Pediatric Weight Estimate Calculator
Estimate a child's weight from age alone using the APLS and Best Guess formulas, and compare the estimate with a measured weight.
두 추정식의 평균 15.0kg · 응급 상황용 근사치입니다
추정식별 결과
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- 연령 기반 추정식은 실측 체중을 잴 수 없는 응급 상황의 근사치입니다. 체중계를 쓸 수 있다면 언제나 실측값을 사용하세요.
- 아이의 성장이 정상인지 평가하는 것은 추정식이 아니라 실측 체중의 성장도표 백분위(질병관리청 2017 소아청소년 성장도표)로 판단합니다.
- 추정 체중으로 약 용량(mg/kg)을 계산하지 마세요. 추정식은 개인차로 ±20% 이상 벗어날 수 있어, 해열제·항생제처럼 체중당 용량을 쓰는 약에서는 과소·과다 투여로 이어질 수 있습니다.
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결과 공유
Overview
This calculator estimates a child's body weight from age alone, for situations where a scale is not available. It runs two validated age-based formulas side by side: the APLS (Advanced Paediatric Life Support) 2011 revision used in paediatric emergency care, and the Best Guess formula validated in Australian children. Entering a measured weight additionally shows the percentage deviation from the estimate. Age-based estimates are approximations only — growth assessment should always use a measured weight plotted on a growth chart.
Formula
APLS (2011) — under 12 months: weight (kg) = (0.5 × age in months) + 4 | 1–5 years: (2 × age) + 8 | 6–12 years: (3 × age) + 7 ‖ Best Guess (2007) — under 12 months: (age in months + 9) ÷ 2 | 1–5 years: 2 × (age + 5) | 5–14 years: 4 × age ‖ Deviation (%) = (measured − mean of the two estimates) ÷ mean × 100 ‖ e.g. age 3 → APLS 2×3+8 = 14 kg, Best Guess 2×(3+5) = 16 kg, mean 15 kg
How to Use
- 1Choose the input basis — months for infants under 1 year, years for children aged 1 and older.
- 2Enter the age in months (0–11) or completed years (1–14); both formulas use whole-number ages.
- 3Read the APLS estimate, the Best Guess estimate, and the resulting range and mean.
- 4Optionally enter a measured weight to see the deviation — within ±10% is close, beyond ±20% is flagged as a large difference.
- 5Use the result only as an emergency approximation; use a measured weight for drug dosing and growth assessment.
Examples
6-month-old infant — estimate 7.0–7.5 kg, measured 8.2 kg
For infants under a year the months-based formulas apply: APLS gives (0.5 × 6) + 4 = 7.0 kg and Best Guess gives (6 + 9) ÷ 2 = 7.5 kg, a range of 7.0–7.5 kg with a mean of 7.3 kg. A measured 8.2 kg is 0.95 kg above that mean, a deviation of +13.1%, which falls in the 'noticeable difference' band. Variation of nearly a kilogram is common in the first year, so a measured weight matters most at this age.
3-year-old — estimate 14.0–16.0 kg, measured 13.2 kg
At age 3 both formulas use their 1–5 year band: APLS gives (2 × 3) + 8 = 14.0 kg and Best Guess gives 2 × (3 + 5) = 16.0 kg, a 2 kg spread with a mean of 15.0 kg. A measured 13.2 kg works out to (13.2 − 15.0) ÷ 15.0 × 100 = −12.0%, i.e. lighter than estimated. A deviation of this size is ordinary; whether growth is on track is answered by a growth-chart percentile, not by this number.
8-year-old — estimate 31.0–32.0 kg, measured 38 kg
From age 6 the APLS formula switches to the Luscombe & Owens rule (3 × age) + 7, giving 31.0 kg at age 8, while Best Guess gives 4 × 8 = 32.0 kg — the two nearly agree, with a mean of 31.5 kg. A measured 38 kg is +20.6%, the 'large difference' band. Age-based formulas are known to underestimate weight substantially in overweight school-age children, which is exactly the pattern here; dosing from the estimate would under-dose this child.
Background
Age-based weight formulas exist so that paediatric emergency teams can pick drug doses and defibrillation energies when there is no time to weigh a child. The long-standing original APLS rule, weight = 2 × (age + 4), was criticised for underestimating weight as childhood obesity increased. Luscombe and Owens published weight = 3 × age + 7 in the Emergency Medicine Journal in 2007, and repeated validations led the APLS manual to adopt a three-band set in its 2011 revision: (0.5 × months) + 4 under 1 year, (2 × age) + 8 for 1–5 years, and (3 × age) + 7 for 6–12 years. The Best Guess formulas come from Tinning and Acworth (Emergency Medicine Australasia, 2007); in an Australian comparison of actual versus estimated weights they produced a smaller mean error than the Broselow tape or the original APLS rule. For assessing whether a child's weight is normal, use national growth references — in Korea, the 2017 Korean National Growth Charts from the KDCA and the Korean Pediatric Society.
Tips
- ✔Use whole-number ages — a child of 3 years 7 months is entered as 3, since the published formulas are defined on completed years.
- ✔At age 5 the two Best Guess bands overlap, but 2 × (5 + 5) and 4 × 5 both give 20 kg, so the result is continuous.
- ✔APLS covers up to age 12 and Best Guess up to age 14; beyond that, use a measured weight.
- ✔Error is largest in obese and underweight children — estimates can fall 5–10 kg short for an overweight school-age child.
- ✔For a child born preterm, corrected age tracks actual weight better than chronological age.
- ✔Many emergency departments prefer a length-based Broselow tape to age formulas; both remain substitutes for an actual measurement.
FAQ
Q. Can I dose medication using the estimated weight?
It is not recommended. Age-based estimates can be off by more than ±20% in an individual child, so for weight-based (mg/kg) drugs such as paracetamol or ibuprofen the estimate can lead to under- or overdosing. These formulas are a clinician's stopgap when no weight is obtainable; at home, use a weight measured on a scale.
Q. Which is more accurate, APLS or Best Guess?
It depends on the population. An Australian comparison found Best Guess had the smallest mean error, while UK and Irish data supported the Luscombe & Owens rule that forms the 6–12 year band of the current APLS set. A wide gap between the two estimates signals greater uncertainty, which is why this calculator shows both as a range rather than picking one.
Q. If the measured weight is far below the estimate, is my child underweight?
Not necessarily — these formulas approximate a population average, not a percentile cut-off. Underweight and overweight are defined by growth-chart percentiles for the child's sex and age (below the 3rd percentile for underweight, at or above the 95th for obesity), with BMI percentiles used from age 2. If the deviation exceeds about 20%, check a growth chart and consult a paediatrician if needed.
Q. Do these formulas work for babies under one year?
Yes — enter the age in months and the calculator applies (0.5 × months) + 4 for APLS and (months + 9) ÷ 2 for Best Guess. Accuracy is lowest in the first year, however, because growth is fastest and individual variation greatest. Weights recorded at routine infant health checks, plotted on a growth chart, are a far more reliable reference at this age.
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