A worked calculation of how much water a length of pipe holds, useful for filling, flushing and dosing.
Knowing how much water a pipe run holds is handy for filling, flushing and working out a disinfection dose. The method is just area times length, with care over units.
Step 1 - the formula. Volume = cross-section area x length, and for a round pipe the bore area = 0.7854 x diameter x diameter, where 0.7854 is pi divided by 4.
Step 2 - inputs. Take an internal bore of 20 millimetres and a run of 25 metres for this example. Work in metres, so the bore is 0.020 m.
Step 3 - area. Area = 0.7854 x 0.020 x 0.020 = 0.7854 x 0.0004 = 0.00031416 square metres.
Step 4 - volume. Volume = area x length = 0.00031416 x 25 = 0.0078540 cubic metres.
Step 5 - convert to litres. There are 1000 litres in a cubic metre, so 0.0078540 x 1000 = about 7.85 litres in the run.
Step 6 - use it. That is how much water the pipe run holds. It is not a flushing volume or a disinfectant dose: follow the specified flow, duration, concentration and contact time. Use the true internal bore, and add fittings and any cylinder or cistern separately when you work out the volume to treat.