Separate pipe capacity, flow rate and tank filling time

Pipe capacity describes internal volume. Flow rate describes volume passing a point per unit of time. Tank filling time depends on that flow being maintained. Keep these quantities distinct when choosing calculator inputs.

Use the internal diameter

An illustrative straight pipe with a 50 mm internal diameter and 10 m length contains π × 0.025² × 10 = 0.019635 m³, approximately 19.6 L. Nominal pipe size or external diameter may not equal the internal diameter needed for this calculation.

Relate area and velocity

At an assumed average velocity of 1 m/s, the same internal cross-section carries approximately 0.0019635 m³/s, or 1.96 L/s. This is a geometric flow calculation from a given velocity. It does not predict velocity from pressure, pump size or pipe length.

Check a tank-fill scenario

If a constant measured inflow is 12 L/min, adding 600 L takes 600 ÷ 12 = 50 minutes. Subtract simultaneous outflow when applicable. A real flow can change with pressure, elevation or control settings, so a nominal appliance figure may not represent the entire filling period.

Record what was supplied and what was inferred

Write down internal diameter, length, measured or assumed velocity, and whether the tank starts empty. The calculators do not design a potable-water or drainage system or verify pressure losses. Use the relevant system design for pipe selection and installation.

Found a measurement or calculation issue? Send the example and your correction.