Don't worry, to a first approximation we can say that no firefighter on a fireground has ever had to make that calculation. It's semi-handy to know in terms of things like this:
5" supply hose holds about a gallon per foot of water when charged, and water weighs just a little more than 8 lbs per gallon, so a charged 100' section of 5" supply hose weighs over 800 lbs. So make sure you have the hose where you want it before charging it, because it's damn hard to move afterwards and the like.
In terms of doing math on the fireground, at least in a structural fire scenario (wildland firefighting has some other concerns) the single biggest one is usually flow-rate. That is, a crew has stretched a 200' pre-connected 2" attack line, and wants to go in flowing 200 gpm to attack the fire. As the engineer, what pressure do you configure on that discharge to give them their 200 gpm? Here's the kicker though... almost nobody sits there thinking "Well, the friction loss per foot of 2" hose at 200 gpm is x, multiply x by 200, assume a desired pressure of 100 psi at the nozzle, blah, blah, blah.". No, instead somebody precomputed the common configurations, wrote them on an index card, laminated it, and taped it to the pump panel. Plus the engineer probably has the really common ones memorized anyway, and/or has figured out a heuristic that's "good enough".
5" supply hose holds about a gallon per foot of water when charged, and water weighs just a little more than 8 lbs per gallon, so a charged 100' section of 5" supply hose weighs over 800 lbs. So make sure you have the hose where you want it before charging it, because it's damn hard to move afterwards and the like.
In terms of doing math on the fireground, at least in a structural fire scenario (wildland firefighting has some other concerns) the single biggest one is usually flow-rate. That is, a crew has stretched a 200' pre-connected 2" attack line, and wants to go in flowing 200 gpm to attack the fire. As the engineer, what pressure do you configure on that discharge to give them their 200 gpm? Here's the kicker though... almost nobody sits there thinking "Well, the friction loss per foot of 2" hose at 200 gpm is x, multiply x by 200, assume a desired pressure of 100 psi at the nozzle, blah, blah, blah.". No, instead somebody precomputed the common configurations, wrote them on an index card, laminated it, and taped it to the pump panel. Plus the engineer probably has the really common ones memorized anyway, and/or has figured out a heuristic that's "good enough".