Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

7 base units are defined in physics and other units are derived from these units.

When we say "3 kg of apples", we mention its mass and mass is a base unit.

But for "3 apples", what is its unit? Is this what unitless is?

share|cite|improve this question

Yes, "3 apples" is a value of a dimensionless (unitless) quantity, the number of apples. Quite generally, quantities that are either integer, or very special if they are integer, are unitless.

All 7 base units of the SI system, or any product of their powers (derived units), has the property that it measures an intrinsically continuous quantity such that there is no a priori preferred normalization (i.e. unit) that everyone in the whole Universe would be likely to use.

share|cite|improve this answer
The SI unit of amount is the mole so he could have 0.5E-23 moles of apples – Martin Beckett Jun 28 '14 at 22:07
Thanks but I don't understand what you mean by "a priori preferred normalization" ? – user50322 Jun 29 '14 at 3:53
Good point, Martin. If an apple is interpreted as a large molecule, indeed, SI has another crazy unit for the number. ;-) user: I mean a choice of the coefficient that everyone without any cultural baggage would consider superior. Like counting apples as 1,2,3. This is very different from 1 meter, 2 meters, 3 meters because others may equally well prefer 1 feet, 2 feet, 3 feet which is something else. – Luboš Motl Jun 29 '14 at 4:37

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.