How Much Should a 4-Month-Old Weigh? Infant Weight Percentile Calculator
Estimate where your baby's weight falls on the WHO growth chart and see expected weight ranges by age. The 4-month example is just a default — works for any infant from birth to 24 months.
- Percentile estimate: Your son is around the 37th percentile for weight at 4 months.
- Reference range: Typical 4-month-old boys weigh 5.6–8.7 kg (12.3–19.2 lb) (3rd–97th percentile).
- Growth pace: Average daily gain since birth is 27 g/day — on track for this age (expected ~18 g/day).
- Feeding context: Feeding pattern looks consistent with expected gain.
- What to watch: A single weigh-in matters less than the trend. Call your pediatrician if weight drops across 2+ percentile bands between visits.
Parents often ask how much should a 4 month old weigh, and the honest answer is that there is a healthy range, not a single number. According to the WHO Child Growth Standards, a typical 4-month-old boy weighs about 6.7 kg (14.8 lb) at the 50th percentile, with a healthy range from roughly 5.6 kg (12.3 lb) at the 3rd percentile to 7.9 kg (17.4 lb) at the 97th percentile. Girls average about 6.1 kg (13.5 lb), ranging from 5.1 kg to 7.3 kg. This calculator compares your baby's current weight to those reference curves.
Beyond the headline number, growth trend matters more than any single weigh-in. A baby tracking steadily along the 15th percentile is just as healthy as one along the 75th — pediatricians look for sudden shifts across two or more percentile bands. For example, a 4-month-old girl weighing 6.2 kg sits near the 55th percentile; if she was at the 60th at 2 months, that's normal variation. This tool estimates percentile rank, expected gain since birth, and flags whether the trend deserves a conversation with your pediatrician.
How it works: Enter your baby's age, sex, birth weight, and current weight. We compare current weight against WHO growth reference data for that exact age and sex, then estimate percentile and average daily weight gain since birth.
This calculator is for informational purposes only and does not replace evaluation by a licensed pediatrician. Growth assessment also requires length/height, head circumference, and feeding history. Seek prompt medical evaluation if your baby has not regained birth weight by 2 weeks of age, is gaining less than 15 g/day in the first 4 months, has dropped across 2 or more percentile bands, or weighs below the 3rd percentile — these can indicate failure to thrive. Do not restrict feeding to lower a baby's weight percentile. Infants under 12 months should never be put on a weight-loss diet; calorie restriction below ~100 kcal/kg/day in infancy can impair brain and organ development. Preterm infants must always be assessed at corrected age until at least 24 months; using chronological age will falsely suggest underweight and can lead to inappropriate intervention.
Understanding Infant Weight at 4 Months — and Every Month Before and After
Babies double their birth weight by around 4–5 months and triple it by their first birthday. But there's a wide healthy range, and the right question isn't 'is my baby heavier than average' — it's 'is my baby growing along their own curve?'
WHO weight-for-age reference (50th percentile, kg and lb)
| Age | Boys median | Girls median | Boys 3rd–97th | Girls 3rd–97th |
|---|---|---|---|---|
| Birth | 3.3 kg (7.3 lb) | 3.2 kg (7.1 lb) | 2.5–4.4 kg | 2.4–4.2 kg |
| 2 months | 5.6 kg (12.3 lb) | 5.1 kg (11.2 lb) | 4.3–7.1 kg | 3.9–6.6 kg |
| 4 months | 7.0 kg (15.4 lb) | 6.4 kg (14.1 lb) | 5.6–8.7 kg | 5.0–8.2 kg |
| 6 months | 7.9 kg (17.4 lb) | 7.3 kg (16.1 lb) | 6.4–9.8 kg | 5.7–9.3 kg |
| 9 months | 8.9 kg (19.6 lb) | 8.2 kg (18.1 lb) | 7.1–11.0 kg | 6.5–10.5 kg |
| 12 months | 9.6 kg (21.2 lb) | 8.9 kg (19.6 lb) | 7.7–12.0 kg | 7.0–11.5 kg |
| 18 months | 10.9 kg (24.0 lb) | 10.2 kg (22.5 lb) | 8.8–13.7 kg | 8.1–13.2 kg |
| 24 months | 12.2 kg (26.9 lb) | 11.5 kg (25.4 lb) | 9.7–15.3 kg | 9.0–14.8 kg |
Expected average daily weight gain by age
| Age range | Typical daily gain | Typical weekly gain | Doubling/tripling milestone |
|---|---|---|---|
| 0–3 months | 20–30 g/day (~1 oz) | 140–210 g (5–7.5 oz) | Regain birth weight by 2 weeks |
| 3–6 months | 15–20 g/day (~0.5 oz) | 100–140 g (4–5 oz) | Double birth weight by ~5 months |
| 6–9 months | 10–15 g/day | 70–105 g (2.5–3.5 oz) | Solids introduced; gain slows |
| 9–12 months | 6–11 g/day | 40–80 g (1.5–3 oz) | Triple birth weight by ~12 months |
| 12–24 months | 5–8 g/day | 35–55 g (1.2–2 oz) | Quadruple birth weight by ~24 months |
How Much Should a 4-Month-Old Weigh, Really?
At 4 months, WHO reference data places the median weight at 7.0 kg (15.4 lb) for boys and 6.4 kg (14.1 lb) for girls. The healthy range — defined as the 3rd to 97th percentile — spans 5.6–8.7 kg (12.3–19.2 lb) for boys and 5.0–8.2 kg (11.0–18.1 lb) for girls. That's a 3+ kg span among healthy infants, which surprises many first-time parents. A 4-month-old at 5.8 kg and another at 8.3 kg can both be perfectly healthy if each is tracking consistently along their own percentile curve. The number alone tells you almost nothing without context.
Why the Growth Trend Matters More Than a Single Weigh-In
Pediatricians are trained to look for percentile crossing — a baby moving across two or more major percentile bands (e.g. from 50th to 15th) between well-child visits. A baby who has been at the 25th percentile since birth and stays there at 4 months is thriving. A baby who was at the 75th at 2 months and drops to the 25th at 4 months warrants investigation, even though 25th is statistically 'normal'. Single data points are noisy: a recent illness, a wet vs. dry diaper, or even time of day can shift a weigh-in by 100–300 g. Always interpret weight as a trend line, not a snapshot.
Breastfed vs. Formula-Fed: Why the Curve Diverges
WHO charts are built from exclusively breastfed infants, while the older CDC charts used a mixed-feeding US sample that ran heavier. From around month 3–4 onward, exclusively breastfed babies typically gain weight more slowly than formula-fed peers — sometimes appearing to 'drop' percentiles on CDC charts even when they're perfectly healthy on WHO charts. The American Academy of Pediatrics recommends WHO charts for the first 24 months precisely for this reason. If your breastfed 4-month-old looks like they're slipping from the 60th to the 40th percentile, check which chart your pediatrician is using before worrying.
Preterm Babies: Always Use Corrected Age
For babies born before 37 weeks, weight should be plotted against corrected (adjusted) age — your baby's age minus the number of weeks they were early — for at least the first 24 months. A baby born at 33 weeks who is 4 months chronologically should be evaluated as a ~2-month-old on growth charts. Skipping this adjustment makes preterm infants look severely underweight when they're growing normally. The calculator above lets you flag prematurity and will subtract an appropriate offset; for babies born before 32 weeks, ask your pediatrician about specialized preterm growth charts like the Fenton or INTERGROWTH-21st curves.
How the Calculator Estimates a Percentile (and What It Can't Do)
This tool interpolates between WHO reference points at standard ages and converts your baby's weight into an approximate Z-score relative to the 3rd/50th/97th percentile band, then maps that Z-score onto a percentile bucket. It's accurate to within roughly ±5 percentile points for typical weights but loses precision at the extremes (below 3rd or above 97th). It does not account for length/height, head circumference, or BMI-for-age — all of which a pediatrician uses for a complete picture. Negative or zero inputs are blocked, and weights outside the 1–40 kg range will produce unreliable estimates.
When to Actually Call the Pediatrician
Most weight concerns resolve with reassurance and a follow-up weigh-in. But call your pediatrician promptly if: weight is below the 3rd percentile or has dropped across two percentile bands; your baby has not regained birth weight by 2 weeks of age; weight gain is under 15 g/day in the first 4 months; your baby has fewer than 6 wet diapers per day after the first week; or weight loss accompanies poor feeding, lethargy, vomiting, or persistent fussiness. Above the 97th percentile is rarely an emergency in infancy but is worth discussing if accompanied by a sudden jump in percentiles.
Common Mistakes Parents Make Reading Growth Charts
First, comparing to a friend's baby instead of the curve — siblings and peers vary by 2+ kg at the same age and it means nothing. Second, weighing at home on bathroom scales, which are typically accurate to ±200 g and can swing the percentile estimate by 10+ points; use the pediatrician's scale or a dedicated infant scale. Third, weighing in different clothes or with different diaper states — always strip the baby to a dry diaper or nothing. Fourth, panicking over a single low reading after a stomach bug; a baby can lose 3–5% of body weight in a day of illness and bounce back within a week.
How This Calculator Works: Methodology & Parameter Explanations
Core formula:
Z = (current_weight_kg − median_kg) / ((P97 − P3) / 4) ; percentile ≈ map(Z) ; daily_gain_g = (current_kg − birth_kg) × 1000 / (age_months × 30.44)where:
current_weight_kg— Baby's current weight, converted to kilograms (kg)median_kg— WHO 50th-percentile weight for the baby's age and sex (kg)P3, P97— WHO 3rd- and 97th-percentile reference values for that age and sex (kg)Z— Approximate Z-score: standard deviations from the medianbirth_kg— Birth weight in kilograms (kg)age_months— Chronological age (or corrected age for preterm) (months)
How to apply: The Z-score is mapped to a percentile bucket (3rd/15th/50th/85th/97th) using a piecewise-linear approximation, since true WHO percentiles follow the LMS method (Box-Cox transform) which is heavier than this tool requires. The percentile output should be read as ±5 points, not as a precise medical figure.
Worked example: Example: a 5-month-old girl weighing 14.3 lb with birth weight 7.2 lb. Convert: current = 14.3 × 0.4536 = 6.49 kg, birth = 7.2 × 0.4536 = 3.27 kg. WHO median for a 5-month-old girl is 6.9 kg, with P3 = 5.4 and P97 = 8.8. Z = (6.49 − 6.9) / ((8.8 − 5.4) / 4) = −0.41 / 0.85 ≈ −0.48, mapping to roughly the 33rd percentile. Daily gain = (6.49 − 3.27) × 1000 / (5 × 30.44) = 3220 / 152.2 ≈ 21 g/day — squarely in the expected 15–20 g/day band for months 3–6.
Alternative formulas
CDC growth charts: Same Z-score approach, but reference based on a mixed-feeding US sample
When to use: AAP recommends WHO charts for ages 0–24 months and CDC charts for 2+ years, since CDC's infant sample ran heavier and was largely formula-fed.
Fenton preterm growth chart: Specialized reference for infants born <37 weeks, plotted by post-menstrual age
When to use: Preferred for babies born before 32 weeks or still under medical follow-up for prematurity.
INTERGROWTH-21st: International reference built from 8 healthy populations, 2014 onward
When to use: Used in research and increasingly in clinics; gives nearly identical results to WHO in the first year.
Parameter explanations
| Input | Unit | What it means | Impact on results |
|---|---|---|---|
| Baby's current age (months) | months | Chronological age in months from date of birth. Use decimals for half-months. | Shifts which row of the WHO reference table is used. Older ages have higher medians and wider absolute ranges. |
| Sex at birth | — | Selects boy or girl reference curves; WHO publishes separate tables. | Boys' median runs ~0.5–0.7 kg heavier than girls' across the first year. Choosing the wrong sex can shift the percentile estimate by 10–15 points. |
| Birth weight (with unit) | lb or kg | Weight at delivery. Combined with current weight, drives the daily gain calculation. | Does not affect the percentile estimate (which uses current weight only), but heavily affects average daily gain — a baby born at 5.5 lb gaining to 14 lb in 4 months has gained ~30 g/day, while one born at 8.5 lb gaining to the same 14 lb gains ~17 g/day. |
| Current weight (with unit) | lb or kg | Most recent weigh-in, ideally on a baby scale with dry diaper. | The primary driver of percentile. A 200 g change near the median shifts percentile by roughly 3–5 points; near the extremes it shifts less. |
| Birth weight unit / Current weight unit | lb or kg | Unit selector for input. Internal calculation always uses kg. | Wrong unit selection (e.g. entering 14.8 as kg instead of lb) will produce a wildly off percentile. Always double-check. |
| Primary feeding method | — | Whether the baby is breastfed, formula-fed, mixed, or eating solids. | Does not change the WHO percentile (WHO is breastfed-based) but conditions the personalized insight: breastfed babies tracking lower after month 4 is flagged as normal, not concerning. |
| Preterm status | weeks | Whether baby was born before 37 weeks, and how early. | Subtracts 1.0–2.5 months from chronological age before looking up the reference. A preterm baby compared at uncorrected age will appear ~15–30 percentile points lower than they actually are. |
Assumptions
WHO Child Growth Standards (2006, still current in 2026) are used as the reference for ages 0–24 months.
The 4-month example in the title is only a default — the calculator works for any infant age from 0 to 24 months.
Percentile is approximated via a piecewise-linear Z-score map, not the official LMS method. — WHO's published percentiles use a Box-Cox (LMS) transformation. The simplified map used here is accurate to within ~5 percentile points across the 3rd–97th range, which is sufficient for parental orientation but not for clinical diagnosis.
Reference values between published age points are linearly interpolated. — Real WHO curves are smooth; linear interpolation between monthly anchor points introduces under 1% error in the first year.
Preterm correction uses a simplified offset based on broad gestational-age bands. — A full correction subtracts (40 − birth gestational age in weeks) ÷ 4.345 months. The bands used here approximate this well enough for screening, but specialists should use exact corrected age.
How to use this calculator
- Enter your baby's basics — Age in months, sex, birth weight, current weight. Pick the unit (lb or kg) that matches the scale you used.
- Flag any prematurity — If born before 37 weeks, select the appropriate band so the chart is read at corrected age.
- Read the percentile and the range, not just the number — Note both the estimated percentile and the 3rd–97th range. Anywhere inside that band is statistically healthy.
- Check the trend over time — Re-run the calculator after the next pediatrician visit. Look for stability across percentile bands, not a specific target.
- Bring concerns to your pediatrician — Use the result as a conversation starter, not a diagnosis. Bring the printout or numbers to your next well-child visit.