl (0.3 g drug, E = 0.24): Total NaCl needed = 0.009 × 30 = 0.270 g; Drug equivalent = 0.3 × 0.24 = 0.072 g; NaCl to add = 0.270 − 0.072 = 0.198 g (198 mg).
Rounding Rules, Clinical Interpretation, Assumptions & Known Limitations
- 1. Purpose: Calculates grams of Sodium Chloride (NaCl) required to render an ophthalmic, nasal, or parenteral drug solution isotonic with blood/tears (0.9% w/v NaCl).
- 2. Required Inputs & Validated Units: Final Prescription Volume (mL, range 1–5000), Mass of Active Drug in Solution (g, range 0.001–500), Drug NaCl Equivalent (E-value) (g NaCl / g drug, range 0.01–2.0)
- 3. Output Unit:
g NaCl (and mg NaCl) - 4. Result Interpretation Notes: Applies the Sodium Chloride Equivalent (E-value) method to render ophthalmic/parenteral solutions isotonic with 0.9% w/v NaCl.
- 5. Rounding Guidance: Express compounding mass in both grams (3 decimal places) and milligrams (1 decimal place).
- 6. Mathematical & Clinical Assumptions: Uses 0.009 × Volume(mL) as total NaCl equivalent needed for lachrymal/blood isotonicity.
- 7. Known Limitations & Contraindications to Formula Use: If boric acid or another buffer is used as the tonicity agent instead of NaCl, divide the remaining NaCl deficit by the buffer's E-value.
- 8. Supporting Compendial & Clinical References: Indian Pharmacopoeia (IP 2022), National Formulary of India (NFI), British National Formulary (BNF), and NCBI Bookshelf Clinical Pharmacology Compendia.