To size an underground cistern, multiply the people in your home by 50 gallons (189 L) a day, then by the days between refills. Add 20% and round up to the next tank size. A family of four with monthly water delivery needs about 7,200 gallons (27,255 L) of storage.
If your home runs on a cistern, the tank is your water utility. Too small, and you are calling the water hauler every week or cutting showers short when a delivery runs late. Too big, and you pay for gallons you never use, plus a bigger hole in the ground.
The good news is that sizing a whole-house cistern takes simple math. This guide walks you through the formula, gives you a sizing chart by household size, and shows how to match your cistern to the way your water arrives. It is built for homes where the cistern is the main water supply, whether you get trucked-in water, catch rain, or back up a slow well.
The Whole-House Cistern Sizing Formula
The formula
Cistern size (gallons) = People × Gallons per person per day × Days between refills × 1.2
The 1.2 adds a 20% safety margin. It covers late deliveries, house guests, a leaky toilet you have not found yet, and the water at the bottom of the tank that your pump cannot reach. After you do the math, round up to the next tank size you can buy.
Worked example
A family of four gets water delivered once a month. 4 people × 50 gallons × 30 days = 6,000 gallons. Then 6,000 × 1.2 = 7,200 gallons (27,255 L). They need a tank setup that holds at least 7,200 gallons.
Step 1: Estimate Your Daily Household Use
Many sizing guides still tell you to plan for 60 to 70 gallons per person per day. That number is out of date. The Water Research Foundation’s 2026 Residential End Uses of Water study found that indoor use in single-family homes now averages 38.5 gallons (146 L) per person per day. In 1999 it was 69.3 gallons (262 L). Newer toilets and washing machines did most of the work.
For cistern sizing, we suggest planning on 50 gallons (189 L) per person per day. That sits above the national average, which gives you room for older fixtures, visitors, and the habits of real families. Use the table below to pick your number.
|
Your Household |
Plan on (per person, per day) |
Why |
|
Careful water users with efficient fixtures |
35 gallons (132 L) |
Fits homes with newer low-flow toilets, showers, and washers |
|
Most homes (recommended) |
50 gallons (189 L) |
The national indoor average plus a cushion |
|
Older fixtures, frequent guests, or a home business |
65 gallons (246 L) |
Covers higher indoor use |
These numbers cover indoor use only. If the cistern also waters a garden, fills a stock tank, or washes vehicles, estimate those gallons and add them to your daily total.
Step 2: Choose Your Refill Window
The refill window is the number of days your tank has to carry the house before more water arrives. It depends on where your water comes from.
- Hauled water: Use the number of days between deliveries. Many rural homes schedule every two weeks or once a month. If your hauler is often booked up, add a few days.
- Rainwater: Use the longest dry spell you expect in a normal year. The roof math is covered in our rainwater harvesting calculator guide.
- Slow or low-yield well: The cistern stores water the well pumps slowly all day, so the house can draw it fast when needed. Size for one to three days of total use, then confirm with your well driller.
Step 3: Add a 20% Safety Margin
Multiply your result by 1.2. Skip this step and a single late truck can leave you dry. The margin also covers the water that sits below your pump intake, which you should not count as usable.
Step 4: Round Up to a Real Tank Size
Tanks come in set sizes, so always round up. If your number lands between two sizes, pick the larger tank or pair two tanks together. Browse our underground water cisterns to see the sizes we carry.
Underground Cistern Sizing Chart by Household Size
This chart uses 50 gallons (189 L) per person per day and already includes the 20% safety margin. Find your household size, then read across to your refill window.
|
People |
Daily Use |
Refill Every 2 Weeks |
Refill Monthly |
Example Setup (Monthly) |
|
1 |
50 gal (189 L) |
840 gal (3,180 L) |
1,800 gal (6,814 L) |
One 2,000-gallon cistern |
|
2 |
100 gal (379 L) |
1,680 gal (6,359 L) |
3,600 gal (13,627 L) |
Two 2,000-gallon tanks (4,000 total) |
|
3 |
150 gal (568 L) |
2,520 gal (9,539 L) |
5,400 gal (20,441 L) |
Two 3,525-gallon tanks (7,050 total) |
|
4 |
200 gal (757 L) |
3,360 gal (12,719 L) |
7,200 gal (27,255 L) |
One 5,025 plus one 2,500-gallon tank (7,525 total) |
|
5 |
250 gal (946 L) |
4,200 gal (15,899 L) |
9,000 gal (34,069 L) |
One 10,000-gallon tank |
|
6 |
300 gal (1,136 L) |
5,040 gal (19,078 L) |
10,800 gal (40,882 L) |
One 10,000 plus one 2,000-gallon tank (12,000 total) |
Using 35 gallons per person? Multiply any number in the chart by 0.7. Using 65 gallons? Multiply by 1.3.
Match Your Cistern to Your Water Truck
If you buy hauled water, your tank and your hauler’s truck should work together. Here is a simple rule: your cistern should hold a full truckload plus about a week of your household’s use. That way you can order water before you run low and still take the whole load, so you never pay for a trip and leave gallons on the truck.
Example
Your hauler carries 3,000 gallons. Your family of four uses 200 gallons a day, so a week of use is 1,400 gallons. 3,000 + 1,400 = 4,400 gallons. You want at least a 4,400-gallon setup, even though your deliveries come every two weeks.
Truck sizes vary from one hauler to the next, so ask yours before you buy. If your hauler charges per trip, a larger tank can pay for itself through fewer deliveries. For large single-tank setups, see our 5,025-gallon underground water tank.
Rainwater and Well Homes: What Changes
The formula works for every water source. What changes is how you pick the refill window.
For rainwater homes, your roof sets the supply and the dry season sets the size. Work out your catch with the Tank Depot rainwater calculator, then use your longest dry spell as the refill window in the formula above.
For homes with a slow well, the cistern acts as a buffer. A well that makes one or two gallons a minute can keep up with a household over a full day, but not with a shower, a washing machine, and a dishwasher running at once. The cistern stores the slow trickle so your pump can deliver a strong flow. Ask your well driller for the well’s real yield, since that number matters most here.
One Large Cistern or Two Smaller Tanks?
Once you have your number, you can reach it with one tank or two tanks joined together. Two tanks often make sense when:
- your total is bigger than the largest single tank that fits your site or delivery route
- you want a backup, so one tank can stay in service while the other is cleaned or repaired
- rock or tight access makes one long, deep hole hard to dig
One larger tank means fewer fittings and only one hole. If you go with two, the tanks must be joined at the bottom so they fill and drain evenly. Our guide on how to connect multiple water storage tanks covers the plumbing.
Usable Capacity: The Gallons You Cannot Pump
A tank’s listed size is not the same as the water you can use. Pump intakes sit above the tank floor so they do not pull up sediment, and many systems shut the pump off at a low-water level to protect it. The water below that point stays in the tank.
This is one more reason the 20% safety margin matters. When you compare tanks, ask how much of the listed capacity your setup can actually draw. Our team can help you plan intake height and shutoff settings when you choose cistern pumps and pressure systems.
Before You Buy: A Quick Checklist
- Daily use: people × gallons per person, plus any garden or livestock water
- Refill window: days between deliveries, or your longest dry spell
- Safety margin: multiply by 1.2
- Tank size: round up to a real size, or plan two tanks
- Site: check access for delivery and digging equipment; see our underground water tank installation guide
- Drinking water: if you will drink it, read our guide to potable vs. non-potable cisterns
- Pump: pick one that fits the tank and the house; compare submersible and surface pumps
- Wastewater: building off-grid? You will likely need a septic tank too; see our septic tank size chart
Get the Right Size the First Time
An underground cistern is a long-term part of your home. The right size means steady water, fewer deliveries, and no panic when the truck runs late. Use the formula, check the chart, and match the tank to how your water arrives.
Talk to a Tank Depot expert
Not sure which size fits your home, or whether one tank or two is the better setup? Call (866) 926-5603 and our team will help you size, plan, and order the right underground cistern for your property. Many of our underground cisterns use FDA-compliant resin, and select models carry NSF/ANSI 61. Check each product page or confirm with our reps.
