Do scalar quantities have magnitude only? I've heard that vector quantities have both magnitude and direction but I've never heard that scalar quantities have magnitude only. Magnitude of vector quantities cannot be negative but what about scalar quantities, like temperature (-1°C)?
If scalar quantities don't have magnitude then what is their "magnitude" called?
Also does the magnitude of a vector quantity include units with the numerical value or only the numerical value?
 A: A vector quantity, $\vec V,$ can be written as $$\vec V=|\vec V|\ \hat V$$in which $|\vec V|$ is the magnitude of the vector, a scalar quantity which is non-negative. $\hat V$ is the unit vector in the same direction as $\vec V.$
The convention is that $|\vec V|$ is the product of a number and a unit, while $\hat V$ has no unit.
A different sort of scalar arises when we express $\vec V$ as the sum of components, say in the x, y and z directions. Using $\hat i,$ $\hat j$ and $\hat k$ for the unit vectors we can write$$\vec V=V_x \hat i + V_y \hat j+V_z \hat k$$
The scalar coefficients $V_{x},\ V_{y},\ V_z$ can be negative, zero or positive. 
"I've never heard that scalar quantities have magnitude only." It is, in fact, quite a common statement in elementary textbooks. Temperature might well be given in such a book as example of a scalar. As you say, (celsius) temperature can be negative, so, clearly, 'magnitude' in this context means real number $\times$ unit, so isn't quite like the magnitude of a vector.
I suspect that temperature wouldn't be given as an example of a scalar in more advanced books, because geometry is not involved in its definition. But this is rather a subtle point.
A: A scalar $x$ has magnitude $|x|$, also known as the absolute value. Celarly, $x \neq |x|$ for negative $x$, but instead of saying the scalar has a direction, we would say that it has a sign (+ or -), which is a much simpler concept. Maybe your confusion arises from the fact that a two-dimensional vector can be described through two scalars, one being magnitude and one being direction? Because this means that all magnitudes are scalars, but a negative scalar does not correspond to a magnitude.
As for units: A vector of two-dimensional direction could look like $\bar v = ( 1 \text{ km}, 2 \text{ km})$. Its magnitude is $$|\bar v| = \sqrt{(1 \text{ km})^2 + (2 \text{ km})^2} = \sqrt{5} \text{ km},$$
so yes, the magnitude includes units.
A: Intuitive description often makes a thing simple, but the level of understanding could be enhanced by going through a rigorous definition. The importance of rigor over intuitive description is that sometimes the easiest questions becomes trouble some to answer intuitively, like this question.
Intuitive description of the vector may lead to question like :-
What is direction?
What is magnitude?
To understand the matter of what are vector and what are scalar rigorously in mathematics sense, it is advised to go through first few may be 1,2 or 3 chapters in any linear algebra text (like sheldon axler linear algebra, friedberg linear algebra etc), or through the wikipedia article on vector space and fields.
A: although its 2 year old post but can be helpful someone have doubt and googleing it :

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*first of all i think you and anyone else have no doubt in vectors . magnitude of vectors is always positive . in vectors -ve means direction which we do not take .

*for that unit thing , yes we take units in magnitude

*now the main confusion occurs in scalars as they do not have direction . so first of all most scalars are positive which have no confusion . but some scalars which have -ve sign are charge , current , temperature ( if we see cel and f ), work etc etc. so we start with current which is scalar and has direction which is convention direction or oppposite to convention direction but current does not follow vector mathematical rules . so it is a scalar . so magnitude of cuurent is positive and we can remove negative while writing magnitude by ignoring direction. now charge is scalar . charge is either positive(more proton) or negative(more electron) . but charge is charge if it is positive or negative . so we can remove the minus sign if magnitude of charge is asked . now work depend on direction of displacement and force (thats why it is +ve and -ve both)but do not follow vector mathematical laws . so we can remove minus because direction can be ignored in magnitude. now comes comes to temperature when it is in si unit (kelvin) it can never be negative . but when taken in celcius and fehrenite it can be negative and here positive or negative does not show neigther any direction nor any difference such as in charge where once proton is more and other time electron is more. it is positive negative just because we take 0 in between anywhere , so when magnitude of temperature is asked in celcius or ferhenite we have to incude -ve sign becoause -5 degree celcius and 5 degree celcius are complete different things .
so when people say magnitude is always positive , i am not saying they are wrong, they are right but when some units are those in which we take zero anywhere in between like celcius , fehrenite . then we have to put -ve in magnitude. now if this is not clear to you that charge also has no direction then why it has +ve magnitude . now you can compare with tempeature. negative charge has a physical meaning of more electrons and same with positive charge but negative temperature and positive temperature in celcius and f do not have any physical meaning .there may be more units where we take 0 of that unit anywhere in between without any direction and physical meaning and then have -ve magnitude but right now celecius and farhenite are coming in my mind . so when units like celcius and ferhenite are there which take 0 anywhere without any direction and physical meaning and they have -ve value and then magnitude can be -ve.

