The answer depends on what characteristic you wish to measure: its length, width, mass, temperature, reflectivity (albedo), thermal or electrical conductivities, and so on.
inch or centimeter
Either a ruler or a scale.
You should use the "math book" to look up the answer to this question as opposed to just having someone answer it on this website.
in ur head
a math book is were u learn math
the mass of your book has about one killogram how do i no cause im cool
The math book was as heavy as a mountain.
it is a book that teaches math.
The metric (SI) unit for weight is the newton, since weight is a force. When people talk about "weight", they sometimes actually mean "mass". The unit for mass is the kilogram.
bar modeling is math
A unit is 1.
The math unit pi (not pie!) is approx 3.14159 which is 314.159%