Well Flow Rate Calculator

1. Pick a mode. Bucket test gives the flow at one tap. Well yield uses a pump test or a water-level recovery test. Household demand checks a yield against your home's peak use.
2. For a bucket test, type the container size in gallons or liters and the fill time in seconds.
3. For a recovery test, pick the casing size and type how many feet the water rose and over how many minutes.
4. For household demand, type the people, the use per person, the peak period and your measured yield. Add the standing water in the well if you know it.
5. Read the gpm, the L/min and the step by step working below the result.

Every rule of thumb in the tool, such as the use per person and the 2-hour peak, is an editable input. Change it if your contractor or local health department gives you a different figure.

Flow rate is volume over time. The formulas below turn a bucket, a pump test or a rising water level into gallons per minute (gpm).

V is the container size in gallons and t is the fill time in seconds. Multiply gpm by 60 for gallons per hour and by 1,440 for gallons per day.

d is the inside diameter in inches, so r = d / 24 is the radius in feet. One cubic foot holds 7.48052 US gallons. A 6 in casing gives pi x 0.25^2 x 7.48052 = 1.469 gal/ft.

h is the rise in the water level in feet, c is gallons per foot and t is the time in minutes.

P is people, u is gallons per person per day, T is the peak period in minutes and Y is the well yield in gpm. w is the standing water in the well in feet. S is the extra storage needed, and it is never less than 0.

1. Use an outside spigot close to the pressure tank, with no hose, filter or softener in the way if you can.
2. Close every other tap and make sure no toilet, washer or irrigation is running.
3. Run the water for a few minutes first, so the pump cycles and the flow settles.
4. Open the spigot fully, place the bucket and start the timer. Stop it when the water reaches the mark.
5. Repeat three or more fills and average the times. One rushed fill can be off by several seconds.

A 5 gallon bucket that fills in 40 seconds is 7.50 gpm. If it takes 75 seconds, the flow is 4.00 gpm. A short test like this mostly measures the pump, the pressure tank and the pipes. It does not show how long the well can keep that rate up.

Flow at the Tap

This is what the bucket test measures. It depends on the pump, the pressure setting, pipe size and any filters. For the first minute or so, part of it comes from the pressure tank.

Pump Capacity

This is the rate the pump is built to move at your well depth and pressure. A pump can be sized larger than the well can supply, so a strong tap flow can hide a weak well until a long peak period.

Well Yield

Penn State Extension defines yield as the most gpm a well can be pumped without the water level dropping below the pump intake. It is measured with a pump test over hours, or by timing how fast the level recovers. Yield often drops in dry seasons and droughts.

Each row is computed from c = pi x r^2 x 7.48052. Liters per meter use 1 gal = 3.785411784 L and 1 ft = 0.3048 m.

Penn State Extension rounds these to about 1.5 gal/ft for a 6 in well and about 4 gal/ft for a 10 in well. So 100 ft of standing water in a 6 in well holds about 147 gallons.

Penn State Extension, citing the EPA, gives 80 to 100 gallons per person per day in the home. It recommends that a system be able to deliver a whole day of use in a 2-hour peak period. It also suggests at least 6 gpm for most single-family homes, and more for larger homes, as in the table below.

Minimum flow rates from Penn State Extension, Table 2 (from the Private Water Systems Handbook, 1992). A dash means the table gives no value.

If the yield is lower, storage can bridge the gap. Water standing in the casing counts, and so does a larger holding tank. Penn State suggests an intermediate tank that ideally holds a full day of use, about 100 gallons per person. It notes that a pressure tank gives only about 20% of its size as usable water.

Call a licensed well contractor if the flow drops over time, the pump runs often or sputters air, or the water turns cloudy after heavy use. These can point to a falling water level, a failing pump or a clogged screen.

Get a professional yield test before you buy a home with a well, add bathrooms or add irrigation. A contractor can pump the well for hours, track the drawdown and report a yield you can rely on. Well rules and licensing vary by state, so check with your state or local health department.

Test the Water Too

Flow says nothing about safety. The EPA advises testing a private well every year for total coliform bacteria, nitrates, total dissolved solids and pH, using a certified laboratory.

Time how long a container of known size takes to fill at an outside spigot, then divide the gallons by the seconds and multiply by 60. A 5 gallon bucket that fills in 40 seconds gives 7.50 gpm, or 450 gallons per hour. The same bucket in 75 seconds gives 4.00 gpm.

No. A bucket test shows what the pump and pipes deliver at that tap right now. Well yield is the rate the well can be pumped without the water level falling below the pump intake, and it takes a longer pump test or a recovery test to measure.

About 1.47 gallons per foot. The volume is pi x (0.25 ft)^2 x 7.48052 gal per cubic foot = 1.469 gal/ft. A 4 inch casing holds about 0.65 gal/ft, a 5 inch about 1.02 gal/ft and an 8 inch about 2.61 gal/ft.

Penn State Extension suggests a minimum of 6 gpm from a well or spring for most single-family homes. Its table raises this with home size, for example 12 gpm for 3 bedrooms and 2 baths. A lower yield can still work if storage covers the peak period.

Penn State Extension, citing the EPA, gives 80 to 100 gallons per person per day in the home, or 200 to 400 gallons a day for a family of four. It suggests sizing the system to deliver the whole day of use in a 2-hour peak period.

When the yield is < the peak demand rate, multiply the peak minutes by the gap. A family using 400 gallons a day needs 3.33 gpm over 2 hours, so a 2 gpm well falls short by 120 x 1.33 = 160 gallons. Penn State suggests an ideal tank of a full day of use, about 100 gallons per person.

The EPA says to test a private well every year for total coliform bacteria, nitrates, total dissolved solids and pH, and to test for other contaminants you suspect. Use a laboratory certified for drinking water testing.

• Penn State Extension: Water System Planning, Estimating Water Needs (80 to 100 gal per person per day, 2-hour peak period, 6 gpm suggested minimum, Table 2 minimum flows, about 1.5 gal/ft in a 6 in well).
• Penn State Extension: Using Low-Yielding Wells (definition of well yield, peak periods of 30 minutes to 2 hours, pressure tank about 20% usable, storage of about 100 gal per person).
• US EPA: Protect Your Home's Water (test a private well every year for total coliform bacteria, nitrates, total dissolved solids and pH; use a certified lab).
• NIST Special Publication 811, Appendix B.8: Factors for Units Listed Alphabetically (US gallon = 3.785412 L; cubic foot = 0.02831685 m3, which gives 7.48052 gal per cubic foot).