How do I say my weight? In which SI units? I see people are using Kg as a unit of denoting weight from my childhood. Answer this question "What's your weight?".

  • $\begingroup$ this is a much better question than the other one... $\endgroup$
    – agentp
    Aug 9, 2017 at 12:18
  • $\begingroup$ There is an important note at the top of the wikipedia page: "In law, commerce, and in colloquial usage weight may also refer to mass." Rest assured the topic has been the subject of unending debate. $\endgroup$
    – agentp
    Aug 9, 2017 at 12:23
  • $\begingroup$ @agentp engineers are very clear about mass and weight.... $\endgroup$
    – Solar Mike
    Aug 9, 2017 at 15:18
  • $\begingroup$ @SolarMike sure but we also need to communicate with non engineers and so understand in many contexts "weight" actually means mass. $\endgroup$
    – agentp
    Aug 9, 2017 at 21:09

2 Answers 2


Weight is a force and is expressed in Newton (N). Mass is expressed in kilogram (kg).

However, in informal (non-scientific) language, people often express weight in kg, although this is not correct strictly speaking.

The relation between the two is $F=mg$, with $F$ the weight (N), $m$ the mass (kg) and $g$ earth's gravity constant.

See also here.

  • $\begingroup$ yeah i got it but if somethings weighs 2kg in weighing machine does that mean 2N $\endgroup$ Aug 9, 2017 at 11:14
  • $\begingroup$ No, $2\mathrm{kg}\cdot9,81\mathrm{m/s^2}\approx 20\mathrm{N}$. $\endgroup$
    – Karlo
    Aug 9, 2017 at 11:16
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    $\begingroup$ okey I understood the concept $\endgroup$ Aug 9, 2017 at 11:27
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    $\begingroup$ I think it would be less confusing to say "in informal (non-scientific) language, people use the word "weight" as a synonym for "mass". In science and engineering, "weight" and "mass" are always different concepts. $\endgroup$
    – alephzero
    Aug 9, 2017 at 18:03

As this is an Engineering stack we use the prescribed units, mass is in kg and weight is in Newtons.

This is a common misconception by the masses (great unwashed...) and it is also covered in this question : Force Required to Lift a WEIGHT of 1Kg

It is also obvious in various phrases such as "I am gong to boil the kettle" which is assumed to mean boil the water that is inside the kettle and not put the kettle in a big cooking pot and boil it...

  • $\begingroup$ So if i say 1 Kg of weight does that mean 1N $\endgroup$ Aug 9, 2017 at 11:11
  • $\begingroup$ 1 N is approx 100g (102g), 1kg is 9.81N, don't mix them... $\endgroup$
    – Solar Mike
    Aug 9, 2017 at 11:21

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