Remote Primary Health Care Manuals

Calculating medicine doses and drip rates

Dose calculations

  • Dosages often written as amount/kg/dose (eg 25mg/kg/dose)
    • This means a dose is made up of 25mg for each kg of body weight
  • Dose needed = amount of mg/kg x weight of person in kg

Example

    • Amount in mg/kg is 25mg/kg, weight of person is 12kg
    • Dose needed = 25 (mg/kg) x 12 (kg) = 300mg

Table 6.4 Calculating doses  

TABLETS

Number of tablets needed =
dose needed [a] ÷ strength of tablet [b]

 Example:
Dose needed is 15mg [a]
Strength of tablet is 10mg [b]
Number of tablets = 15mg ÷ 10mg = 1.5 = 1½ tablets
MIXTURES
OR INJECTIONS — small volume IM or IV push

Volume needed (mL) =
(dose needed [a] ÷ strength of mixture or injection [b]) x volume this strength is in mL [c]

OR

dose needed (a) x volume this strength is in mL (c)
strength of mixture or injection (b)

Example 1:
Dose needed is 300mg [a]
Strength is 250mg/5mL [b/c]
Volume needed = (300 ÷ 250) x 5mL = 1.2 x 5 = 6mL

OR Volume needed = 300 x 5 =  6mL                                                    250

Example 2:
Dose needed is 20mg [a]
Strength is 30mg/mL [b/c]
Volume needed = (20 ÷ 30) x 1 = 0.67mg x 1 = 0.67mL

Dosage examples given in mg, but same formulas can be used for other strengths (eg microgram). Must use same unit for strength and for dose needed (eg mg and mg, microgram and microgram)

Quick calculations

  • Dose needed = amount of medicine per kg  x  body weight (kg)

  • Number of tablets needed = dose needed ÷ strength of tablet

  • Volume of mixture or injection needed (mL) =
                     dose needed                     x   volume this strength is in (mL)
    strength of mixture or injection              

Drip rate and infusion rate calculations

Table 6.5 Calculating drip rates and infusion rates for IV fluids  

GRAVITY ADMINISTRATION SET
Remember: Check drop rate on infusion set packet (eg 20 drop/mL, 60 drop/mL)
Rate (drops/minute) =
(total volume of solution (mL) [a] x number of drops/mL [b]) ÷ time in minutes [c]		 Example
Volume of fluid to give is 1,000mL (1L) [a]
Set delivers 20 drop/mL [b]
Time to give is 5 hours = 5 x 60 = 300 minutes [c]
Rate (drops/min) = (1,000mL x 20 drops/mL) ÷ 300min = 20,000 drops ÷ 300 = 67 drops/min
INFUSION PUMP — setting dials
Remember: Always check instructions for your machine
Rate (mL/hr) =
total volume of solution (mL) [a] ÷ time in hours [b]		 Example
Volume of medicine is 5mL, volume of fluid is 1000mL (1L). Total volume of solution to give is 1,005mL [a]
Time to give is 5 hours [b]
Rate (mL/hr) = 1,005mL ÷ 5 hours = 201mL/hr

Units and concentrations

  • 1 litre (L) = 1,000 millilitres (mL)
  • 1 gram (g) = 1,000 milligrams (mg)
  • 1 milligram (mg) = 1,000 micrograms
  • 1% solution = 1g of solute dissolved in 100mL of solution
  • 1:1,000 = 1g solute dissolved in 1,000mL of solution = 1mg solute dissolved in 1mL of solution

Converting units

  • Grams (g) to milligrams (mg) = g x 1,000
    • OR Move decimal point 3 numbers to right (1g = 1.000g = 1,000mg)
  • Milligrams (mg) to grams (g) = mg ÷ 1,000
    • OR Move decimal point 3 numbers to left (1mg = 0001.0mg = 0.001g)
  • Milligrams (mg) to micrograms = mg x 1,000
    • OR Move decimal point 3 numbers to right (1mg = 1.000mg = 1,000microgram)
  • Micrograms to milligrams (mg) = microgram ÷ 1,000
    • OR Move decimal point 3 numbers to left (1microgram = 0001.0microgram = 0.001mg)
  • Litres (L) to millilitres (mL) = L x 1,000
    • OR Move decimal point 3 numbers to right (1L = 1.000L = 1,000mL)