Osmolality (mOsm/kg) = (peptide MW × concentration in mg/mL) ÷ (molecular weight × 1000). Isotonic solutions (280-310 mOsm/kg) cause minimal discomfort. For example, a 2mg/mL solution of a 3000 Da peptide has osmolality of ~667 mOsm/kg (hypertonic). Diluting to 1mg/mL reduces osmolality to ~333 mOsm/kg, closer to physiological.
USP tonicity calculations provide detailed formulas.