Newborn Weight Loss Calculator NHS
This independent UK newborn weight loss calculator works out the percentage change from a baby’s birth weight to a current weight. Some weight loss is common in the first days after birth, but the number must be considered alongside feeding, hydration, age and the baby’s overall health.
Newborn weight loss calculator
What is newborn weight loss?
It is common for babies to lose some weight during the early days after birth, largely because of normal body-fluid adjustments. NICE says this early weight loss usually stops after about 3 or 4 days, and most infants have returned to their birth weight by 3 weeks.
How to use the calculator
- Enter the baby’s recorded birth weight in grams.
- Enter the latest measured weight in grams.
- Optionally enter age in completed days for additional context.
- Select Calculate to see the gram change and percentage change from birth weight.
Formula
Percentage weight loss = (birth weight − current weight) ÷ birth weight × 100. If current weight is above birth weight, the calculator reports weight gain instead.
Worked example
A baby weighs 3,500 g at birth and 3,150 g at the latest weighing. The loss is 350 g. 350 ÷ 3,500 × 100 = 10.0%.
What does more than 10% mean?
NICE recommends clinical assessment if an infant in the early days of life loses more than 10% of birth weight. Assessment includes looking for dehydration, illness or another disorder, taking a detailed feeding history and considering direct observation of feeding. The percentage is therefore a trigger for assessment, not a diagnosis by itself.
When should birth weight be regained?
The NHS says babies are weighed during their first 2 weeks to check that they are regaining birth weight, and most babies are at or above their birth weight by 3 weeks. A midwife or health visitor can support you if a baby loses a large amount of weight or has not regained birth weight by then.
Important limitations
This calculator does not assess hydration, jaundice, feeding effectiveness, urine or stool output, illness, gestational age or growth centiles. It cannot decide whether supplementary feeding is required. If your baby seems unwell, unusually sleepy, is feeding poorly, has fewer wet nappies, or you are worried for any reason, seek advice from your midwife, health visitor, GP, NHS 111 or urgent services as appropriate.
Authoritative sources
- NICE NG75: Faltering growth – weight loss in the early days of life
- NHS: Your baby’s weight and height
- RCPCH: UK-WHO growth charts
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FAQs
How do I calculate newborn weight loss percentage?
Subtract current weight from birth weight, divide the difference by birth weight, then multiply by 100.
Is some weight loss after birth normal?
Yes. NICE says some weight loss is common in the early days of life and usually stops after about 3 or 4 days.
What happens if a newborn loses more than 10%?
NICE recommends clinical assessment, including assessment for dehydration or illness and a detailed review of feeding.
When should a baby regain birth weight?
Most babies are at or above their birth weight by around 3 weeks, according to NHS and NICE guidance.
Medical disclaimer: This independent calculator is for general information only. It is not an NHS diagnostic service and does not replace assessment by a midwife, health visitor, doctor or other qualified healthcare professional.