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 I'm sure I 'll be told off by someone but its important (to me at least) and does relate to the forum sort of..


Like many in greater Auckland and beyond, I have a water issue at home (i.e I'm on tank water and supply is difficult).


I am trying to confirm my math with regard to a) capacity of my 2 tanks which are connected - at the bottom (and so in my mind 1 body of water for the purpose of this exercise) and B) Usage.



I believe that to work out the amount of water I do the following;

3.14 (pii) x radius ® x radius ® = this result is the square metre measurement of the tank floor


x depth in millimetres = which gives the quantity (litres) of water


3.14 x 1.6 x 1.6 x (2 tanks)  = 16.0m2 to the nearest

16.0 x 1000 = 16000 Litres


I'm sure that's right because a cubic metre  contains 1000 Litres, and if I have 2 of em I have 2000 L


However here is the bit that's getting me out of shape and I'm hoping someone can confirm for me.



Daily, I measure the drop in water level with a staff, and yesterday the tank level in one tank changed by 30 mm.


So, using the same formula as above;

3.14 x 1.6 x 1.6 x 60mm (there are 2 tanks, sitting side by side on flat ground & connected, so the water level would be change at the same rate. So if one changes by 30 mm then the other must too - which means a 60mm drop) 


so 3.14 x1.6 x1.6 (x2) = 16.0m2

x 60 = 960 Litres 


Sounds like a bucket load of water to me, and I'm hoping I am wrong for obvious reasons.


Educated opinion would be gratefully received.


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your tank volume in m3 is pii x r squared x height. 

Thumbsuck rule for house hold consumption is 200litres/person/day. Tank users are usually more like boaties in minimising water usage, however if you have teenage daughters who all love long showers the sky is the limit.

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we are tank water also, average about 500l a day in summer with two preteen kids,  so i guess 125l per person per day.  our 50k of storage is at 25% and i ordered water today for the first time in 15years of being on a tank so it is definately a dry year.

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So do u see any error with my end result - esp the usage calc?


Should be

total area =(Pi*r squared)*2

Then, volume = total area * height (depth)

Usage = total area * change in height


If your r is 1.6 m (realistic, my old concrete tank is 3.36m diameter, measured by climbing inside and getting the internal diameter), then the area of one tank is 8m2. 

area of 2 tanks (total area)=16 m2

Then, your usage is exactly the change in height on your staff, times the total area. 16m2 x 0.03m (i.e. 30 mm) = 0.48 m3

to get m3 to l, divide by 1,000, so you used 480 l per day. Sounds like a 2 or 3 person household, or a 2 person household and you did some washing... That is a normal and realistic amount of usage.


On a side note, I've ordered my 3rd watercart this year. I just ordered one, less than a week from the last delivery, asked for the last week of Feb (planning ahead and being super well planned etc), the earliest they can do is 4th March. We are ok, our neighbours are on town supply if we need water urgently.


I've got mates in Matakana who have a 25 day wait, and have about 20 days of water left. And I have mates on Waiheke who have already ran out. There are no water carts on Waiheke, because there is not enough source water. Apparently they are allowed 20 l a day from the Council tap or something crazy?

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 i ordered water today for the first time in 15years of being on a tank so it is definately a dry year.

There was 6.6 mm of rain for January. Average is 66 mm for Jan. There has been no rain so far in Feb... (North Auckland area)


I reckon the only way we will get significant rain is if we catch a cyclone. And so far it looks like the first one will hit fiordland, like the one place in the country that has already had enough rain....

If we do catch a cyclone, the slips will be wild, there are such huge cracks in the ground, water will go pouring in and everything will let go. Glad I don't own cliff top property (or a farm, for that matter)

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Thanks all. So i shouldnt double the 'drop'.

480 L per day isn't as frightening but now all i gotta do is convince the fairer sex members of the house that we do need to save...


I can't even make an arrangement for a load. Seems like most water truck operators believe that the best way to manage extreme demand is to stop taking bookings - as if ignoring it makes the need magicly go away. I get they are busy, i get they can only do so many loads a day and all the rest, but, the closing of bookings I can't fathom. I'd be over the moon if my forward bookings were under pressure-its called certainty.


So anyway we'll have to wait till we got nearly none in the tank and have faith in human nature.

Cheers all for your replies.

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