Baby weight converter

Converting a baby's weight between kilograms and pounds

Birth weight is one of the last places in British life where imperial units are still the emotional default. The hospital records 3.4 kilograms; the family group chat wants 7 pounds 8 ounces; the grandparents will only ever think in pounds and ounces. So the conversion gets done, repeatedly, and different websites give slightly different answers, which is oddly unsettling when it is the number attached to your baby.

Why the answers disagree

The conversion itself is exact and not in dispute. A pound is defined as exactly 0.45359237 kilograms, and has been for decades. There is no approximation in the definition.

The disagreements come from what converters do with it. Some use 2.2 as the kilograms-to-pounds factor, which is convenient and wrong by about a quarter of a percent. On a birth weight that is enough to move the ounces figure. Others round to pounds first and then work out the ounces from the rounded number, compounding a small error into a visible one. And some fail to carry properly when the ounces round up to sixteen, producing the impossible-looking “7 lb 16 oz”.

None of this matters clinically. It matters when you are trying to write the number on an announcement and two websites disagree.

Doing it by hand

Divide the kilograms by 0.45359237 to get total pounds. The whole number is your pounds. Multiply what is left over by 16 to get ounces. For 3.5 kg: 7.7162 total pounds, so 7 pounds, and 0.7162 times 16 is 11.5 ounces — 7 lb 11 oz once rounded.

Going the other way, remember that the ounces are sixteenths, not decimals. Seven pounds eleven ounces is 7 plus 11/16, which is 7.6875 pounds, not 7.11. Multiply by 0.45359237 and you get 3.487 kg. Treating it as 7.11 would give you 3.225 kg, well over 250 grams out, and this is easily the most common error people make.

Why hospitals use kilograms

Every clinical calculation involving a baby's weight is metric, and weight-based calculations are among the most safety-critical things done in paediatrics. Keeping one unit throughout removes an entire category of error. Growth charts are plotted in kilograms, and international growth standards are published in metric.

So your notes, your red book and your charts will be in kilograms and grams. The pounds and ounces version is for people, not for records, and it is worth keeping the metric figure as the one you actually write down.

Grams for the small numbers

For weight changes in the early days, grams are far easier to think in than fractions of an ounce. A newborn losing 200 grams is a clearer statement than one losing 7 ounces, and the loss and regain of birthweight in the first weeks is normally discussed as a percentage of the birth weight rather than in either unit.

What the number does and does not tell you

A converted weight means exactly what the original meant. Converting 3.1 kg into 6 lb 13 oz does not make it more or less reassuring; it is the same weight in different words.

Whether a weight is typical is a separate question and one that a converter cannot answer. It depends on gestational age at birth, on sex, on the population the growth chart was built from, and after birth on the trend across several measurements rather than any single one. Babies born at the same gestation vary widely, and the majority of babies who are smaller or larger than average are simply smaller or larger than average.

If you want to know where a weight sits on a centile chart, use a tool built for that, with the right chart for the right population and prematurity corrected where it should be. Reading significance into a converted number is a good way to worry about nothing.

A note on the scales themselves

If you are weighing at home, expect disagreement with the clinic. Baby scales vary in calibration, and the same baby weighed on two sets of scales minutes apart can differ by enough to alarm you. Full nappy versus empty, before a feed versus after, clothed versus not — each of these moves the number more than a day's growth does.

This is one reason the NHS advises against frequent weighing of a healthy baby: no more than monthly up to six months, no more than every two months from six to twelve, and no more than every three months after the first birthday. The extra data points do not contain extra information, and they turn ordinary noise into a story.

Stones, and other units that still appear

Older relatives and some older records use stones, where one stone is 14 pounds. Babies are almost never described in stones, but the unit turns up when converting a mother's weight, and it is a common source of error because the fourteen is easy to forget. Ounces are sixteenths of a pound, stones are fourteen pounds, and neither divides into anything neatly, which is much of why medicine standardised on grams.

You may also meet weights written as decimal pounds, such as 7.69 lb. That is not 7 pounds 69 ounces, and it is not 7 pounds 6.9 ounces either. Multiply the decimal part by 16 to get the ounces: 0.69 times 16 is about 11, so 7.69 lb is 7 lb 11 oz.

The quick reference

One kilogram is 2.2046 pounds. One pound is 453.592 grams. One ounce is 28.35 grams. Half a kilogram is a little over a pound and a hint. And the number that matters in your notes is the metric one, whatever the family group chat prefers.