Properly sizing a well pump involves knowing a few key variables  including well depth, where the top of any existing well screens are in the well, what the Static Water Level ('SWL') is, and what the well produces in Gallons Per Minute ('GPM'). If you know the original well driller, he probably can provide you with a well drilling log or report that shows most of the above information.
Well Depth and Screen Depth
Well depth, and if applicable, screen depth, influences how deep we can set the pump. Motor manufacturers recommend that any pump installed in a well be installed at the top of a sand screen, or a few feet up from the bottom of the well if there isn't a screen. If the pump must be installed lower than the top of the screen/perforations in the casing, then the well pump should be installed in a 'shroud'  a PVC tube that allows the wires and drop pipe to come out of the top, but only allows water in the bottom so that it is forced to travel past the motor.
Using your available information, write down your well screen depth, or the bottom of your well if you don't have a screen, and subtract 5 feet  this will be your pumping depth, where you install the pump.
Total Dynamic Head
Total Dynamic Head ("TDH") is determined by adding all the 'head losses' and/or 'head requirements' together. For example, if you need to build 60 PSI of pressure in the home  this is 'head' you'll need to build (1 PSI = 2.31 Feet of Head). If you have 1000 feet of 1" pipe and you're pumping 10 GPM, you'll have 59 feet of head loss in friction.
TDH = 'friction head loss' + 'pressure at delivery, in feet of head' + 'vertical lift from PWL'
Start up TDH should include 'vertical lift from SWL' if it varies greatly from 'PWL'. If you find that during start up the pump is really far to the left/right of the curve, but it gradually moves closer on the curve after the water level gets closer to the pumping water level, you may want to change the designs for greater pump longevity.
Determine the friction loss in the well by measure the total length of pipe, determing the flow rate, and referencing the below chart.
Determine feet of head in vertical lift from the PWL (and the SWL) to the discharge level. For example, say the top faucet in the house is 30ft above ground surface, the PWL is 100 feet, and the SWL is 50 feet. During start up, the feet of head in lift would be 80 feet (30ft + 50ft), once the pump has run for awhile though, the feet of head will be 130 feet (100+30).
Total Dynamic Head = "TDH" = Friction Loss + Pressure Required + Lift Required
Determine TDH under start up, and under normal operating conditions.
For example:
If you have a home that is 30 ft at the highest fixture, is 50 feet up from the well, with 100 feet of trench to the well, and the unscreened well is 200 feet with a 100ft SWL and a 150ft PWL, we can determine the below information. You need to develop 10 GPM @ around 60 PSI at the top fixture.
 Pump Set: 190 feet (10ft above the bottom of the well)
 Pumping lift 230 feet (150ft PWL + 50ft from well UP to house + 30ft to top fixture)
 Start up lift: 180 feet (100ft SWL + 50ft from well UP to house + 30ft to top fixture)
 Pressure in feet of head: 60 PSI = 2.31 (FT OF HEAD / PSI) X 60 (PSI) = 138.60 feet of head = 140ft
 Friction loss (see below chart): based on the required flow rate & pipe lengths = 190 feet of drop pipe + 100 feet of trench + house plumbing = 300 feet of pipe @ 10 GPM. We'd be best served by using 11/4" pipe, but could get away with 1" pipe @ 5.56 feet of head per 100 feet = 16.68 feet of head in friction loss, plus extra loss in check valves, ball valves, etc. Lets round up to 20 feet.
Total Head Required @ Start Up = Startup Lift + Pressure + Friction Loss = 180ft + 140ft + 20ft = 340ft
Total Head Required During Run = Pumping Lift + Pressure + Friction Loss = 230ft + 140ft + 20ft = 390ft of head
Pump Curves
The below pump curve is for a 10 GPM well pump series from Flint & Walling. Note the dot on each curve  this is the ideal operating point for these particular models at 10 GPM of flow. Based on the TDH figure, you can easily look on the left hand side, draw a line from the TDH calculated over to the curve, and try to pick the pump that most closely matches your paticular TDH at the required flow rate. Ideally, that particular pump will also closely match at the start up TDH also.
If the pump runs 'to the right' of the curve, meaning the well pump is pumping out in the white area on the right hand side of the curve (using a 3HP pump at 20 feet, for example), but not on the curve, the pump will develop a condition known as 'up thrust' which will destroy bearings & cause premature failure. It is very important to properly select the pump & curve, and to match this to the characteristics of the well.
Based on the above example head calculation, we can determine that the best fit pump would be a 10 GPM 1 HP, or maybe a 10 GPM 11/2 HP, if 10 GPM was absolutely imperative during the longer run times (as the 1HP will drop down to about 8 GPM).
Friction Loss Chart
The below chart shows friction loss in plastic pipe, in feet of head loss per 100ft of pipe. Friction loss varies by material  steel, copper, pex, etc. However, most mainlines these days tend to be plastic. If you need additional friction loss charts, please contact us.
Pressure Friction Head Loss (ft H_{2}O/100 ft pipe) 
Volume Flow 
Nominal Pipe Diameter (inches) 
Gallons Per Minute (GPM)^{1)} 
Gallons Per Hour (GPH)^{2)} 
3/8 
1/2 
3/4 
1 
1 1/4 
1 1/2 
2 
2 1/2 
3 
4 
6 
Nominal Inside Diameter (inches) 
0.493 
0.622 
0.824 
1.049 
1.380 
1.610 
2.067 
2.469 
3.068 
4.026 
6.065 
1 
60 
3.3 
1.1 
0.3 








2 
120 
11.8 
3.8 
1.0 
0.3 
0.1 






4 
240 
42.5 
13.7 
3.5 
1.1 
0.3 
0.1 





5 
300 
64.2 
20.7 
5.3 
1.6 
0.4 
0.2 





6 
360 

29.0 
7.4 
2.3 
0.6 
0.3 





8 
480 

49.5 
12.6 
3.9 
1.0 
0.5 
0.1 




10 
600 

74.7 
19.0 
5.9 
1.6 
0.7 
0.2 
0.1 



20 
1200 


68.6 
21.2 
5.6 
2.6 
0.8 
0.3 
0.1 


30 
1800 




11.8 
5.6 
1.7 
0.7 
0.2 


40 
2400 




20.1 
9.5 
2.8 
1.2 
0.4 
0.1 

50 
3000 





14.4 
4.3 
1.8 
0.6 
0.2 

60 
3600 





20.1 
6.0 
2.5 
0.9 
0.2 

70 
4200 






7.9 
3.3 
1.2 
0.3 

80 
4800 






10.2 
4.3 
1.5 
0.4 

90 
5400 






12.6 
5.3 
1.9 
0.5 

100 
6000 







6.5 
2.3 
0.6 
0.1 
125 
7500 







9.8 
3.4 
0.9 
0.1 
150 
9000 








4.8 
1.3 
0.2 
^{1)} GPM = gallons per minute
^{2)} GPH = gallons per hour