# Rainfall/cistern calculation - results

```First, thanks to the various kind souls who sent me responses on this.  I
hope over time I'll be able to "give back" with contributions of my own.

I was asked by one list member to post any response I got.  I've received
two versions of this, one in English and one in metric terms, and I think
I'll quote both here.  (I haven't tested the metric one.)

From Jeff Jacobsen in Washington state (US)(English measure):

>Here is what I do for my calculations:
> Roof size has to be the calculated equal to the ground area the roof
>covers.  Hence, do your measurements parallel to the ground; if you
>measure at an angle your result will be too high.  Determine area, i.e.
>length x width.  Add all roofs or roof segments together to get your
>catchment area.  If you work in inches you won't have to worry about
>conversion. Include your gutters too, since they catch water too.
>
>Divide by 231, the number of cubic inches in a gallon.
>
>This gives you the number of gallons for one inch of rainfall.
>
>Mulitply by annual rainfall to get annual catchment.

And from Murray Hopkins in Australia (Metric):

>  If you want to do the conversions, the metric formula is:
>
> 1 litre of water per 1 mm of rainfall per square metre of roof.
>
>(4.5 litres per gallon, 305 mm per foot, approx 39 inches per metre.)
>
>You then need to subtract a percentage for losses. This depends upon how
>hot the roof is; how much water ends up in the gutters etc etc.
>We work on about 10% here but thats very variable.

I also got some suggestions as to storage media, from stock tanks to 40 gal.
plastic barrels to ferrocement to pre-formed large plastic tanks (we've got
one already -- they're not cheap!).  We're leaning toward ferrocement at
this point, but with at least a year before we have the resources (time and
labor) to essay this, anything is still possible.

Again, thanks to those who wrote!

Loren
Loren Davidson
loren@wombat.net
http://www.batnet.com/beauty/
The First Amendment went too far.  It should have said, "Congress shall
make no law".

```