Turning a storage volume and a fill flow rate into a fill time, with unit conversion shown.
How long a cistern takes to refill is a simple flow calculation, and it is the same idea as filling a bath or a tank. This example shows the method with chosen figures.
The formula. Fill time = volume / flow rate. The only catch is keeping the units consistent.
The situation. A storage cistern needs 240 litres put back into it. The float valve admits water at a flow rate of 0.2 litres per second.
Step 1 — divide. Time = 240 litres / 0.2 litres per second = 1200 seconds.
Step 2 — convert to minutes. 1200 / 60 = 20 minutes.
Working in other units. If the flow had been given as litres per minute, you would multiply 0.2 L/s by 60 to get 12 litres per minute, then 240 / 12 = 20 minutes, the same answer.
Takeaway. Fill time is just volume divided by flow, as long as the units match. A part-closing valve or low supply pressure lowers the flow rate and so lengthens the fill time, which is worth remembering when a cistern seems slow to recover. Use measured flow rates for real assessments.
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