The Field Calculators → Water
Water
Water storage: how long it lasts
Tank size against real daily use, including the stock — and what it takes to get through a dry spell.
Result
Storage capacity1500 gallons
People4
Per person per day50 gal
Livestock use0 gal/day
Garden / irrigation0 gal/day
Hold back as reserve15 %
Days of supply
6.4 days
Under a week. That is a thin margin for a pump failure or a dry spell.
Total daily draw
200 gal/day
Usable volume
1,275 gal
Holding 15% back — you never want a tank truly empty.
To reach 30 days you'd need
7,059 gal
Refill rate required
0.14 gal/min
Continuous flow that would exactly keep up.
What this will not do
- Measure your real use before you size anything. A water meter on the main for one month beats every estimate in this calculator, including the defaults.
- The midnight-to-5 a.m. rule: if your tank level falls while everybody is asleep and nothing is irrigating, you have a leak. That's the single most valuable thing a level sensor ever tells you.
- Livestock figures move enormously with heat: a dairy cow drinks 20–35 gal/day and more in August. Size for the hot week, not the average.
The full build and the reasoning are in Chapter 13 of the guide.