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  

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  

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)