Is your well big enough for your house? A peak demand calculator

A well can make plenty of water over a whole day and still leave you with a trickle at 7 a.m. when two people are showering and the washer is filling. What matters is the busy stretch, not the daily total. This calculator puts your household's busiest-hours demand next to what your well can deliver, using planning figures published by Penn State Extension and University of Nebraska Extension (both drawn from the MidWest Plan Service Private Water Systems Handbook). The yield and depth figures you need are usually on your well's driller's report.

The three numbers the calculator compares

Water planners don't size a home water system from a single figure, so the calculator shows three, and each answers a slightly different question.

1. Minimum flow by bedrooms and baths

The Private Water Systems Handbook (MidWest Plan Service) gives a minimum flow for a house based on its bedrooms and bathrooms, on the logic that more bedrooms means more people and more bathrooms means more taps open at once. Penn State Extension reprints the full grid:

Penn State's own starting point for an ordinary single-family home is at least 6 gpm, which is 360 gallons an hour. Nebraska Extension's planning guide lists the same handbook figures and adds a shortcut for bigger houses: roughly 2 more gpm for every extra bedroom and every extra bath.

2. A whole day's water in two hours

Most household water goes out in a couple of short bursts, morning and evening. Penn State suggests planning so the system could hand over the entire day's water inside a two-hour peak, and Nebraska Extension likewise says the supply should keep up with peak demand for about two hours. For the daily total, the calculator uses Nebraska's planning figure of 100 gallons per person per day; Penn State cites EPA's range of 80 to 100.

3. Fixtures running at the same moment

The third check is the one you feel in the shower. Each fixture needs a certain flow to work properly, and these are the handbook figures as printed by Nebraska Extension:

Newer low-flow showerheads and fixtures use less than these older planning numbers, so a house full of them will do a bit better than the calculator suggests.

Where your yield figure comes from

Yield is the fastest rate a well can be pumped without pulling the water in the hole down below the pump intake. Tennessee drillers record an estimated yield, the total depth and the static water level on the report they file with TDEC, and that report is public. If yours isn't in the closing papers, the well depth and records page explains how to look it up, and it also shows the median reported yield for each county around Knoxville.

Two cautions. A yield written down on drilling day may not match what the well gives today, especially after years of use or in a dry autumn. And in the fractured limestone and dolomite under East Tennessee's valleys, the USGS reports that many springs flow around ten times harder in wet periods than in dry ones, so treat a marginal well as more marginal in a drought. A licensed installer can run a flow test if you need a current number.

TDEC's general guidance is that 3 to 5 gpm or better usually serves a family of four to six. That sits well below the handbook grid, which is why the water stored in the borehole matters so much on wells around here.

Stored water: borehole, pressure tank and holding tank

When the well's flow alone can't match a busy hour, stored water makes up the gap. There are three places it can sit.

• The borehole itself. Penn State figures a 6-inch drilled well holds about 1.5 gallons for each foot of standing water and a 10-inch well about 4. A 6-inch well with 100 feet of water above the bottom holds roughly 150 gallons. Penn State's fact sheet on low-yielding wells also warns that the same well might have under 20 feet of water during a drought, which is why the calculator calls its storage number a best case.
• The pressure tank. Useful, but small. Penn State's fact sheet puts the usable water at around a fifth of the tank's size: a 42-gallon tank gives up about 8 gallons before the pump kicks on. A bigger tank mostly helps the pump run less often; see pressure tank replacement.
• An intermediate storage tank. This is the standard fix for a well that truly can't keep up. The well pump trickles water into an unpressurized tank or cistern all day, and a second pump feeds the house from that tank. Penn State's rule of thumb is about 100 gallons of tank for each person in the home, so a family of five would want around 500 gallons, kept from freezing. The well pump on such a setup is sized a little below the well's yield and protected by a low-water cutoff.

If the calculator says you come up short

A shortfall is common and usually fixable. In rough order of cost:

1. Move the load around. Run laundry on different days, stagger showers between morning and night, and keep sprinklers off during the morning rush. It costs nothing.
2. Cut what each use takes. Penn State notes a front-loading washer alone saves more than 20 gallons a load compared with a top-loader, and low-flush toilets use 1.6 gallons where older ones used 5.
3. Add storage. A holding tank and booster pump, as described above, lets a slow well fill the tank overnight.
4. Deepen or redrill. Drilling deeper adds borehole storage and may hit another fracture, but it costs more and can change the water quality, so it is a conversation for a licensed driller.

A bigger pump is not on that list on purpose. A pump can only move what the well yields; one that pulls harder than the well refills just drops the water to the intake faster. If the pump has already failed and you're choosing a new one, the yield and peak numbers above are exactly what the installer will ask about; see well pump replacement. If water quits during heavy use and comes back after a rest, start with no water from the well.

Good questions

How many gallons per minute does a house need from a well?

The MWPS handbook figures reprinted by Penn State and Nebraska Extension run from 6 gpm for a 2-bedroom, 1-bath house to 18 gpm for 6 bedrooms and 3 baths. TDEC says 3 to 5 gpm or more usually serves four to six people, with stored water covering short bursts.

Can a 1 gpm well supply a house?

Over a full day it makes 1,440 gallons, more than most families use. Penn State's example is that the same well gives only 120 gallons in a two-hour morning rush, so a low-yield well usually needs a storage tank and some spreading out of heavy water use.

Can I link to this calculator?

Yes. Real estate agents, home inspectors, builders and neighborhood groups are welcome to link to it. It is free and asks for no contact details.