How big a water storage tank do you need?

By The Downspout · Updated Oct 11, 2026 · 8 min read

Multiply what you use in a day by the days you want to cover, then add any fire reserve. A family of four at the US average of 82 gallons each needs about 2,300 gallons for a week. Try your own numbers in the calculator.

Run your own numbers first in the water storage tank size calculator. This guide explains what is behind it. The method works anywhere. The permit rules and rain figures are for Sonoma County.

Step 1: what you use in a day

Few people know their daily gallons. So the calculator asks what you do, and does the math with published figures. Count everyone on the property in one number, so a ranch with a main house and a cottage works the same as one home.

Showers. Showers per person, times minutes, times flow. EPA says standard showerheads run 2.5 gallons a minute, and WaterSense models 2.0 or less. We use 2.5. The 2016 Residential End Uses of Water study found the average shower is 7.8 minutes.
Toilets. The federal standard is 1.6 gallons a flush, and the 2016 study found 5 flushes per person per day. We use both and do not ask. Old toilets can use up to 6 gallons a flush, so round up for an old house.
Sinks. Standard faucets run 2.2 gallons a minute. EPA's planning benchmark is about 4 minutes of tap per person per day. We use both and do not ask.
Dishes. ENERGY STAR says a dishwasher saves about 8,400 gallons a year over washing by hand, or about 23 gallons a day. We count a dishwasher at 3 gallons a day and hand washing at 26.
Laundry. EPA's planning range is 1.4 to 3.5 loads per person per week. We count 20 gallons a load, the standard washer. ENERGY STAR washers use about 14, and the 2016 study found an average of 31.
Hot tub. Tubs hold from about 150 gallons for a small one to 600 or more for a large one. Swim and Soak says to change the water every 3 to 4 months for typical use, so we count 4 refills a year, averaged over the year. Each refill is a few hundred gallons at once, so time it for when the tank is full. Top-ups are small and we did not find a figure.
Pool. EPA says an uncovered 500 square foot pool can lose 12,000 to 31,000 gallons a year to evaporation, depending on climate. We use the low end, 24 gallons per square foot, for Sonoma County. A cover cuts evaporation by up to 95 percent, so a covered pool needs very little.
Animals. USDA NRCS lists a horse at 12 gallons a day, a beef animal 8 to 12, a sheep 2, a goat 2 to 4 and a hog 3 to 5. We count 12 for horses and cattle, and 3 for sheep, goats and pigs.
Garden and lawn. The Santa Rosa lawn watering schedule is 45 minutes a week in July at 2 inches an hour, which is about 1.5 inches a week. The other months follow the same schedule's shape, scaled to what you enter for July. The calculator starts at 1.5 inches a week in July and you can change it. Lawn and vegetable beds get all of it. Fruit trees, shrubs and natives get about 40 percent, our estimate between the UC plant factor ranges for moderate and low water plants.

What we left out

Leaks. The 2016 study found 7.9 gallons per person per day on average. Check your toilets and meter. We do not count them.
Hens. 100 laying hens drink about 9 gallons a day, so a small flock is a rounding error.

A family of four on these figures uses about 43 gallons per person indoors, well below the USGS average of 82 at home. The 82 includes outdoor use and leaks, so it is a check, not an input.

Step 2: how many days to cover

Storage covers three things: a pump or power failure, a slow well, and a dry season. Pick a number for the worst of those you face.

A few days covers a pump repair or an outage.
A week or two covers a slow well that needs time to recover.
A month or more is for springs, hauled water and rain-fed systems that go dry.

If you have a well, see how its yield compares with your use in the rural water guide. A well that makes less than you use in a day needs the most storage.

Step 3: fire reserve is separate

Some properties must keep water for firefighting. Tanks installed for fire protection must meet NFPA 22 and follow the Sonoma County Fire Safety Ordinance. Ask for the gallons your parcel needs. Do not guess, and do not count fire water as household water. Contact Fire Prevention at (707) 565-2361, or your own fire district.

Step 4: how much rain can refill it

Rain figures here are Sonoma County. Use your own yearly rainfall elsewhere.

Rain on a roof gives 0.623 gallons per square foot per inch. Most roofs catch about 80 to 90 percent of that. A 1,000 square foot roof in 30 inches of rain gives about 15,000 gallons. See our rain catch calculator for your ZIP.

Rain comes in winter. Use runs all year. A tank sized to hold only a year's total will miss the dry months, so size for the longest dry stretch, not the yearly average. For drinking water from rain, read our rainwater section.

Step 5: shape and space

A gallon is 0.134 cubic feet. A round tank 10 feet wide holds about 590 gallons per foot of height. A 5,000 gallon tank 10 feet wide stands about 8.5 feet tall. Divide your gallons by 590, or ask your dealer for the spec sheet. Check that it fits the spot, and that a truck can reach it if you buy hauled water.

Step 6: one tank or several

The calculator shows one, two, three and four equal tanks, rounded up to common sizes. Both ways work. Here is the trade.

Several tanks. If one is damaged, the others keep your water. They fit awkward spots better, and you can add one later. The cost is extra piping and fittings to link them.
One tank. Fewer parts to maintain and nothing to connect. If it fails, you have no storage at all.
Our view. A second tank also gives you somewhere to hold water while you clean the first. That is our reasoning, not a sourced figure.

Fire water can go in a tank of its own or share the household tanks. Ask your fire district which they accept. Sonoma County does not publish one gallon number for every parcel.

Step 7: what kind of tank

Polyethylene (plastic). The common choice at home. It is the cheapest rigid option, light, and does not rust. One dealer lists sizes from 120 to 11,000 gallons, in black, California green and dark green. Dark colors let in less light, which slows algae. For drinking water, look for resin rated for potable water. Sun can wear it, so shade helps.
Steel. Galvanized, stainless or corrugated. It costs more than plastic, especially large, and lasts for decades if kept up. It can still corrode and needs inspecting. Some regions require steel for fire water.
Concrete. Lasts for decades, comes in large sizes, and does not burn. It needs skilled installation, and a poorly built one can crack and leak. Often used underground.
Fiberglass. Light and low maintenance, often priced between plastic and steel. The shell can be damaged during installation.
Bladder. A flexible bag, cheap and easy to fit in odd spaces. It tears more easily than a rigid tank and holds less.

For fire water, material matters more. A tank maker's own note says a strict reading of NFPA 22 does not authorize plastic tanks, though many jurisdictions allow them. Heat can damage plastic in a fire, and the outcome depends on how close the fire is and how much clear ground surrounds the tank. Ask Fire Prevention, (707) 565-2361, whether plastic is accepted for your fire water.

Step 8: rules and permits in Sonoma County

Tanks over 5,000 gallons need a permit from the Sonoma County Building Department and Fire.
Tanks of 5,000 gallons or more must sit on a concrete foundation.
Fire tanks have set outlet and valve sizes. Ask Fire Prevention before you buy fittings.

Rules change, and the county bulletin we read is from 2018. Call before you build. See who to call about water.

Step 9: wildfire

No water storage system is fire resistant. The Sonoma Resource Conservation District suggests a noncombustible, ember-resistant surface within 5 feet of the tank, no branches overhead, open low landscaping out to 30 feet, and a blue reflective marker visible from the road. Tank guidance from your fire district comes first.

Keep the water good

Stored water can grow algae and collect sediment. Clean the tank on a schedule and test it. See water stored in a tank and our filter guide.

What we do not know

We did not find a Sonoma County rule for how many gallons a home needs, apart from fire. Ask the county for your parcel.
The Santa Rosa lawn schedule says 2 inches an hour for sprinklers. We read that as inches. The 40 percent share for trees, shrubs and natives is our estimate from UC plant factor ranges, not a measured figure.
The 2016 study and EPA give different laundry numbers, 31 and 14 to 20 gallons a load. We use 20.
The calculator rounds to common plastic tank sizes from one dealer's list. Your dealer's sizes may differ.
The county bulletin lists one valve size in two places and may have a typo. Confirm sizes with Fire Prevention.

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Sources

Checked 2026-10-11. Rules and programs change. If something here is out of date, the source above wins. Spot an error? See our corrections policy.