Water storage tank size calculator

By The Downspout · Updated Oct 11, 2026 · 6 min read

Answer a few questions about your household, animals and garden. The calculator works out your daily use, then the tank size for the days you want to cover, and whether one tank or several fits.

How to use it

Count everyone who uses the tank, across all homes on the property.
Answer the household questions. Nobody knows their daily gallons, but most people know their showers and loads of laundry. The calculator does the rest.
Add a hot tub, pool, animals and garden if you have them. Pick the closest size.
Fire gallons are only for if your fire department asks for them.
Pick days of reserve, then read the total and the tank options at the bottom.

Why those numbers, and what else to check, is in our guide how big a water storage tank do you need.

Defaults are for a family of four. Change what is not true for you. Tap the ? if you are not sure.

1. Your household

Everyone living on the property, all homes together

Count everyone who lives there most of the year, in every home on the property. A part-time guest counts as a fraction. Someone who stays about a quarter of the year counts as 0.25.

Showers per person per week

7 is one a day. Every other day is about 3 or 4. Count baths as a shower.

Minutes per shower

The average shower in the 2016 national water-use study was about 8 minutes. Teenagers run longer. Time one if you are not sure.

Laundry loads per person per week

Count the loads in a normal week and divide by the number of people. EPA's planning range is about 1.4 to 3.5 loads per person per week. Families with young children are at the high end.

Dishes

Pick the way most of your dishes get washed. ENERGY STAR says a dishwasher saves about 23 gallons a day over washing by hand.

2. Hot tub, pool, animals and garden

Hot tub

Tubs hold about 150 gallons for a small one to 600 or more for a large one. Check your model. We count the water drained and refilled 4 times a year, which is typical for regular use. Top-ups are small and not counted.

Pool

This counts water lost to evaporation, uncovered. A pool cover cuts that by about 90 percent, so a covered pool needs very little.

Horses or cattle

Leave at 0 if you have none. A horse drinks about 12 gallons a day, and a beef animal 8 to 12.

Sheep, goats or pigs

Leave at 0 if you have none. Sheep drink about 2 gallons a day, goats 2 to 4, pigs 3 to 5. We count 3 for each.

Lawn and vegetable beds

Mowed grass and beds you water often through summer. Pick the size they add up to. Leave at None if you have neither. A few raised beds is about a parking space.

Fruit trees, shrubs and native plants

Plants that need less water than lawn. For a tree, count the ground under its branches. Pick the size they add up to.

Garden watering in July, inches a week

How much water your lawn and beds get each week in the hottest month. Santa Rosa's watering schedule works out to about 1.5 inches. Cooler coastal areas need less and hot inland areas need more. Your water agency or UC Cooperative Extension publishes a local watering guide.

Include July garden watering in my reserve

3. Your tank

Days of reserve you want

A few days covers a pump repair or a power outage. A week or two covers a slow well. A month or more is for a spring or a rain-fed system that goes dry.

Gallons set aside for fire protection

Only if your fire district or permit asks for it. Sonoma County does not publish one gallon number that fits every parcel, so ask Fire Prevention, (707) 565-2361, or your own fire district. Leave at 0 if not.

Where the fire water goes

Fire water kept in its own tank cannot be drawn down by the house. Water kept in the same tanks only stays reserved if the household outlet sits above the fire amount. Ask your fire district which they accept.

Footprint of your roof, to see how much rain refills the tank

Optional. The flat area the roof covers on the ground. For a one story house it is about the same as the floor area. For two stories, about half the floor area. A garage or barn roof counts too if it feeds the tank.

Rain in a year, inches

Santa Rosa averages 31.5 inches a year. For another place, look up your town's yearly average on weather.gov or ask your water agency. Our rain catch calculator has Sonoma County ZIP codes.

Figures are from EPA, ENERGY STAR, the 2016 Residential End Uses of Water study, USDA NRCS and Santa Rosa's lawn watering schedule. Toilets are counted at the federal standard of 1.6 gallons a flush, showerheads at 2.5 gallons a minute and washers at 20 gallons a load. Sinks are counted at EPA's benchmark of 4 minutes per person a day. Rain uses 0.623 gallons per square foot per inch at 80% catch.

What this does not do

It does not check the rules for your parcel. The permit notes here are for Sonoma County: tanks over 5,000 gallons need a county permit. Ask Permit Sonoma and your fire district, or your own county outside Sonoma.
It does not model a year of rain month by month. Rain falls in winter and use runs all year, so you may need more storage than the yearly total suggests.
It counts fixtures at today's standards: 2.5 gallon a minute showerheads, 1.6 gallon toilets and 20 gallon washers. Older ones use more, so round up for an older house.
It leaves out leaks, hens and other small uses. The hot tub is counted as 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.
The pool figure is evaporation only, uncovered, at a low estimate for a mild climate. It does not count top-ups, backwashing or heating.
Garden watering follows the Santa Rosa schedule through the season. You set the July amount, 1.5 inches a week by default, and the other months scale from it. Rain is yours to set, with the Santa Rosa average as the default.
It rounds to common plastic tank sizes. Your dealer's sizes, and steel or concrete tanks, may differ.
It does not size the pump or pressure tank. See the rural water guide.

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.