Baby Weight Percentile Calculator

Baby weight and length growth tracking uses percentile charts from the World Health Organization (WHO) for children birth to age 2, and CDC charts from age 2 onward.
These charts establish whether a baby is growing at a healthy rate, and both halves of that matter: the current measurement, and the trajectory it has been on.

Weight gain reference rates: Newborn to 3 months: ~1 oz (28g) per day = ~7 oz/week 3–6 months: ~0.7 oz (20g)/day = ~5 oz/week 6–12 months: ~0.5 oz (14g)/day = ~3–4 oz/week 1–2 years: ~0.25 oz (7g)/day ≈ 5.5 lbs (2.5 kg) per year

Length gain reference rates: 0–3 months: ~1 inch (2.5 cm)/month 3–6 months: ~0.75 inch (2 cm)/month 6–12 months: ~0.5 inch (1.5 cm)/month Year 1 total: approximately 10 inches (25 cm)

Weight-for-length ratio: Weight-for-Length = (Actual Weight ÷ Median Weight at same length) × 100 Below 80% = underweight; above 120% = overweight for length.

Head circumference growth: 0–3 months: ~0.5 inch (1.2 cm)/month Healthy total growth year 1: from ~34 cm at birth to ~46 cm at 12 months

What each variable means:

• WHO vs. CDC: WHO charts reflect children raised in optimal conditions (breastfed, non-smoking home, well-nourished); CDC charts reflect the actual US population; WHO is recommended for birth–24 months internationally
• Percentile vs. absolute value: a consistently 10th-percentile baby who has always tracked there is healthy; a baby who drops from 60th to 20th percentile in 3 months warrants investigation
• Birth weight regain: newborns typically lose 5–10% of birth weight in the first week; expect birth weight to be regained by day 10–14

Reference: average birth measurements:

• Weight at birth: 7.5 lbs (3.4 kg) average; healthy range 5.5–9.5 lbs
• Length at birth: 20 inches (50.8 cm) average; healthy range 18–22 inches
• Head circumference: 13.5 inches (34.3 cm) average

What this calculator covers, and what it does not: It reads weight for age only, which is the measurement parents check most often and the one you can take at home with a bathroom scale. Length and head circumference need a printed WHO chart, because measuring an infant’s length accurately takes two people and a proper length board. The reference rates above will tell you whether those are moving at a sensible pace.

Worked example: Baby girl, 4 months old, weighing 6.0 kg, which is a shade over 13.2 lb.

• WHO median for a 4-month girl is about 6.4 kg, with roughly 0.7 kg to one standard deviation
• Her z-score is (6.0 − 6.4) ÷ 0.7 = −0.57
• Reading that off the normal curve gives roughly the 28th percentile: normal, a little below the median
• Trajectory matters more than the number. If she tracked near the 50th at 2 months and is at the 28th now, that is a single crossing and usually nothing. Crossing two percentile bands and staying there is what gets a pediatrician’s attention, and the questions are feeding frequency and milk supply.