Weight-based dosage calculation: mg/kg step by step
Weight-based orders give a dose per kilogram, and the calculation has three steps: kilograms, dose, volume. Each step is short; the points are lost when one is skipped. This page shows the sequence with two worked examples.
Formula
Convert to kilograms first. Find the dose, then the volume. Round the dose to the tenth and the volume to the tenth of a mL, once, at the end of each.
Example 1 · pounds given
Order: gentamicin 2 mg/kg IV q8h. Patient 154 lb. Available: 40 mg/mL. Dose and volume?
- 154 lb ÷ 2.2 = 70 kg
- 70 kg × 2 mg/kg = 140 mg
- 140 mg ÷ 40 mg/mL = 3.5 mL
Answer: 140 mg, 3.5 mL
Example 2 · kilograms given
Order: enoxaparin 1 mg/kg SC q12h. Patient 80 kg. Available: 100 mg/mL. Dose and volume?
- 80 kg × 1 mg/kg = 80 mg
- 80 mg ÷ 100 mg/mL = 0.8
- Under 1 mL → 0.8 mL (0.80)
Answer: 80 mg, 0.8 mL
Mistakes that cost points
- Pounds used as kilograms. 154 × 2 mg = 308 mg, 7.7 mL. Convert first: 70 kg, 140 mg, 3.5 mL.
- Multiplying by 2.2. 154 × 2.2 is 338.8. Pounds to kilograms divides.
- Daily dose given once. 40 mg/kg/day is a 24-hour total. Divide by the doses per day.
Practice for this topic opens soon
Generated weight-based problems join the drills shortly. Until then, check your own working with the mg per kg calculator, and practise the steps it builds on: Conversions.
Step 1: kilograms
Every mg/kg order uses the weight in kilograms. If the weight is in pounds, divide by 2.2. A 154 lb patient weighs 70 kg; a 198 lb patient weighs 90 kg.
Use the weight the question gives, usually the current or admission weight. In practice, some drugs are dosed on ideal or adjusted body weight; exams state it when that applies.
Step 2: the dose
Multiply kilograms by the ordered mg/kg. Gentamicin 2 mg/kg for 70 kg is 140 mg. Vancomycin 15 mg/kg for 90 kg is 1,350 mg.
Read whether the order is per dose or per day. mg/kg/dose is the amount for each dose. mg/kg/day is the total for 24 hours, divided into the number of doses in the frequency. The pediatric dosage page covers the divided-dose form, which is where most errors happen.
Step 3: the volume
Divide the dose by the concentration on the label. 140 mg of gentamicin 40 mg/mL is 3.5 mL. 1,350 mg of vancomycin at 1 g / 20 mL, which is 50 mg/mL, is 27 mL.
In dimensional analysis the whole problem is one chain: x mL = 154 lb × 1 kg / 2.2 lb × 2 mg / 1 kg × 1 mL / 40 mg. Every unit cancels except mL.
Safe dose ranges
Weight-based questions often ask whether an order is safe. Multiply the low and high ends of the reference range by the weight to get a range in mg, then compare the order. If it falls outside, the right action is to hold the dose and clarify the order with the prescriber, not to adjust it yourself.
Exam writers build the incorrect answer choices from three slips: pounds used as kilograms, kilograms multiplied by 2.2 instead of divided, and a daily dose given as a single dose.
A safe dose check in numbers
Suppose the reference range for a drug is 5 to 10 mg/kg per dose and the patient weighs 30 kg. The safe range is 150 to 300 mg per dose. An order for 100 mg is below the range, which may be ineffective; an order for 350 mg is above it. Either way the action is the same: hold and clarify with the prescriber.
Compare like with like. If the range is per day, convert the order to a daily total first by multiplying by the number of doses.
Rounding the weight: two conventions
Some programs round the weight in kilograms to the tenth before calculating the dose. Others keep the full value and round once at the end. The two can differ in the last digit for awkward weights. Dose Drills follows round-once-at-the-end; if your program rounds weight first, follow your program, and the difference will show as a Rounding result rather than an error.
How exams test it
Weight-based items almost always give the weight in pounds, because the conversion is the step being tested. Many add a safe dose question or a volume step. The incorrect answer choices are built from the pounds-as-kilograms and multiply-by-2.2 slips, so an answer more than double what you expected is a signal to re-check the first step.
Maximum doses and capped weights
Many references set a maximum dose regardless of weight, for example a maximum single dose or a maximum per day. When a weight-based calculation exceeds the maximum, the maximum applies. A child dosed at 15 mg/kg of a drug with a 1,000 mg maximum single dose receives 1,000 mg once they weigh more than 66.7 kg, not 1,050 mg at 70 kg. Exams that include a maximum expect you to compare the calculated dose against it before you calculate the volume.
Check before you submit
Is the weight in kilograms, divided by 2.2 if it started in pounds? Is the order per dose or per day, and did you divide a daily total? Did you multiply the weight by the mg/kg, not divide? Did you use the label concentration for the volume? Is the dose inside the safe range, if one is given? Is the volume rounded to the tenth, or the hundredth under 1 mL?
A useful estimate: for an adult, kilograms are a little under half of pounds. 154 lb is about 70 kg. If your kilogram figure is larger than the pound figure, the conversion went the other way.
Questions people ask
How do you calculate a weight-based dose?
Convert the weight to kg, multiply by the mg/kg ordered to get the dose, then divide by the concentration to get the volume.
Do I round the weight in kilograms?
Keep the full value unless your program rounds weight first. Round the dose and volume once, at the end.
What is the difference between mg/kg/dose and mg/kg/day?
Per dose is given each time. Per day is the 24-hour total and is divided by the number of doses.
What weight do I use for weight-based dosing?
The weight the question gives, in kilograms. In practice some drugs use ideal or adjusted body weight; exams state it when they do.
What if the dose is outside the safe range?
Do not give it and do not change it yourself. Hold the dose and clarify the order with the prescriber.
Is mg/kg the same as mg per kg per day?
No. mg/kg on its own is usually per dose. mg/kg/day is the 24-hour total, divided by the number of doses in the frequency.