What size pressure tank does your well need?

What the number on the label is not

A pressure tank is a buffer. The Texas Well Owner Network describes the job in one line: most home well systems have a pressurized storage tank so that small demands do not continually activate the pump Source 4: Texas Well Owner Network, Well Owner's Guide to Water Supply4. Everything on this page follows from how much water that buffer hands you between pump starts.

Washington's design manual gives the quantity its engineering name: withdrawal capacity is the portion of tank volume that can be usefully withdrawn between pumping cycles while still maintaining 30 psi throughout the house under peak demand Source 2: Washington State Water System Design Manual, DOH 331-1232. The everyday word is drawdown. The second half of that definition is the half that matters. Water that leaves the tank after the pressure has already collapsed is not capacity. By then, the pump has restarted.

Three tanks, three fractions, same physical volume. Michigan puts numbers on it.

• A bladder or diaphragm tank with a correct pre-charge, running at 30 to 50 psi, makes about 25 percent of its total volume available.
• A plain steel tank with a pre-charge, holding its high water level at 55 percent of capacity, makes about 20 percent.
• A plain steel tank with no pre-charge at all makes about 10 percent Source 1: State of Michigan Water Well Manual1.

The tank has not changed size between those three lines. What changed is how much air is in it and where the air is kept.

This is the conversion the sizing charts skip. It runs in the direction you need, not the direction a catalogue runs. You do not start from a tank and ask what it gives; you start from the drawdown you need and divide.

Twenty-eight gallons from a pre-charged bladder tank is 28 divided by 0.25, a 112-gallon nameplate. The same 28 gallons from an unpre-charged steel tank is 280.

Michigan does that division in its own worked example and lands on a 200-gallon tank for a 50-gallon requirement Source 1: State of Michigan Water Well Manual1.

One boundary before the arithmetic starts. This page sizes a tank on plan. It does not diagnose one that is misbehaving.

A pump restarting every few seconds, water spraying from the air valve, or a waterlogged tank is a different question, with a different order of checks. See the short-cycling guide, which also carries the dated cost of every repair named here.

There are two sizing rules, and which one applies depends on your well

Most homeowner guides that size a tank at all use one rule. Michigan uses two, and the fork between them is whether the pump can meet the house's peak demand on its own Source 1: State of Michigan Water Well Manual1. Getting the fork wrong turns a correct formula into a tank too small for the house it was bought for.

First, the fixture count. Michigan's shortcut for pump capacity is that the minimum output in gallons per minute should equal the number of water-using fixtures in the house. Its own example counts two bathrooms, a kitchen sink, a disposal, a dishwasher, a washing machine, a laundry sink and three hose bibs, reaches 14 fixtures, and calls for a 14-gallon-per-minute pump Source 1: State of Michigan Water Well Manual1. Count yours before reading further, because the fork depends on it.

Rule A, pump protection. This is the rule when the pump meets or exceeds peak demand. Drawdown equals the pump's output multiplied by a minimum run time, and Michigan tabulates the run time by pump size where the manufacturer states none: one minute for 10 to 20 gallons per minute, two for 21 to 50, three for 51 to 75, and four for 76 to 100 Source 1: State of Michigan Water Well Manual1.

Larger pumps get longer run times because starting them draws more electrical energy. A 14-gallon-per-minute pump on a one-minute cycle needs 14 gallons of drawdown, and 28 if you want a two-minute cycle.

Rule B, peak demand. This is the rule when the pump cannot keep up. It produces a larger tank. Michigan gives the seven-minute peak demand by bathroom count, with the pump size that meets it unaided: 1 bathroom, 45 gallons, 7 gpm; 1.5, 70 gallons, 10 gpm; 2 to 2.5, 98 gallons, 14 gpm; 3 to 4, 122 gallons, 17 gpm, with farm and irrigation use added on top where it overlaps the peak Source 1: State of Michigan Water Well Manual1.

Required drawdown is peak demand minus what the pump delivers in those seven minutes. A 10-gpm pump in a two-bathroom house produces 70 of the 98 gallons, so 28 must come from the tank.

And the ceiling on all of it. Arizona states the limit that neither rule contains: a tank cannot compensate for demand greater than your pump or well capacity Source 5: University of Arizona Superfund Research Program, Well Owner's Guide5. Storage buys minutes, not gallons per day.

If the well itself is the constraint, no tank on this page is the answer. The question to ask is about yield, not storage.

The pre-charge number, and why you will find two of them

Pre-charge is the air put into the tank before water reaches it. Read it at the air valve, with a tire gauge. Michigan's glossary gives the rule in one sentence: the pressure should be 2 psi less than the cut-in pressure of the pump pressure switch, and a correct pre-charge prevents interruption of water and extends the life of the pump and tank Source 1: State of Michigan Water Well Manual1. On a 30/50 switch that is 28 psi.

You will also find five psi. It is in the same manual. Michigan's sizing chapter writes that if the pre-charge is about 5 psi below the pump cut-in, supplemental supply is obtained from the tank when demand exceeds the pump's capacity Source 1: State of Michigan Water Well Manual1. Washington's design manual makes the same figure a standing assumption, taking bladder tanks to be pre-charged about 5 psi below pump-on pressure Source 2: Washington State Water System Design Manual, DOH 331-1232.

The two answer different questions. The placement is the tell. Two psi below cut-in is the normal-operation setting. It keeps the air cushion working at the moment the pump restarts, and it is what you check your own tank against.

Five psi appears where it appears, inside a supplemental-storage passage and inside a manual's sizing of banks of tanks for a utility. It is a deliberate detune that buys volume at peak and pays in delivered pressure. If you run out mid-shower while the washing machine fills, that trade is on the table. Otherwise 2 psi is your number.

The same tank, set two ways, is two different tanks

The instinct when a house runs short at peak is to buy a bigger tank. Michigan's own worked example shows what the settings are worth first. The numbers are larger than most people expect.

A 120-gallon plain steel tank, run at 30 to 50 psi with no pre-charge, makes about 10 percent of its volume available: 12 gallons. Lower the operating pressure to 20 to 40 psi and pre-charge the tank to 15 psi, and the usable capacity rises to 37.4 gallons, or 31 percent Source 1: State of Michigan Water Well Manual1. Same tank, nothing bought, roughly three times the water.

The cost is real, and belongs in the same breath. 20 to 40 psi delivers less pressure at the shower head than 30 to 50. A household that already finds its pressure weak would be trading the wrong way. That is one reason the settings are a contractor's call.

Where a second tank is added for storage instead, Michigan's rule is that it is pre-charged lower than the first and the switch differential is set closer together Source 1: State of Michigan Water Well Manual1.

When none of this arithmetic applies

One system type voids the whole page. Almost nothing written for homeowners says so. Michigan is explicit: conventional pressure tank sizing methodology is not applicable to variable-frequency-drive systems, because the need to achieve a minimum pump run time is not a concern on one, and a much smaller tank can control such a system Source 1: State of Michigan Water Well Manual1.

These are sold to homeowners as constant-pressure systems. If you have one, every run-time figure above is irrelevant to your tank. Following a generic sizing chart buys a tank several times larger than the system needs.

The manufacturer's installation specification is the authority there. It is the one place on this page where the answer is a document, not a calculation.

What is yours, and what is not

Three verdicts, no spectrum. The split on this topic is unusual and worth stating plainly: the arithmetic is entirely yours, and almost none of the physical work is.

What a good quote shows you

A pressure tank is the one component on a well system most often replaced like for like. A like-for-like swap of a tank that was undersized to begin with reproduces the original problem at full price. Two questions close most of that gap.

• Ask what pump output and what minimum run time the tank was sized against. Michigan's tables turn on the pump's gallons per minute and a run time, not on the dimensions of the tank being carried out Source 1: State of Michigan Water Well Manual1. If the answer is the model number of the old tank, the sizing has not been done. If the answer is a gpm figure and a run time, you can check it against this page in a minute.
• Ask for the pre-charge, cut-in and cut-out to be set and written on the invoice. Those three numbers are the system and only work as a set: a tank fitted without its pre-charge matched to the switch is a correctly sized tank behaving like a small one. NGWA's guidance applies directly. Ask the contractor for the warranty and maintenance information on individual components, pump and pressure tank included Source 3: National Ground Water Association, Well Owner's Guide 20213. The figures belong in your records.

Rules, and who is allowed to do the work

The two manuals cited throughout are Michigan's and Washington's, which are two states' documents rather than national rules. They are used here for pressure system mechanics, which are physics, and not for their permitting requirements, which are local.

Michigan's Well Construction Code, for instance, requires a pressure tank to sit in an approved pump room, well house, crawl space, basement offset or basement, requires buried tanks to be an approved model, and requires a relief valve where the pump can exceed the tank's working pressure Source 1: State of Michigan Water Well Manual1. Its land-division rules ask a well to demonstrate a sustained 10 gallons per minute for four hours, or a combination of yield and storage meeting peak demand, which is rule B's fork written into a rule Source 1: State of Michigan Water Well Manual1. Several states license well contractors separately from plumbers. They also restrict wellhead work to that trade.

Requirements vary by county. Your county health department has the final word on permits, inspections, and what is allowed on your property.

Opening a system for tank or pump work is also a water-quality event: what to test for and how often covers the retest, and shock chlorination covers disinfection afterwards. More sourced guides are in the guide library.