Flow Rate Calculator

Volumetric flow rate measures how much fluid volume passes through a cross-section in a given time. For steady flow through a full circular pipe, flow rate equals cross-sectional area multiplied by average fluid velocity. This calculator converts the entered inside diameter from millimeters to meters, finds the circular area using pi times diameter squared divided by four, and multiplies that area by velocity. It reports the same result in cubic meters per second, liters per second, and liters per minute. The velocity must represent the area-averaged speed, not necessarily a single local reading. Real pipe velocity varies across the section because friction slows fluid near the wall. In fully developed laminar flow the profile is parabolic, while turbulent flow has a flatter center region and steep gradients near the wall. A properly positioned and calibrated flow instrument may report average velocity directly or apply a correction to sampled velocity. Using an uncorrected point measurement can produce a biased flow estimate. Use the actual inside diameter rather than a pipe's nominal trade size. Wall thickness, schedule, lining, deposits, corrosion, and manufacturing tolerance can all change the open area. Because area depends on diameter squared, even a modest diameter error can noticeably alter calculated flow. The equation also assumes the pipe is completely filled and that velocity is normal to the cross-section. Partially full gravity pipes, open channels, compressible gas streams, and strongly pulsating flows require additional methods or time averaging. This calculator is useful for quick checks in plumbing, irrigation, process piping, ventilation, laboratory systems, and pump planning. It determines kinematic flow from known diameter and velocity; it does not predict velocity from pressure, pump head, pipe length, roughness, fittings, or elevation. Those problems require an energy-loss model such as Darcy-Weisbach together with fluid properties. Always keep units consistent, use operating rather than nameplate conditions, and consider measurement uncertainty. For safety-critical design, verify the result against applicable codes, equipment curves, pressure limits, and an appropriately detailed hydraulic analysis.