Accurately estimate a child's weight based on age using standard Advanced Paediatric Life Support (APLS) formulas. Essential for medical dosing and growth verification.
Months (< 1 Year)
Years (1 – 12 Years)
Select whether to calculate by months (infants) or years (children).
Enter the age in the selected unit above.
Please enter a valid age (Months: 0-12, Years: 1-12).
Estimated Weight
20.0 kg
Formula: (2 × Age) + 8
Reference Range (-1yr)
18.0
kg (Approx)
Reference Range (+1yr)
22.0
kg (Approx)
Example Drug Dose
300
mg (Paracetamol @ 15mg/kg)
Estimated weight progression based on APLS guidelines.
Age
Calculation Formula
Estimated Weight (kg)
Est. Weight (lbs)
What is a Paediatric Weight Calculator?
A paediatric weight calculator is a critical clinical tool used to estimate the body weight of infants and children when direct measurement is not immediately possible. This is particularly vital in emergency medicine, resuscitation scenarios, and pre-hospital care where accurate weight is required to calculate safe drug dosages and equipment sizes.
While a calibrated scale remains the gold standard for obtaining weight, emergency situations often preclude weighing a distressed or critically ill child. In these instances, healthcare professionals rely on age-based formulas to provide a "best guess" estimation. This calculator uses widely accepted mathematical models, primarily the Advanced Paediatric Life Support (APLS) formulas, to derive these estimates.
Common misconceptions include the idea that these calculators can replace regular growth monitoring. They cannot. They are estimation tools designed for immediate decision-making or cross-referencing, rather than tracking long-term nutritional health.
Paediatric Weight Calculator Formula and Mathematical Explanation
The calculation logic relies on linear regression models derived from population growth data. The most current and widely taught formulas in life support courses (APLS) segment the calculation based on the child's age group to account for different growth velocities.
The APLS Formulas
This calculator employs the following updated logic:
Infants (< 1 year):Weight = (0.5 × Age in Months) + 4
Children (1 – 5 years):Weight = (2 × Age in Years) + 8
Children (6 – 12 years):Weight = (3 × Age in Years) + 7
Variable
Meaning
Unit
Typical Range
Age (m)
Age of infant in months
Months
0 – 12
Age (y)
Age of child in years
Years
1 – 12
Weight
Estimated body mass
Kilograms (kg)
3kg – 50kg+
Coefficient
Growth multiplier (0.5, 2, or 3)
Unitless factor
Fixed by age group
Practical Examples (Real-World Use Cases)
Example 1: Emergency Triage for a Toddler
Scenario: A 3-year-old child arrives at the ER in status epilepticus. The parents do not know the child's current weight. The medical team needs to administer medication immediately.
Input: Age = 3 Years
Calculation: (2 × 3) + 8
Result: 14 kg
Application: The nurse calculates the Diazepam dose (0.5mg/kg) as 0.5 × 14 = 7mg.
Example 2: School-Aged Checkup Estimate
Scenario: A general practitioner is reviewing the file of an 8-year-old before an appointment and wants a quick mental benchmark for expected weight.
Input: Age = 8 Years
Calculation: (3 × 8) + 7
Result: 31 kg
Application: If the child weighs significantly more or less than 31 kg upon measurement (e.g., 45kg or 20kg), the GP knows to investigate potential obesity or failure to thrive issues.
How to Use This Paediatric Weight Calculator
Follow these steps to generate an accurate weight estimation:
Select Age Unit: Choose "Months" for infants under 1 year, or "Years" for children aged 1 to 12.
Enter Age: Input the specific age. Ensure the number corresponds to the unit selected (e.g., do not enter '12' in the Years field if you mean 12 months).
Review Results: The primary box displays the estimated weight in kilograms.
Check Reference Range: Look at the intermediate values to see estimates for a child one year younger and one year older to understand the growth trajectory.
Copy Data: Use the "Copy Results" button to save the data for medical notes or transfer to a dosage calculator.
Key Factors That Affect Paediatric Weight Results
While the formula provides a statistical average, individual weight varies significantly due to:
Genetics and Ethnicity: Parental height and build are strong predictors of a child's size. Population-specific growth charts often vary from global averages.
Nutrition Quality: Caloric intake, breastfeeding duration, and diet quality directly impact growth velocity and weight gain.
Pre-existing Health Conditions: Chronic conditions like asthma, cardiac issues, or malabsorption syndromes can lead to lower weight-for-age.
Recent Illness: Acute illnesses often cause temporary weight loss due to dehydration or reduced appetite.
Gender: Boys and girls have slightly different growth curves, though emergency formulas often generalize this for speed.
Prematurity: Children born prematurely may have a "corrected age" that differs from their chronological age, affecting where they sit on standard curves.
Frequently Asked Questions (FAQ)
Is this calculator accurate for all children?
No formula is 100% accurate for every individual. This calculator provides an estimation based on the 50th percentile of population data. It is an approximation tool, not a measurement replacement.
Can I use this for children over 12?
For children over 12, puberty causes growth rates to vary so wildly that simple linear formulas become unreliable. Adult ideal body weight formulas or BMI charts are preferred.
Why are there different formulas (APLS vs Luscombe)?
Different formulas are developed based on different population datasets or time periods. The updated APLS formulas used here are designed to reflect the increasing average weight of children in modern populations.
Does this calculator account for obesity?
No. These formulas estimate "ideal" or average weight. In populations with high rates of childhood obesity, these formulas may underestimate the actual weight of the patient.
What is the "Example Drug Dose"?
The calculator displays a sample calculation for Paracetamol (15mg/kg) to demonstrate how the estimated weight translates to clinical utility. Always verify doses with an official formulary.
How do I convert kilograms to pounds?
To convert the result to pounds, multiply the kilogram value by 2.20462. (e.g., 10 kg ≈ 22 lbs).
Is this appropriate for premature infants?
Use with caution. For premature infants, corrected age (actual age minus weeks premature) should be used for better accuracy, though growth charts are far superior for this group.
Why is weight estimation important in emergencies?
Many resuscitation drugs (like adrenaline/epinephrine) and defibrillation energies are calculated per kilogram. A significant error in weight estimation can lead to toxicity or therapeutic failure.
Related Tools and Internal Resources
Explore our other medical and health calculation tools: