Convert st to kg

A stone is fourteen pounds, and it is used for body weight in Britain and Ireland and essentially nowhere else — which is why an American reading a British weight usually needs this conversion.

The kilogram was the last unit defined by a physical object — a cylinder in a vault near Paris that was slowly losing mass. Since 2019 it has been fixed to the Planck constant instead.

The factor is 6.350293, and almost nobody carries that around. Rounded to 6.4 it is off by 0.78 % — which stays invisible on small numbers and turns into a whole unit somewhere around 1,000 st.

That is the number worth knowing before you round: not the error itself, but where it stops being ignorable. Below that point the shorter factor is the sensible one; above it, use the field above, which never rounds until it prints.

A stone is exactly 14 pounds, and the pound has been fixed in metric terms since the international yard and pound agreement of 1959. In this page's units that is 0.45359237 kg to the pound and 6.35029318 kg to the stone — stipulated figures rather than measured ones, which is why every digit of them stops.

The stone never received a metric definition of its own; it inherits the pound's, 14 times over. That is also where you meet it — Britain and Ireland quote body weight this way and almost nowhere else does, so a figure in stone has usually crossed a border to reach you.

The form is the problem, not the conversion. A weight known as "eleven stone four" has to go into a field that takes a single figure, and there is no way to enter it that preserves both parts. Some forms offer stone and pounds as two boxes and convert behind the scenes, which is the design that works; most take kilograms and leave the arithmetic to the person filling it in.

What that arithmetic needs is one conversion, not two roundings. Eleven stone four is 158 lb, which is 71.67 kg — go to pounds first, then to kilograms, and the pounds step is a multiplication by fourteen with nothing to round. Converting the stone and the pounds separately and adding gives the same answer, and gives it in a form that is easier to check when the number looks wrong.

Somebody who weighs fifteen stone nine and is asked for a decimal will very often write 15.9. That reads as fifteen point nine stone, which is 222.6 lb — the real weight is 219 lb. The gap is 3.6 lb, or 1.6 kg, and it is entirely produced by treating a fourteen-part remainder as a tenth.

The error is largest for pounds between about four and thirteen, and it always runs the same way: the decimal figure is too high, because nine tenths is more of a stone than nine pounds is. Nine pounds is 0.643 of a stone, so fifteen stone nine is 15.643 stone or 99.34 kg. Where a field genuinely wants decimal stone, that conversion is the one to do; where it wants kilograms, go through pounds and never write a decimal stone figure at all.

It is rarely stored to be read back. A weight entered in kilograms is an input: to a BMI, to a drug dose written per kilogram, to a fluid or anaesthetic calculation, to an equipment or restraint rating, to an airline's passenger figures. Each of those multiplies the number by something, which is why the institutions asking for it are unwilling to accept a unit they would have to convert first.

That also explains the asymmetry British people notice. The nurse asks for the weight, hears the stone figure, writes down the kilogram one — and later reads the kilogram figure back as stone in conversation. Neither unit is being replaced; one is the unit of speech and the other is the unit of arithmetic, and the conversion happens at the boundary every time.

One pound is 0.4536 kg, so a scale stepping in pounds is more than twice as fine as one stepping in whole kilograms. A person watching their weight in stone and pounds sees changes a whole-kilogram readout would hide, which is one honest reason the unit survives among people who have no attachment to it in principle.

It also sets the sensible precision for the converted figure. Fourteen stone two is 89.81 kg, and reporting it as 89.8 keeps every bit of information the original had; reporting it as 90 throws away nearly half a pound. Where a form takes one decimal place, use it — and where it takes only whole kilograms, know that the rounding is the form's choice rather than a limit of the conversion.

The number to carry is 6.35 kg to the stone and 0.45 to the pound. From there, ten stone is 63.5 kg and every further stone adds a little over six: eleven is 69.9, twelve is 76.2, thirteen is 82.6, fourteen is 88.9, fifteen is 95.3. Pounds are then roughly two per kilogram in the other direction, which is close enough to keep in your head.

Working from ten stone rather than from zero keeps the arithmetic small and the error visible. Fifteen stone four is 95.3 plus four times 0.45, so 97.1 kg against a true 97.07 — accurate to within a tenth using two remembered constants. That is enough for a sanity check on anything typed into a field; the converter above is what the field itself should get.

Driving licence medicals, insurance applications, airline special-assistance forms, gym and physiotherapy intake sheets, and most health apps take kilograms and nothing else. Some accept pounds as a second option, generally because the software was written for an American market; almost none accept stone, because outside these islands the unit is unknown to the developers.

That makes the conversion a one-way obligation rather than a preference. It is worth doing once, carefully, and remembering the result — a personal weight changes slowly enough that a converted figure stays usable for months, and the errors on these forms come from repeated hurried conversions rather than from any difficulty in the conversion itself.

The unit switches with age in Britain, and it switches only in speech. A birth weight is announced in pounds and ounces, growth charts and clinical records are in kilograms throughout childhood, and somewhere in the teens the spoken unit becomes stone. Nobody decides this and no document marks the transition; the kilogram figure has been the recorded one the whole time.

That has a practical effect on a parent converting anything. A child's weight from a red book or an app is already metric and needs no conversion at all, while the figure a grandparent asks for is in pounds. Converting stone to kilograms is an adult problem specifically, which is why the range that matters on this page runs from about seven stone upward and the arithmetic below that is of no interest to anybody.

Most bathroom scales sold in Britain switch between stone-and-pounds, pounds and kilograms, and the switch is usually a small button underneath that gets pressed by accident. A reading that has moved by an implausible amount overnight is far more often a unit change than a weight change: 12 st 4 becoming 78.0 is the same body in a different unit.

Health apps mostly store kilograms internally and display whatever the user chose, which is the right design and produces one visible artefact. A weight entered as stone and pounds, stored in kilograms and displayed again in stone can come back a pound different, because the stored value was rounded on the way in. That is a display rounding rather than a lost pound, and it is worth recognising before it is treated as a measurement.