Is NH_3 a weak or strong base?

In aqueous solution, ammonia, $N {H}_{3}$ acts as a weak base.
$N {H}_{3} \left(a q\right) + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s N {H}_{4}^{+} + H {O}^{-}$
$p {K}_{b}$ $=$ $4.75$