Thankfulness in gallons per inch

My daughter occasionally used to say 'we never use math in a practical
way'. We've been really short of water these days, and today it rained,
hard but short. It rained about 2mm worth. Not much. I wanted to show
everyone 'although it did just rain, we still need to conserve.'

So... FIRST you measure the width and length of your roof.
Then multiply those two numbers to get your square footage.
My roof has 3 parts, so I did this for all three parts, and then added
them all together.

1756 sq.ft of roof.

Then I multiplied by .56 which will give you the amount of water
produced per inch of rain assuming 90% efficiency. Since my gutters are
only slightly leaky I'm comfortable assuming 90%.

That gave me 983.36 gallons p/inch or, 39.3 gallons p/mm or 148 liters
p/mm (I'm rounding for simplicity)

So now the kids know, if it rains a single millimeter, we'll get near 40
gallons of water.

the stats I used for the average water consumption (drinking/potable,
dishes,laundry, etc) per person per day is: 150litres (that is using UK
stats, US stats are much higher). or 40 gallons. Our family consumption
p/day would be 120 gallons p/day when NOT in conservation mode.

80litres for a bath
45litres for a 5 min. shower
50-100litres for a wash of laundry
15 litres for doing dishes (we don't have a dish washer)
+ drinking water around 3 litres p/day.

Given that we only produce nearly 148litres/40 gallons for a mm of
water, then that little flash rain we got covers the need for only 1 of us.

That opened their eyes.
THEN, I decided to calculate how long it would take to fill our tanks.

PRAISE to God, that in the midst of this drought, we have not run out of
rain water in our tanks. This is because we have a LOT of storage
capacity. I do not know if we were the only ones to not run dry here,
but it is thanks to the fact that we were bought a new tank a while ago,
and kept it through the move. When we looked for houses to move to (2
yrs ago) water storage was a huge issue for me. Knowing we'd have 2
teenagers, and knowing that a drought was predicted 4 years ago, water
storage was more than once a reason I said 'no' to house.

God provided us with water storage, and we are thankful. We got to
within 2 weeks of running out, enacted our emergency stores, and then it
rained and now we're at 1/2 capacity again (most people are full).

Our storage capacity is 4000 gallons of water, plus an emergency 400
With that in mind, it would take us 4inches of rain to completely fill
our tanks if they were empty. So far, we've got a little under 2 inches
of rain.

I also designed a clever little water tank meter, so that we can easily
monitor our water level. Otherwise you have to tap on a metal water tank
to tell where the water level is (or pour hot water down the side and
watch the condensation mark).

(you tend to learn a few tricks regarding water).

That all said... the way I figure it, if we go 3 months without rain, we
will have depleted our reserves. This assumes tight water conservation
methods kick in once we reach half tank.

So we're thankful for the rain, and we're able to actually measure that
thankfulness in gallons per inch. (or litres per mm)