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Many doses depend on the patient’s size. Children’s doses are almost always based on weight, and some adult drugs are too. Because a child’s safe range is narrow, nurses also check every pediatric order against the recommended range before giving it.
This lesson covers doses per kilogram, daily doses split into several doses, checking whether an order is safe, body surface area (BSA), and maintenance fluids. The numbers in the examples are for practice only; always use the range in your drug reference and your facility’s policies.
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When the order or reference says mg/kg (or mg/kg/dose), multiply by the weight in kilograms to get one dose. Then calculate the volume to give with the dosage formula.
Change the weight to kg (lb ÷ 2.2), rounded to the nearest tenth.
Multiply by the mg/kg.
Calculate the amount to give.
A child weighs 44 lb. Order: 15 mg/kg per dose. Label: 100 mg per 5 mL. Weight: 442.2 = 20 kg → Dose: 15 × 20 = 300 mg → Volume: 300100 × 5 mL = 15 mL
Watch out: Multiplying by the weight in pounds. That gives a dose 2.2 times too large.
Micrograms per kilogram
Very small patients and strong drugs are often dosed in mcg/kg. The math is the same; just keep track of mcg versus mg.
A newborn weighs 3,200 g. Order: 10 mcg/kg. → 3,200 g = 3.2 kg → 10 × 3.2 = 32 mcg
An adult weighs 80 kg. Order: 5 mg/kg. → 5 × 80 = 400 mg
When the amount is per day, find the daily total first, then divide by the number of doses.
Order says
Doses per day
Order says
Doses per day
every 12 hours (q12h), BID
2
every 6 hours (q6h), QID
4
every 8 hours (q8h), TID
3
every 4 hours (q4h)
6
mg/kg/day in divided doses
Doses per day = 24 ÷ the number of hours between doses. Multiply the mg/kg/day by the weight to get the total for the day, then divide by the doses per day.
Order: 25 mg/kg/day in 2 divided doses. Child: 20 kg. Daily: 25 × 20 = 500 mg/day → Each dose: 500 mg2 = 250 mg
Before giving a pediatric medicine, nurses check the order against the recommended range in a drug reference.
Checking a safe dose range
A reference gives a range, like 50 to 100 mg/kg/day. Calculate the low end and the high end for this child, then see whether the order falls between them. Compare like with like: a daily amount with a daily amount, or a single dose with a single dose.
Change the weight to kg.
Low end: low mg/kg × kg. High end: high mg/kg × kg.
If the range is per day, divide both by the doses per day to get the safe single-dose range (or multiply the ordered dose by the doses per day).
Compare. Inside the range: safe to calculate and give. Outside: hold it and contact the provider.
Reference: 50 to 100 mg/kg/day divided q6h. Child: 10 kg. Order: 200 mg q6h. Daily range: 50 × 10 = 500 mg to 100 × 10 = 1,000 mg → per dose (÷ 4): 125 to 250 mg 200 mg is inside the range → safe
Reference: 25 to 50 mg/kg/day divided q6h. Child: 22 kg. Order: 350 mg q6h. Daily range: 550 to 1,100 mg → per dose: 137.5 to 275 mg 350 mg is above the range → not safe; hold and call the provider
Watch out: Comparing a single dose with a daily range. 200 mg looks safe next to “500 to 1,000 mg,” but that range is for the whole day.
Maximum doses
Some references give a maximum, like “do not exceed 4 g per day.” Multiply the ordered dose by the doses per day and compare with the maximum.
Order: 650 mg every 4 hours. Maximum: 4 g/day. → 6 doses × 650 = 3,900 mg = 3.9 g → under 4 g, within the maximum.
Multiply the height by the weight, divide, then take the square root (the √ key on a calculator). Use the 3,600 formula with cm and kg, or the 3,131 formula with inches and pounds. Round BSA to the hundredth.
Child: 120 cm and 25 kg → √120 × 253,600 = √3,0003,600 = √0.8333 = 0.9129 ≈ 0.91 m²
Adult: 175 cm and 70 kg → √12,2503,600 = √3.4028 ≈ 1.84 m²
Child: 48 in and 55 lb → √2,6403,131 = √0.8432 ≈ 0.92 m²
Watch out: Mixing the formulas, such as cm and lb with 3,600. Use cm with kg, or inches with pounds.
The same idea per hour, for setting an IV pump. The two rules give slightly different answers because 4-2-1 is rounded; use the one your program teaches.