Haaland equation provides the [[Darcy friction factor]] similar to [[Colebrook equation]]. This was derived for a full flowing circular pipe for both laminar and turbulent flows. It is easier to use than Colebrook equation since it can be solved explicitly $\frac{1}{\sqrt{f_D}}=-1.8log\left[\left(\frac{\varepsilon /D}{3.7}\right)^{1.11} + \frac{6.9}{Re}\right]$ where $\varepsilon /D$ is the pipes relative roughness and $Re$ is the [[Reynolds number]] Haaland eqn can be use for both liquids and gases. For gases both the terms inside the brackets can be raised to the power of three and entire right hand side of equation divided by three for more accurate approximation like this $\frac{1}{\sqrt{f_D}}=-\frac{1.8}{n}log\left[\left(\frac{\varepsilon /D}{3.7}\right)^{1.11n} + \left(\frac{6.9}{Re}\right)^{n}\right]$ n = 3 for gases Reference: [Review of explicit approximations to the Colebrook relation for flow friction (archives-ouvertes.fr)](https://hal.archives-ouvertes.fr/hal-01586547/document)