Storage solves a rate problem, not a volume problem
This is the idea the whole page rests on. A household that uses 150 gallons a day needs a source averaging about a tenth of a gallon a minute. Almost any well on earth manages that. What a household also does is want four gallons a minute for eleven minutes while somebody showers, and that is an entirely different demand which most low yield wells cannot meet.
A tank sits between the two. The source fills it slowly and continuously; the house empties it quickly and intermittently. The well is asked for a daily total instead of an instantaneous rate, and the daily total was never the hard part. This is why a well delivering three quarters of a gallon a minute — a figure that sounds unlivable — supports a household comfortably with a tank and a booster pump, and why people replace pumps trying to solve a problem that a tank solves.
Two ways to size it, and you want both
By reserve: daily demand times the days you want to be independent of the source. That is the usable volume. Divide by the usable fraction of the tank to get the tank you buy.
By refill: can the source put back what the household takes, within the hours the source may run? If yes, the reserve figure governs. If no, no tank size saves you and the shortfall has to be closed some other way.
Run both. The first is the one people do; the second is the one that determines whether the plan works at all.
| Situation | What sets the reserve |
|---|---|
| Low yield well, filling continuously | Peak simultaneous demand plus a margin for the day the pump is out. Days of reserve can be modest because refill is constant. |
| Hauled water | The longest realistic gap between deliveries, not the ordinary one. Weather, road access and the schedule of whoever hauls it, and the tank should accept a full load. |
| Spring or seasonal surface source | The dry period. The reserve has to cross the months when the source gives little or nothing. |
| Rainwater | The dry spell between useful rains — see the rainwater harvesting calculator, which sizes from catchment rather than from a refill rate. |
| Outage backup on a municipal supply | Days you want to be unaffected. The emergency water storage calculator handles the container-and-shelf version of this. |
The usable fraction, and why the nameplate lies
A tank does not give you its rated volume. Water below the outlet connection never leaves under gravity. Sediment settles and you do not want to be drawing off the bottom of it. There is freeboard at the top that never fills. A float or level switch cuts out above the true bottom. Between them these routinely account for ten to twenty percent of a tank, and it is better to build that in at the sizing stage than to discover it the first time you actually run the reserve down.
The other thing the nameplate may not tell you is whether it is nominal or actual. Some tanks are sold by a round number that is not the volume to the brim, let alone to the outlet. Take the manufacturer figure and check what it refers to.
Weight, and where a tank can go
Water weighs 8.34 pounds a gallon. A 2,500 gallon tank holds a little over ten tons. On an eight foot circle that is a little over four hundred pounds per square foot, sustained, forever, on whatever is under it.
That figure rules out most casual placements. A prepared, level, compacted, load-bearing base is not an optional refinement, it is the difference between a tank that lasts and one that splits a seam or settles out of true. Partially buried and buried tanks add their own set of problems — buoyancy when empty and the ground is wet, which floats tanks out of the ground, and soil loading on the walls, which is why buried tanks are specific products and not surface tanks put in a hole. Anything on a structure, in a loft or on a stand is a structural engineering question. If you are excavating for a buried tank, the excavation volume calculator covers the spoil, and the dig line gets called first either way.
The pieces around the tank
A cistern system has more parts than the tank. There is a source and its control — a float switch, a level probe, a timer — and something protecting the source pump from running dry, which for a low yield well is essential rather than nice to have. There is a booster or pressure pump taking water from the tank to the house, with its own pressure tank, and that side of it is the pressure tank calculator. There is an overflow, which has to go somewhere sensible and not into the tank base, the foundation, or across a drainfield. There is an access hatch, a vent and an insect screen. And there is a level indicator, because the single most useful thing about a cistern is knowing how full it is before the day it is not.
Nothing here says anything about whether the water is fit to drink, and it should not. A private well is not tested by anyone unless the owner arranges it, and what to test for, how often, and what to do about a result are questions for your local or state health department and an accredited laboratory. Treatment selection follows the lab report, not a web page. Whether a given tank, fitting or pipe is suitable for water that will be drunk is part of that same question and is answered by the product listing and your health department, not here.
If the reason you are here is that the well cannot keep up, work the source figure out properly first on the well yield and drawdown calculator. Sizing a cistern against an optimistic yield figure is the most common way this exercise goes wrong.
Questions people ask
How big should a cistern be for a house?
Multiply your daily demand by the days you want to be independent of the source, then divide by about 0.85 to allow for the part of the tank you cannot use. Everything else is judgement about how many days that should be, and that comes from the source rather than from the house: a well filling continuously needs far less reserve than a hauled supply on a road that closes in winter. The one input worth getting right is the daily demand, because everything scales off it, and the way to get it right is to build it from fixtures or read the meter rather than accept a per-person figure from the internet.
Can a tank fix a well that runs dry?
It fixes the common version of the problem, which is a well that cannot meet peak demand even though it produces plenty over a day. Storage converts a rate problem into a volume problem and low yield wells are almost always rate problems. What it cannot fix is a source that genuinely produces less per day than the household consumes, because a tank stores water, it does not make it. The test is arithmetic: recovery rate times the minutes the source may run, against daily demand. If the source loses that comparison, more tank only lengthens the interval before you run out.
What does a full water tank weigh?
Multiply gallons by 8.34. A 1,000 gallon tank is 8,340 pounds, a 2,500 gallon tank a little over ten tons, a 5,000 gallon tank over twenty. The number that matters more is the load per square foot, because that weight sits on a small footprint: a 2,500 gallon tank eight feet across puts something over four hundred pounds on every square foot beneath it. That governs the base, and it rules out placing a tank on ground that has not been prepared, on an existing slab that was not designed for it, or on any structure without an engineer looking at it.
Should the tank be above ground or buried?
Above ground is cheaper, far easier to inspect, easy to repair and easy to see the level in. It also freezes where winters are cold, degrades in sunlight unless the tank is made for it, and is visually and spatially in the way. Buried solves the freezing and the appearance and creates its own problems: a buried tank has to be rated for burial because the soil load will collapse a surface tank, it will float out of the ground if it is emptied when the water table is high unless it is anchored, and everything about inspecting, cleaning and repairing it becomes harder. Partially buried is a common compromise. This is worth deciding on the basis of your climate and your access rather than on cost alone, because the cheap option in year one is not always the cheap option in year ten.
How do I know how full the tank is?
Fit something that tells you, because guessing is how people run out. The simple mechanical answer is a float and a sight indicator, and it works without power. Electronic level senders with a display or an alert are common and more informative. On a hauled supply this matters more than anything else on the system: the whole discipline is ordering before you need to, and that is only possible if the level is visible from somewhere you actually look. A tank you have to climb up and open to check is a tank nobody checks, and opening the hatch to look is its own hazard on a large tank.