Off-grid cistern size calculator

How this is calculated

The need. Storage is consumption times the interval you have to cover. There are no constants in it; every term is something you choose or measure. The gallons-per-day figure is the one people get wrong. The EPA WaterSense program puts the average U.S. family at more than 300 gallons a day at home, about 70 percent of it indoors. A household that has moved onto hauled water does not use water like that. A careful single occupant with low-flow fixtures and a composting toilet runs 30 to 50 gallons a day; a couple or small family typically draws 75 to 150 gallons a day in total. Enter the number for how you will live, then check it against the meter or the first two deliveries and come back.

The standard size. Poly cisterns come in a handful of common capacities. The calculator uses 1,500, 2,500, 5,000, 10,000 and 20,000 gallons and reports the first one that is at least as large as the need. It always rounds up: a 6,000-gallon need returns 10,000, and a 12,000-gallon need returns 20,000 because there is nothing on the ladder between 10,000 and 20,000. That jump is where you should look at two tanks instead.

Tanks needed and total installed capacity. If the need is at or under 20,000 gallons, tanks needed is 1 and total capacity is the standard size. If the need is larger than the largest tank, the calculator divides the need by 20,000, rounds up, and reports that many 20,000-gallon tanks and their combined capacity. A 43,200-gallon need is 3 tanks and 60,000 gallons. The count is there so a large need is never silently capped at one tank.

What the count does not say. Total capacity is the figure that matters. How you split it is a logistics decision: two 5,000-gallon tanks instead of one 10,000 are easier to deliver down a rough road, easier to set on a pad, and one can be drained for cleaning or repair while the other carries the house. The calculator will not suggest that split; you have to.

Cochise County defaults

• Days between fills: 60. Haulers have a minimum delivery, so a tank smaller than a load wastes a trip, and per-trip costs mean a bigger load costs less per gallon. A practical floor is 2,500 gallons for one person and 5,000 for a family. Hold at least two weeks of use in reserve at all times; a month is better. If the road washes out in August you will want it.
• Gallons per person per day: 50. A middle figure for a household with flush toilets and low-flow fixtures. Drop to 30 to 40 with a composting toilet; raise it for a garden or animals, which this calculator does not model.
• Rain is a supplement, not the supply. Valley-floor rainfall normals run 11.8 to 13.2 inches a year (NOAA 1991–2020: Pearce–Sunsites 12.19, Willcox 12.40, Sierra Vista 13.01), and 51 to 57 percent of it falls July through September. A 1,200 ft² roof yields about 8,200 gallons in an average year; see the rainwater catchment calculator. If the tank is rain-fed, days between fills is the dry stretch from October to late June, not 60.
• Weight. Water is 8.34 lb per gallon, so a full 2,500-gallon tank is 20,850 lb, over ten tons, and a 5,000-gallon tank is nearly 21 tons. Poly tanks need a flat, uniformly compacted pad the full diameter of the tank, not blocks.
• Freeze. Thousands of gallons of water are slow to freeze; the exposed fittings, pump and the line to the house are not, and valley-floor winter nights go well below freezing. Plan the plumbing for it; see the related guide.

A worked example

Four people at 50 gallons a day, with deliveries every 60 days: 4 × 50 × 60 = 12,000 gallons. The next standard size up is 20,000 gallons, 1 tank, 20,000 gallons installed. That is 8,000 gallons of headroom, which is either a 40-day cushion or a reason to price one 10,000-gallon tank plus a 2,500 instead.

Two people at 40 gallons a day with 90 days between fills: 7,200 gallons, next size 10,000, 1 tank. Six people at 80 gallons a day and 90 days: 43,200 gallons, which exceeds the largest standard tank, so the calculator reports 3 tanks of 20,000 gallons and 60,000 gallons installed. At that scale, tighter fixtures or more frequent deliveries are the cheaper variable.

What this does not cover

• Livestock, irrigation and dust control. Each can exceed the household draw; add them to the daily figure yourself.
• Fire reserve. Some insurers and rural fire districts want a dedicated volume with a dry-hydrant fitting; it is on top of this number, not inside it.
• Rain inflow. The calculator treats storage as a bucket that only drains. Combine it with the catchment calculator for a rain-fed system.
• Dead storage. The outlet sits above the tank floor and you keep a reserve, so usable volume is less than nominal.
• Pump, pressure tank, filtration and freeze protection.
• Pad, anchoring and delivery access for a truck that weighs as much as the full tank.
• Permits and setbacks. Whether a tank on a pad needs a county permit is a building-code question; ask Cochise County Development Services before you set one.
• Price. Tank, pad, plumbing and per-load hauling costs change often enough that we do not quote them.

Sources

• U.S. EPA WaterSense — How We Use Water — average U.S. family uses more than 300 gallons per day at home; roughly 70 percent indoors.
• NOAA National Centers for Environmental Information — U.S. Climate Normals, 1991–2020 — annual and monthly precipitation for Pearce–Sunsites (USC00026353), Willcox (USC00029334) and Sierra Vista (USC00027880).
• University of Arizona Cooperative Extension, AZ1344 — Harvesting Rainwater for Landscape Use (PDF) — supply calculation and runoff coefficients used for the rain-fed case.
• USGS Water Science School — Rain and Precipitation — gallons per inch of rain per acre, behind the 0.623 conversion.

Related guides

• Hauled Water and Storage Tanks for Cochise County Off-Grid Land
• Hauled Water vs Drilling a Well: The Real Numbers
• Winter on the Valley Floor: Freeze Protection for Tanks, Pumps and Lines