12mm V 18mm Hose Flow Rate for Spa Fill

Hi all

We have a new spa being delivered in the coming days and will be ready to fill early October. Question is what the economies of scale would be in terms of using 12mm garden hose over 18mm garden hose to fill it. The spa takes 1250L of water all up and the garden tap where it will be filled from is about 2 metres away.

Any gurus out there have any wizardry in this area to see whether its worthwhile getting some bigger diameter hose and fittings or whether the time saving wouldn't be that much.



  • +3

    18mm will be over twice as fast assuming the copper leading to the tap is a similar diameter.

    Cross section of 18mm is 1017 vs 452 for 12mm so over twice as much water can get through.

    • Thanks for the info. Good point on the copper to the tap. That is something I am not sure about given it is in the brickwork.

      Do you know what standard household copper plumbing usually is?

      • Either 13mm or 19mm. Not certain what the standard is for a garden tap unfortunately.

  • +3

    Just have the tap wide open. Difference will be minutes not hours. Just use the hose you have.

    • +1

      I beg to differ…

      A 30m 13mm ID garden hose would take 2.5 hours.

      A 2m 19mm ID garden hose would take 12 minutes…

      The length of the hose used to fill the tub would have the greatest affect on how fast it fills if they only have one hose.

      Sometimes, a short hose really is better… ;)

  • +2

    I have a big spa around 2000L, takes me less than 2 hours to fill with a "standard" (don't know diameter, looks normalish) hoselink hose.

  • +7

    Largest diameter and shortest hose. Don’t run it through 30m of hose if it only has to go 2m. There is far more internal friction in a 15 or 30m hose than there is in one that is 2m long.

    To give you an idea…

    13mm hose @ 5bar (average household tap pressure)
    2m = 28l/m (45ish mins for 1,250l spa)
    15m = 11l/m
    30m = 8l/m (2.5 hours for 1,250l spa)

    19mm hose @ 5bar

    2m = 91l/min
    15m = 40l/m
    30m = 28l/m

    • Wow!

      I wonder if the house plumbing contributes commensurately? Most spas are gonna be in the backyard at the end of the run.

      • Yes, house plumbing would contribute to how fast it would fill. The home plumbing system is full of bends, restrictions, long lengths of pipe and eventually the tap all causing restrictions to flow.

        My calculations above were ideal conditions and just to show how the diameter and length of the hose can make a difference. To give your already restricted system the best chance to fill something the fastest, just use the largest diameter and the shortest piece of hose required to reach what you are trying to fill.

  • Thanks for the info everyone. I will leave as is and just fill.

  • Maybe ask them to bring it filled?
    It depends on water pressure not just the pipe diameter.
    Either way a standard internal tap will often have a restricted rate of 9ltrs a minute, external will be faster.
    Will be less than 2hrs easy. Not worth buying a hose.
    Have you factored in the volume of the people that will use it, Archimedes?