For household purposes, an American Standard Teaspoon has been established by the American National Standards Institute* as containing 4.93 ± 0.24 mL. In view of the almost universal practice of employing teaspoons ordinarily available in the household for the administration of medicine, the teaspoon may be regarded as representing 5 mL. Preparations intended for administration by teaspoon should be formulated on the basis of dosage in 5-mL units. Any dropper, syringe, medicine cup, special spoon, or other device used to administer liquids should deliver 5 mL wherever a teaspoon calibration is indicated. Under ideal conditions of use, the volume error incurred in measuring liquids for individual dose administration by means of such calibrated devices should be not greater than 10% of the indicated amount.
Household units are used often to inform the patient of the size of the dose. Fifteen milliliters should be considered 1 standard tablespoonful; 10 mL, 2 standard teaspoonfuls; and 5 mL, 1 standard teaspoonful. Doses of less than 5 mL are frequently stated as fractions of a teaspoonful or in drops.
Because of the difficulties involved in measuring liquids under normal conditions of use, patients should be cautioned that household spoons are not appropriate for measuring medicines. They should be directed to use the standard measures in the cooking-and-baking measuring spoon sets or, preferably, oral dosing devices that may be provided by the practitioner. It must be kept in mind that the actual volume of a spoonful of any given liquid is related to the latter's viscosity and surface tension, among other influencing factors. These factors can also cause variability in the true volumes contained in or delivered by medicine cups. Where accurate dosage is required, a calibrated syringe or dropper should be used.
* American National Standards Institute, 1430 Broadway, New York, NY 10018.