Biot-Savart's law gives the magnetic field $d\vec{B}$ due to a small current element $Id\vec{l}$ at a point P at distance $r$:
$$d\vec{B} = \frac{\mu_0}{4\pi} \cdot \frac{I(d\vec{l} \times \hat{r})}{r^2}$$
Magnitude:
$$dB = \frac{\mu_0}{4\pi} \cdot \frac{I\, dl\sin\theta}{r^2}$$
where $\theta$ is the angle between $d\vec{l}$ and $\hat{r}$.