Quantity calculus
The multiplication and division rules of quantity calculus are applied to SI base units ( which are measurable quantities ) to define SI derived units , including dimensionless derived units , such as the radian ( rad ) and steradian ( sr ) which are useful for clarity , although they are both algebraically equal to 1 . 
<triplet> SI derived unit <subj> SI base unit <obj> based on <triplet> radian <subj> SI derived unit <obj> instance of <triplet> steradian <subj> SI derived unit <obj> instance of