This document shows the metric ystem, the common system, conversion equivalents, calculation of doses, IV flow rate calculation, pediatric calculation: Clark's rule, Young's rule, Cowling's rule, Fried's rule for infants, and calculations based on surface area (BSA).
DOSAGE CALCULATIONS I. Metric System Kilo, hecto, deka, meter, deci, centi, milli, micro, nano, pico II. The Common System Apothecaries Fluid Measure 60 minims 8 fluidrachms 16 fluidounces 2 pints 4 quarts 1 fluidrachm (fz) 1 fluidounce (fz) 1 pint (pt) 1 quart (qt) 1 gallon (gal) Apothecaries Measure of weight 20 grains 3 scruples 8 drachms 12 ounces 1 scruple ( ) 1 drachm ( ) 1 ounce ( ) 1 pound (lb) III. Conversion Conversion Equivalents of Volume 1 ml 1 minim 1 fz 1 fz 1 pt 1 gal (US) 16.23 minims 0.06 ml 3.69 ml 29.57 ml 473 ml 3785 ml Conversion Equivalents of weights 1g 1kg 1 gr 1 oz (avoir.) 1 lb (avoir.) 15.432 gr 2.2 lb 65 mg 28.35 g 454 g IV. Approximate equivalents V. Calculation of doses A. Basic Formula D = desired dose H = dose on Hand/ Labeled dose/ Available dose Q = unit of measure that contains the dose on hand; for tablets and capsules, it is always 1; varies in liquids X= unknown dosage that you need to administer Formula: B. D/H x Q = X Ratio & Proportion H Q :D X ; HX = DQ Example: Order: 125 mg Amoxicillin suspension every 8 hours Available: 250mg/5ml Amoxicillin suspension How many ml should be given to the patient every 8 hours D = 125 mg H= 250mg Q=5ml Therefore: 125mg/250mg x 5ml = 2.5 ml should be given to the patient every 8 hours Or by ratio and proportion: 250mg = 125mg , 5ml X X = 2.5 ml VI. Intravenous Flow rate Calculation 2-step Step 1 = amount of fluid divided by hours to administer = ml/hr Step 2 = ml/hr Xgtts/ml(IV set) = gtts/min 60 min 1-step Amount of fluid X gtt/ ml (IV set) Hours to administer X minutes/hour Or V1 x V2 , T1 T2 V1 = volume to infuse V2 = drop factor T1 = time to infuse (minutes or hours) T2 = time in minutes – always 60 (min/hr) = gtts/min Example: Ordered: D5W 1000 ml to infuse in 6 hours. The drop factor of your tubing is 10 gtts/ml. How many gtts/min will you infuse? V1=1000 ml V2=10 gtts/ml T1= 6 hr T2 = 60 min/hr 1000 ml X 10 gtt/ml = 1000 X 1 = 1000 = 27.8 = 28 gtts/min 6 hr 60 min/hr 6 6 36 VII. Pediatric Calculation Two methods are used to calculate pediatric dosages: According to the weight in kilograms (kg) According to the child's body surface area (BSA) Calculations based on body weight 1. The first step is to convert the child's body weight into kg. The formula is 2.2 lb. = 1 kg. 2. The second step is to calculate the medication dose. a. Calculate the daily dose. b. Divide the daily dose by the number of doses to be administered. c. Use either the ratio-proportion or formula method to calculate the number of tablets or capsules or volume to be administered with each dose. Example: A child weighing 76 lbs. is ordered to receive 150 mg of Clindamycin q6h. The pediatric drug handbook states the recommended dose is 8-20 mg/kg/day in four divided doses. The Clindamycin is supplied in 100 mg scored tablets. How many tablets should you give the patient? 1. What is the weight in kg? 34.5 kg 2. What is the safe total daily dose? Minimum: 8 mg/kg/day X 34.5 kg = 276 mg/day Maximum: 20 mg/kg/day X 34.5 kg = 690 mg/day 3. Is this a safe dose? Yes 4. Calculate the number of tablets to give. 100 mg : 1 tablet = 150 mg : x = 100 x = 150 = x = 1.5 tablets Clark’s Rule, based on weight Weight (in lb) x Adult dose = 150 (average weight of adult in lb) Dose for child Calculations based on Age 1. Young’s Rule = Dose for child Age X adult dose Age + 12 2. Cowling’s rule Age at next birthday (in years) x adult dose = Dose for child 24 3. Fried’s Rule for Infants Age (in months) x Adult dose 150 = Dose for infant Calculations based on Body Surface Area (BSA) BSA is determined from a nomogram using the child's height and weight. Child's BSA X adult dosage 1.73 M2 Example: The child has a BSA of 0.67 M2. The adult dose is 40 mg. The physician ordered 8 mg. Is the dosage correct? 0.67 X 40 = 26.8 = 1.7 1.7 15.8 mg , the dose is too small