Weight-Based Dosage Calculations
Weight-based problems give you a dose per kilogram and a weight. You multiply them to find the ordered dose. The dose per kg always comes from the order or problem, never from this site.
In this lesson
- The dose-per-kg × weight setup
- Converting pounds to kilograms with 2.2
- An optional second step to mL using a supplied concentration
Important: every value here is given in a fictional problem. Dosage Calcs performs the arithmetic from supplied values and does not determine an appropriate dose for any patient.
What you're solving for
The ordered dose (for example in mg) that results from the given dose per kg and the given weight.
Ordered dose per kilogram, times weight in kilograms, equals ordered dose
If the weight is in pounds, convert first:
pounds divided by 2.2 equals kilograms
Dosage Calcs uses exactly 2.2 lb = 1 kg and carries the unrounded kg value forward unless a problem says otherwise.
Worked examples
Example 1: Weight already in kg
The order supplies 2 mg/kg. Weight: 30 kg. Calculate the ordered dose.
Step 1: Formula
Dose per kg × Weight in kg = Ordered dose
Step 2: Calculation
2 mg/kg × 30 kg
= 60 mg
Step 3: Check your units
kg cancels with /kg, leaving mg.
Answer: 60 mg
Example 2: Convert lb to kg
The order supplies 5 mg/kg. Weight: 44 lb. Calculate the ordered dose.
Step 1: Convert weight to kg (2.2 lb = 1 kg)
44 lb ÷ 2.2 = 20 kg
Step 2: Formula
Dose per kg × Weight in kg = Ordered dose
Step 3: Calculation
5 mg/kg × 20 kg
= 100 mg
Step 4: Check your units
kg cancels with /kg, leaving mg.
Answer: 100 mg
Example 3: Two steps — dose, then mL
The order supplies 3 mg/kg. Weight: 66 lb. The liquid on hand is labeled 30 mg / 1 mL. How many mL?
Part 1: ordered dose = 90 mg
Step 1: Convert weight to kg (2.2 lb = 1 kg)
66 lb ÷ 2.2 = 30 kg
Step 2: Formula
Dose per kg × Weight in kg = Ordered dose
Step 3: Calculation
3 mg/kg × 30 kg
= 90 mg
Step 4: Check your units
kg cancels with /kg, leaving mg.
Part 2: use the calculated dose as D
Step 1: Calculation
90 mg ÷ 30 mg × 1 mL
= 3 mL
Step 2: Formula
Desired ÷ Have × Quantity = Amount
Step 3: Check your units
mg cancels with mg, leaving mL.
Answer: 3 mL
Common mistakes
Multiplying pounds by 2.2
- What goes wrong
- 44 lb × 2.2 = 96.8 kg
- Why it fails
- A kilogram is heavier than a pound, so kg must be the smaller number.
- Correct approach
- 44 lb ÷ 2.2 = 20 kg
Using pounds as kilograms
- What goes wrong
- 5 mg/kg × 44 = 220 mg
- Why it fails
- The dose is per kg; 44 is in lb.
- Correct approach
- Convert to 20 kg, then 5 mg/kg × 20 kg = 100 mg
Quick self-check
Given 10 mg/kg and 22 lb, what is the ordered dose?
Show the answer
22 lb ÷ 2.2 = 10 kg; 10 mg/kg × 10 kg = 100 mg
Calculate and practice
Related lessons
- Nursing Unit Conversions — Move between metric units, pounds and kilograms, and hours and minutes.
- Dimensional Analysis for Dosage Calculations — Chain conversion factors so unwanted units cancel.
- Liquid Medication Calculations — Read a concentration correctly and calculate the mL.
Sources
- Nursing Skills — Math Calculations — Open RN / NCBI Bookshelf
- Pharmacology for Nurses — 2.4 Dosage Calculations — OpenStax
Explanations are original Dosage Calcs wording. See all sources and the methodology.
Educational Use & Safety
Educational content only. These lessons teach medication math using fictional values and do not determine an appropriate medication, dose, infusion rate, or treatment for an actual patient.
Educational Use & Safety