A newborn weight loss calculator using the NICE 10% guide
Most babies lose a little weight in the first few days, and midwives check how much at the early weigh-ins. This newborn weight loss calculator turns a birth weight and one or more later weights into the percentage lost or gained, the grams (or ounces) still to regain, and where the baby sits against the two markers in the NICE guideline on faltering growth: a loss of more than 10% of birth weight, and being back to birth weight by 3 weeks.
Enter weights in kilograms, grams or pounds and ounces, add as many weigh-ins as you have, and the chart and table show the curve from birth. Nothing leaves your browser.
This page is information only, not medical advice, and it does not diagnose anything. A percentage is one part of the picture; your midwife, health visitor or doctor looks at the whole baby.
How to use the Newborn Weight Loss Calculator
- Choose the unit: kg, g or lb + oz.
- Enter the birth weight.
- For each weigh-in, enter the day of life (day 1 is the first 24 hours) and the weight. Use Add a weigh-in for more.
- Read the percentage change, the amount to regain and the message about the NICE markers.
- Check the chart and table; weigh-ins over the 10% line are highlighted.
Newborn Weight Loss Calculator features
Percentage and amount
Change from birth weight in %, the grams or ounces lost, and how much is left to regain.
NICE markers
The more-than-10% line and the 3-week mark from NICE NG75, shown on the chart and in plain words.
Several weigh-ins
Up to ten weighings, sorted by day, with the lowest point picked out.
Three units
Kilograms, grams or pounds and ounces; switching converts what you have typed.
Saved on this device
Your numbers stay in this browser so you can add the next weigh-in later.
How the percentage is worked out
% change = (weight now − birth weight) ÷ birth weight × 100. A negative number is a loss.
Worked example: a baby born at 3,500 g weighs 3,220 g on day 3. The loss is 280 g, and 280 ÷ 3,500 = 0.08, so 8.0% below birth weight. The 10% mark for this baby is 3,150 g. If the same baby weighs 3,280 g on day 5, they are 6.3% below birth weight with 220 g to regain. In pounds: 7 lb 11 oz at birth and 7 lb 0 oz on day 4 is 8.9% below.
What NICE says about early weight loss
Source: NICE guideline NG75, Faltering growth: recognition and management of faltering growth in children, recommendations 1.1.1–1.1.5 (published 27 September 2017; checked 25 October 2026).
- It is common for babies to lose some weight in the first days of life, and the loss usually stops after about 3 or 4 days.
- Most babies are back to their birth weight by 3 weeks of age. Many NHS leaflets add that lots of babies are back by about 2 weeks.
- If a baby loses more than 10% of their birth weight, health professionals are asked to assess the baby (including for dehydration), take a detailed feeding history, consider watching a feed, and give feeding support.
- If a baby has lost more than 10%, or is not back to birth weight by 3 weeks, professionals consider whether to refer to a paediatric team or when to weigh again — usually no more often than daily.
Getting the most accurate numbers
Weigh on the same scales where possible, with the baby naked or in a clean nappy, and note the day of life. Home baby scales and clinic scales can differ by tens of grams, which matters when the whole loss is a couple of hundred grams. Wet and dirty nappies are an everyday check between weigh-ins — the newborn diaper chart shows what to expect each day, and the baby tracker logs feeds and nappies together. After the newborn weeks, growth is followed on centile charts; the child growth percentile calculator covers that.
Newborn Weight Loss Calculator FAQ
How do I calculate newborn weight loss percentage?
Subtract the current weight from the birth weight, divide by the birth weight and multiply by 100. The newborn weight loss calculator does this for each weigh-in.
How much weight loss is normal for a newborn?
Some loss in the first few days is common and usually stops by day 3 or 4. NICE asks professionals to assess any baby who has lost more than 10% of their birth weight.
When should a baby be back to birth weight?
NICE says most babies are back to birth weight by 3 weeks; many NHS leaflets say lots of babies get there by about 2 weeks. If your baby is not back by 3 weeks, talk to your health visitor or GP.
Is 10% weight loss in a newborn bad?
It is the point where NICE asks for a closer look, not a diagnosis. A baby who has lost more than 10% should be checked and offered feeding support — contact your midwife, health visitor or doctor the same day.
Can I use kg, grams or pounds?
Yes. Pick kg, g or lb + oz; switching converts the numbers you have entered.
Does this replace my midwife’s weigh-in?
No. It is information only. Weigh-ins, feeding checks and your baby’s overall health are for your midwife, health visitor or doctor to judge.