Pull out the pump curve you wish to interpret -- you can also find one in any book on hydraulics. Notice the head -- height of the liquid -- is plotted on the y-axis, or vertical axis. The capacity or flow is plotted on the x-axis, or horizontal axis.
Look carefully at the measurement of the head on the y-axis, and take note of the scale. See if the head is being measured in feet, inches, centimeters, meters or even in a large scale like miles or kilometers for a huge pump.
Look carefully at the measurement of the flow on the x-axis and take note of the scale. See if the flow is being measured in cubic feet per second, cubic yards per minute, cubic meters per hour, cubic centimeters per second or some other scale.
Notice that the curve bends downward from the starting point; this makes sense, as you would expect the flow rate to increase as the head or height the pump must push the liquid decreases. Take a pencil and mark some random points on the graph to get used to reading it. You will notice, as an example, if at one point the head is 50 feet and the flow rate is 3 gallons per minute, at another point where the head is 20 feet, the flow rate might increase to 5 gallons per minute.
Find the best efficiency point by following the curve about 1/3 of the way down from the point where it begins on the y-axis. Use your calculator to figure out the horsepower of the pump by taking the head measured in feet, multiplying it by the gallons per minute, then dividing your answer by 3960.