Editing GNU Units (section)

=== Complex units expressions ===
One form of the [[Darcy–Weisbach equation]] for [[fluid flow]] is
: <math>\Delta P = \frac {8} {\pi^2} \rho fL \frac {Q^2} {d^5} \,,</math>
where Δ''P'' is the pressure drop, ''ρ'' is the mass density, ''f'' is the (dimensionless) friction factor, ''L'' is the length of the pipe, ''Q'' is the volumetric flow rate, and ''d'' is the pipe diameter. It might be desirable to have the equation in the form
: <math>\Delta P = A_1 \,\rho fL \frac {Q^2} {d^5}</math>
that would accept typical US units; the constant ''A''<sub>1</sub> could be determined manually using the [[Units conversion by factor-label|unit-factor method]], but it could be determined more quickly and easily using ''units'':<ref>GNU units version 2.10, Windows XP SP3, PTC MKS Korn shell</ref>
<syntaxhighlight lang="console">
$ units "(8/pi^2)(lbm/ft^3)ft(ft^3/s)^2(1/in^5)" psi
        * 43.533969
        / 0.022970568
</syntaxhighlight>
Crane Technical Paper No. 410,<ref>
{{Cite book|title=''Technical Paper No. 410,'' Flow of Fluids through Valves, Fittings, and Pipe|publisher=Crane Co|location=New York|year=1985|url=http://www.flowoffluids.com/publications/crane-tp-410.aspx}}
</ref>
Eq.&nbsp;3-5, gives the multiplicative value as 43.5.