Water Tank Design

Domestic users in the UK consume about 0.1m³ per person per day, which includes bath, shower, washing, etc. For a work washroom it would be much less.

In the UK, cold water is usually supplied from the "town mains" under pressure. The water is chlorinated and thereby rendered potable. Storing it in a tank prior to use for consumption is contrary to local Regulations; connection to the cold tap and the level control valve in a toilet cistern would be direct, maintaining an air gap between the valve and the maximum level in the toilet cistern. It is not required by the Regulations to supply potable water for hand washing and toilets, provided that the taps/faucets that could become inadvertently used for drinking are marked "Not Drinking Water" or something similar. The use of rainwater and grey water for domestic applications is not entirely unknown. It is common to flush toilets on ships with filtered seawater.

Provided the supply available locally is reasonably secure, 0.02-0.05m³ should be more than enough for this application.

The one factor we need, you left off, how often will you replenish the tank? What rate of flow? If you can refill the tank with in 15 to 30 seconds, you don't need a tank. If it takes an hour, 2 US gal.'s should do. If they plan on using the rest room every 15 min. you need to get new employees. If they are going to hold it all day.... you don't need a restroom.

OK, enough joking around. Figure one visit every hour for a woman and twice a day for a man. you need a gallon and a half per flush plus 1/2 gallon to wash, so, you need 2 gallons per visit, or 10 gallons a day.

If it was me, I'd put up a 55 gallon drum and forgetaboutit...

Your road sign is absolutely wonderful!

I am going forth to get a similar local one for my avatar-emblem!

Consumption per day is taken about 25 litres per day per person here in Mexico. You may take thid figure as a first approximation.

So we know standards:

In UK ; 100 L (=0.1 cubic meter)

Mexico 25 L (=0.025 m^3)

Any other standards from other coutries? What about USA? Germany? France?

And warm water? From solar energy?