Baby weight and length

Why is your baby’s weight important?

Knowing the weight of your baby or young child is important for a number of reasons:

  • it is important to understand whether your baby is under- or over-weight;
  • it is important to follow your baby’s growth and development;
  • essentially safety information can relate to a child’s weight;
  • in case of emergency it may be important to provide you baby’s weight to a doctor; medicine doses are calculated based on weight.

Why use kilograms?


Kilograms are much simpler to understand than pounds and ounces – there are 1,000 grams in a kilogram, so any changes in your child’s weight can be easily calculated, and you don’t need to try to remember how many ounces are in a pound and pounds in a stone.

For example, babies are classified as under weight in the UK if they are born weighing below 2.5 kg. If you don’t know the birth weight of your child in kilograms, how easy is it to work out if your baby is under weight, or close to being under weight?

Baby boy weight and length

The Department of Health uses weight of babies in kilograms, as well as head and body size in centimetres to monitor baby growth.


Critical safety equipment for your baby or small child often requires knowledge of their weight for their safety. If your car booster seat is suitable for children up to 18 kg and your child is 1 stone 4 pounds and 11 ounces, are they safe in that seat in the event of an accident?

In addition, body weight in kg is used to calculate medicine doses both in Britain and abroad, especially so for babies and small children, where the margin of error for medicines is smaller than for adults.

Mistakes made in converting a baby’s weight from lb/oz to kg in the heat of the moment could have catastrophic consequences, especially overseas where doctors will not understand pounds and ounces.

Don’t take a chance, know your child’s weight in kilograms.

Why is your baby’s length important?

In addition to weight two other indications of baby size are important. One is the length and the other is head circumference. Both measures, like weight, indicate whether the baby’s development is normal.

Why use centimetres?

Centimetres are used internationally for measuring baby length and head circumference by the medical profession (even in the USA). It is easier to work out percentage growth with centimetres.

What about Granny?

It is often said that although health records in the UK have been metric for years, hospitals routinely tell parents birth weight in imperial because grandparents say they do not understand it in metric.

Of course grandparents will have heard about birth weights for decades and will want to compare those of newly born children with those in the past. However converting everything back into imperial is not the sensible way. The same grandparents will have had decades of experience of old money but in 1971 managed to pick up decimal currency.

Get Granny to write down the weights she wants to compare with and convert them to kilograms.