Pediatric Weight Estimation Calculator
Estimate pediatric weight from age using Broselow, APLS, and Nelson formulas for emergencies. Enter values for instant results with step-by-step formulas.
Formula
APLS: Infant = (0.5 x months) + 4 | 1-5yr = (2 x years) + 8 | 6-12yr = (3 x years) + 7
Multiple validated formulas estimate pediatric weight from age. APLS uses three age-group equations. Nelson uses (months+9)/2 for infants and (years x 7-5)/2 for older children. Best Guess uses (months+9)/2 for infants, 2x(years+5) for 1-4 years, and 4x(years) for 5-14 years. Average of available formulas provides the best composite estimate.
Worked Examples
Example 1: Emergency Weight Estimation for 3-Year-Old
Problem: A 3-year-old child is brought to the ED after a motor vehicle accident. No parent is present and the child cannot be weighed. Estimate the weight using available formulas for emergency medication dosing.
Solution: APLS (1-5 yr): (2 x 3) + 8 = 14 kg\nNelson (1-6 yr): (3 x 2) + 8 = 14 kg\nLeffler: (3 x 2) + 10 = 16 kg\nArgall: (3 + 4) x 2 = 14 kg\nBest Guess (1-4 yr): 2 x (3 + 5) = 16 kg\n\nAverage: ~14.8 kg\nFluid bolus (20 mL/kg): 296 mL\nET tube: (3/4) + 4 = 4.75 (use 4.5 uncuffed)
Result: Estimated weight: 14-16 kg (average 14.8 kg) | Use for emergency drug dosing and equipment sizing
Example 2: Infant Weight Estimation at 6 Months
Problem: A 6-month-old infant requires emergency intubation. Estimate weight for drug dosing and equipment selection.
Solution: APLS (Infant): (0.5 x 6) + 4 = 7 kg\nNelson (3-12 mo): (6 + 9) / 2 = 7.5 kg\nBest Guess (Infant): (6 + 9) / 2 = 7.5 kg\n\nAverage: ~7.3 kg\nET tube size: 3.5 (for 6 months)\nET depth: ~9 cm at lip\nFluid bolus: 7.3 x 20 = 146 mL\nDefib: 7.3 x 2 = 14.6 J initial
Result: Estimated weight: 7-7.5 kg (average 7.3 kg) | ETT 3.5 at 9 cm | Bolus 146 mL
Frequently Asked Questions
Why is estimating pediatric weight important in emergency settings?
Accurate weight estimation is critical in pediatric emergencies because nearly all pediatric drug doses, fluid volumes, defibrillation energies, and equipment sizes are weight-based. Unlike adults where standard doses are commonly used, children span a weight range from 3 kg newborns to 80+ kg adolescents, making weight-based dosing essential for both safety and efficacy. In emergency situations such as cardiac arrest, trauma resuscitation, or status epilepticus, there is often no time to weigh the child on a scale, and delays in treatment can have devastating consequences. Overestimation of weight can lead to drug toxicity, while underestimation can result in subtherapeutic doses. Studies have shown that weight estimation errors greater than 10 percent can lead to clinically significant dosing errors for medications with narrow therapeutic indices.
What is the APLS formula for estimating pediatric weight?
The Advanced Paediatric Life Support (APLS) formula is one of the most widely taught age-based weight estimation methods in pediatric emergency medicine. It uses three separate equations based on age groups. For infants up to 12 months, the formula is: Weight (kg) = (0.5 times age in months) + 4. For children aged 1 to 5 years, the formula is: Weight (kg) = (2 times age in years) + 8. For children aged 6 to 12 years, the formula is: Weight (kg) = (3 times age in years) + 7. The APLS formulas are designed for simplicity and rapid mental calculation during emergencies. However, studies conducted since the formulas were developed have shown they tend to underestimate weight in modern pediatric populations, particularly in regions with increasing childhood obesity prevalence.
How does the Broselow tape compare to age-based weight estimation formulas?
The Broselow tape (Broselow-Luten system) is a length-based weight estimation tool that uses a measured recumbent length to estimate weight, which then corresponds to color-coded zones providing pre-calculated drug doses and equipment sizes. The Broselow tape has been shown to be more accurate than age-based formulas for children weighing up to approximately 25 kg (roughly 8 years old) because length correlates more strongly with weight than age alone. Studies demonstrate that the Broselow tape estimates weight within 10 percent of actual weight in approximately 55 to 75 percent of children. However, the tape may underestimate weight in obese children and may be less accurate in children over 36 kg. The tape is particularly valuable because it eliminates the need for mathematical calculation, reducing cognitive load during high-stress resuscitation scenarios.
What are the limitations of age-based weight estimation formulas?
Age-based weight estimation formulas have several significant limitations that clinicians must understand. First, they assume average body habitus and do not account for individual variation, meaning they can be highly inaccurate for children who are significantly above or below the 50th percentile for weight. Second, many formulas were derived from population data that is now decades old and may not reflect current pediatric weight distributions, particularly the increasing prevalence of childhood obesity. Third, the formulas use population averages and inherently cannot capture ethnic, nutritional, and socioeconomic differences that affect weight. Fourth, most formulas were validated in specific populations and may perform differently in other demographic groups. Fifth, formulas require knowledge of the exact age, which may not be available in emergencies involving unaccompanied or nonverbal children.
Which weight estimation method is most accurate for modern pediatric populations?
Research comparing multiple weight estimation methods has found that no single formula is universally most accurate across all age groups and populations. The Best Guess formula (Tinning and Acworth, 2007) was specifically developed using modern Australian pediatric weight data and tends to estimate higher weights than older formulas, potentially better reflecting contemporary childhood weights. For infants, the Nelson and APLS formulas perform comparably well. For children aged 1 to 5 years, most formulas produce similar estimates. For older children (6-14 years), the Best Guess formula (4 times age) and updated APLS formula tend to outperform older formulas. The ideal approach in clinical practice is to use multiple methods and consider the range of estimates, use the Broselow tape when available for children under 36 kg, and adjust estimates based on visual assessment of the child's body habitus.
How should weight estimates be adjusted for obese or underweight children?
For obese children, standard weight estimation formulas will significantly underestimate actual weight, potentially leading to subtherapeutic drug doses. Clinicians should use ideal body weight (IBW) rather than actual weight for most medication dosing in obese children, because drug distribution into adipose tissue varies by medication. For lipophilic drugs (such as benzodiazepines), adjusted body weight using a fraction of excess weight above IBW may be appropriate. For hydrophilic drugs (such as aminoglycosides), dosing based on IBW is generally preferred. Visual assessment suggesting obesity should prompt the clinician to increase the formula-based estimate by 10 to 20 percent for actual weight estimation, while using IBW for dosing. For visibly underweight or malnourished children, formula estimates should be reduced by 10 to 20 percent and medications dosed conservatively with plans to reassess.