For an infinitely long straight wire carrying current $I$, the field at perpendicular distance $d$: $B = \mu_0 I/(2\pi d)$. The field lines are concentric circles centered on the wire. Direction: right-hand thumb rule (thumb along current, fingers curl in the direction of $B$). This is the most commonly used formula in magnetism. For a finite wire subtending angles $\alpha$ and $\beta$ at the point: $B = \frac{\mu_0 I}{4\pi d}(\sin\alpha + \sin\beta)$. Semi-infinite wire (one end at the point): $B = \mu_0 I/(4\pi d)$.