Well Yield and Drawdown Test Calculator

How it works

A constant-rate well yield test tracks two steps. First, you measure how far the water drops while the pump runs. Second, you time how fast the hole refills after the pump stops. In its Water Basics Glossary, the U.S. Geological Survey (USGS) defines drawdown as "The difference between the water level in a well before pumping and the water level in the well during pumping." Dividing the pumping rate by drawdown gives specific capacity, which the USGS glossary defines as "The yield of a well per unit of drawdown." At 10 gpm with a 40 ft static level and 70 ft pumping level, drawdown is 30.0 ft. That gives a specific capacity of 0.333 gpm/ft.

In Water System Planning: Estimating Water Needs, Penn State Extension defines well yield as "the maximum rate in gallons per minute (GPM) that a well can be pumped without lowering the water level in the borehole below the pump intake." This tool gives an upper-end estimate of that yield in a straight line: specific capacity times the feet of water between the static level and the pump intake. Specific capacity often drops at higher flow, so treat the figure as an upper-end estimate and confirm it with a test run near that rate. As Penn State Extension notes, "A typical 6-inch-diameter well will store about 1.5 gallons of water for every foot of standing water in the borehole and a 10-inch well stores about 4 gallons of water per foot." By exact cylinder math, a 6-inch pipe holds 1.469 gal/ft.

Casing water is not new supply. Drawdown times gallons per foot is the water the pump took from standing storage, and the rest of the pumped volume came from the rock. The refill figure divides the water rise after pumping stops by the minutes you watched. It is an average over that interval. Inflow slows as the water rises, so a longer window gives a lower average. The figure is not a yield. If the pumping level is below the pump intake, the estimate comes out lower than the test rate, before rounding. Send your pumping rate to the well pump head calculator to find total head. You can also check power bills in the well pump electricity cost calculator.

Formula

Drawdown (ft) = Pumping water level − Static water level
Specific capacity (gpm/ft) = Pumping rate ÷ Drawdown
Casing volume (gal/ft) = π × (Casing diameter ÷ 24)^2 × (1,728 ÷ 231)
Average refill rate (gpm) = (Recovery rise × Casing volume) ÷ Recovery time
Upper-end yield estimate (gpm) = Specific capacity × (Pump intake depth − Static water level)
Water from the rock (gal) = Water pumped − Drawdown × Casing volume

Example

Take a 6-inch home well with a static water level at 40 ft and a pump intake at 120 ft. You pump at 10.00 gpm for 60 minutes, and the water drops to 70 ft. Drawdown is 70 − 40 = 30.0 ft. Specific capacity is 10.00 ÷ 30.0 = 0.333 gpm/ft, and water stays 50.0 ft above the 120 ft intake. In 60 minutes the pump moves 600 gal. Of that total, 30.0 ft × 1.469 gal/ft = 44.1 gal (7.3%) comes from casing drawdown storage. The other 555.9 gal flows in from the rock, an average of 9.27 gpm. At rest, the 80.0 ft column above the intake holds 80.0 × 1.469 = 117.5 gal.

Between the 40 ft static level and the 120 ft intake lie 80.0 ft of water. At 0.333 gpm/ft, the upper-end yield estimate is 0.333 × 80.0 = 26.7 gpm, 2.7 times the tested rate. That is 14.7 gpm above Penn State Table 2's 12 gpm minimum flow for a 3-bedroom, 2-bathroom home. The 10.00 gpm test rate sits 2.00 gpm below it. Because specific capacity often drops at higher flow, a test run near 12 gpm would confirm that the well meets 12 gpm. After you shut the pump off, the water rises 29.0 ft in 15 minutes, climbing from 70 ft back to 41 ft. That rise holds 29.0 × 1.469 = 42.6 gal. Dividing by 15 minutes gives an average refill of 2.84 gpm. That average is below the test rate because rock inflow slows as the level rises, so it is not the yield.

Assumptions and limitations

• Casing volume per foot uses exact cylinder math (1 US gallon = 231 cubic inches; 1 cubic foot = 1,728 cubic inches). This gives 1.469 gal/ft at 6 in and 4.080 gal/ft at 10 in, rather than rounded 1.5 and 4 gal/ft approximations.
• The yield estimate equals specific capacity times the feet between the static level and the pump intake, rounded to 3 figures. It assumes drawdown stays in proportion to the pumping rate, which wells often do not follow at higher flow, so it is an upper-end figure.
• Specific capacity (gpm/ft) applies to your test rate and time. At higher flow rates or longer runs, specific capacity often drops due to well friction and aquifer drawdown.
• Home flow targets come from Table 2 of Penn State Extension's Water System Planning: Estimating Water Needs (MWPS-14, 1992), which titles it minimum flow rates. Five bedroom and bathroom pairs have no listed value in Table 2.
• FHA and HUD loan rules follow 24 CFR 200.926d(f)(2)(i) (5 gpm for 4 hours on new homes). Existing homes follow HUD Handbook 4000.1 II.A.3.a.ii(N)(2) (3 gpm). These are published minimums, not a forecast of the well in a dry season.
• The estimate is flagged when half or more of the pumped water came from casing storage, when it exceeds 3 times the tested rate, when drawdown is under 5 ft, or when the readings are impossible. Readings are impossible when the pumping level is below the intake or the casing holds more drawn water than was pumped. A flagged estimate is left out of the Table 2 comparison.
• The pumping rate shows what the well supplied only if the pumping level had stopped falling by the end of the run. The calculator cannot check that from one reading. Water levels also move with the seasons: Penn State Extension notes that "a well that typically has 100 feet of water may have less than 20 feet of water storage during a drought."

What to do with the result

Compare the 4-hour column with 24 CFR 200.926d(f)(2)(i), which states 5.00 gpm over at least 4 hours for new homes. For existing homes, HUD Handbook 4000.1 II.A.3.a.ii(N)(2) states a continuous 3.00 gpm, and the 4-hour test is stated for new construction and shared wells. For new homes, only a test that pumped 5.00 gpm for 4 hours shows it, and an estimate does not. Then size your pump in the well pump head calculator.

Table 2 is titled "Minimum flow rates (GPM)", so it is a flow target and not a measured demand. Penn State Extension adds: "you may need to rely on water storage to meet peak demand periods." To size storage, take your day's projected use (400 gallons in the source's example) and compare it with the well's gpm × 120 minutes, the 2-hour peak. This page's own rough guide is the shortfall against the Table 2 flow times 120 minutes. A 42-gallon tank gives only about 8 gallons of drawdown between 20 psi and 40 psi. Check tank options in the pressure tank drawdown calculator, or add a 100-gallon-per-person holding tank (500 gallons for 5 people). If the estimate meets the flow but the test pumped less, run a longer test near that flow.

Check the margin above the pump intake, which is 50.0 ft in the default example. If the pumping level reached or passed the intake, recheck the levels and the intake depth before relying on the result. Penn State Extension says a low-water signal should stop the pump when the level reaches a critically low point. Check floor height losses in the water pressure elevation calculator and power bills in the well pump electricity cost calculator.

Sanitize and test the water after any well work. When you open a well cap, find your bleach dose in the well shock chlorination calculator. Check softener draw in the water softener size calculator, and test every 12 months for coliform bacteria, nitrates, total dissolved solids, and pH.

Frequently asked questions