Locate the velocity of your nozzle. This velocity will likely be listed in the original documentation that came with your sink or garden hose (depending on the type of nozzle you're looking for). This will be measured in some variation of "meters per second," "feet per second" or "inches per second" depending on the origin of the hose. The actual measurement doesn't matter.
Choose a number of seconds to use for your calculator. Calculating how far water will spray out of a nozzle requires to to decide how long you're willing to give that water to reach its destination. For example, if your nozzle typically reaches its destination in three seconds, use the number 3.
Multiply your nozzle's velocity by the time from Step 2. The number you get is the distance the water will spray out of the nozzle. For example, if the nozzle sprays water at 5 meters per second and you're using 3 seconds as your gauge, 5 x 3 equals a distance of 15 meters.