The magnetic field at a perpendicular distance $r$ from a long straight current-carrying wire of current $I$ is given by:

From Ampère's law applied to a circular loop of radius $r$ centred on the wire: $\oint \vec{B} \cdot d\vec{l} = \mu_0 I \Rightarrow B (2\pi r) = \mu_0 I \Rightarrow B = \dfrac{\mu_0 I}{2\pi r}$. Direction: right-hand rule. Curl fingers along the current, thumb points along the magnetic field circulation. The $1/r$ dependence (not $1/r^2$) is characteristic of a *line* source — same shape as the electric field of an infinite line charge.