Worked example
A 1,000-litre tank filled by a pump delivering 50 LPM takes about 20 minutes (0.33 hours).
Calculate your fill time
Water Tank Filling Time Calculator
How long your tank takes to fill
Estimated fill time
20 min
≈ 0.33 hours
A pump's rated flow (LPM) is usually measured at zero head. Lifting water to an overhead or rooftop tank adds resistance that lowers real flow — so actual fill time is often somewhat longer than this figure, more so for taller lifts or narrower pipework.
How this is calculated
Fill time (minutes) = tank capacity (litres) ÷ pump flow rate (litres/minute). Simple volumetric arithmetic — the only real-world variable is getting an accurate flow rate for your actual setup, since lift height and pipe size both reduce flow below a pump's rated (zero-head) figure.
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Frequently asked questions
How do I find my pump's flow rate?
It's printed on the pump's nameplate or spec sheet, usually in LPM (litres per minute) or LPH (litres per hour, divide by 60 for LPM). A typical 0.5 HP domestic pump delivers roughly 40-70 LPM and a 1 HP pump roughly 80-120 LPM at low head — but this varies a lot by model, so check your specific pump where possible.
Why might my tank actually take longer to fill than this estimate?
Pump nameplate flow rates are usually measured at zero or low head (no vertical lift). Pumping water up to an overhead or rooftop tank, through narrow or long pipework, or through a partially closed valve all reduce real flow below the rated figure.
Does tank shape affect fill time?
No — fill time depends only on volume and flow rate, not shape. A tall narrow tank and a short wide tank of the same litre capacity fill in the same time at the same flow rate.