Pipe velocity and flow rate, and how they connect
Why flow rate depends on both pipe size and water speed, how to pick a design velocity, and what happens when a pipe runs too fast or too slow.
Flow rate is area times velocity
Volumetric flow rate is the amount of water that passes a point each second, and it equals the pipe's cross-sectional area multiplied by how fast the water moves. The area of a round pipe is pi times the inner radius squared, so a 1 inch bore has an area of about 0.785 square inches. If water moves through that pipe at 5 feet per second, the flow works out to roughly 12.24 gallons per minute. Change either the size or the speed and the flow changes with it.
Why diameter matters more than it looks
Because area depends on the square of the diameter, small changes in bore have an outsized effect on flow. A 2 inch pipe has four times the area of a 1 inch pipe, so at the same velocity it carries about four times the water. This is the reason a modest jump in pipe size can relieve a starved fixture, and why using nominal size instead of true inner diameter can throw a result off noticeably.
Choosing a sensible design velocity
Engineers usually target water velocities of about 5 to 8 feet per second for supply piping. Staying in this band keeps friction loss manageable while avoiding the problems of extreme speeds. Slower than a couple of feet per second and sediment or air can linger in the line, while faster than 8 feet per second and the flow starts to erode pipe walls, generate noise, and increase the shock of water hammer when a valve shuts.
Reading the extra outputs
Alongside gallons per minute, the tool shows the flow area in square inches and the flow in cubic feet per second. The area helps you confirm you entered the diameter you intended, since it should match pi times the radius squared. Cubic feet per second is handy for larger systems and for cross-checking against hydraulic references, and it converts to gallons per minute by multiplying by 448.831.