Water Tank Refill Time Calculator

Work out water tank refill time instantly with clear inputs, formula shown and shareable results.

About Water Tank Refill Time Calculator

Fill time is the volume to add divided by the net inflow — supply rate minus any draw-off during filling. Ignoring simultaneous use is the classic error: a 14 L/min supply with 3 L/min being used fills at only 11 L/min, taking a quarter longer.

Formula

Refill time

volume to add = capacity x (1 - level); time = volume / (inflow - outflow)

Worked example

With the default values below, Water Tank Refill Time Calculator returns Time to fill of 127. Change any input above and every figure here is recalculated in your browser.

Inputs used

Tank volume: 2,000 L
Current level: 30 %
Fill flow rate: 14 L/min
Usage during filling: 3 L/min

Results

Time to fill: 127 min
Volume to add: 1,400 L
Net fill rate: 11.00 L/min
Time to fill: 2.12 h

Step by step

1Values used
2Refill time
3Time to fill= 127 min
4Volume to add= 1,400 L
5Net fill rate= 11.00 L/min
6Time to fill= 2.12 h

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What if usage exceeds the fill rate?

The tank never fills. The calculation returns zero net flow, and in practice the level continues to fall until demand drops.

How do I measure the fill rate?

Time how long a 10 litre bucket takes at the inlet tap and scale up. Mains flow varies with time of day and pressure.