Well Yield & Recovery Test Calculator

How it works

gal/ft = π (D/2)² × 7.48; recovery yield = recovered ft × gal/ft ÷ minutes; inflow = (pumped gal − drawdown × gal/ft) ÷ minutes; specific capacity = gpm ÷ drawdown; safe yield = 70% of estimate

A driller's "yield" is often just what the airlift produced on the day; a proper estimate comes from pumping the well down and watching it recover. Part of what the pump removed was simply water stored in the casing — 1.47 gallons per foot for a 6-inch bore — so the true aquifer inflow is what was pumped minus the storage emptied, spread over the test time. Recovery gives a second estimate: the feet regained per minute times gallons per foot. Dividing the pumping rate by the drawdown gives specific capacity, which projects how much the well could deliver if drawn down further, and a 70% safety factor covers seasonal water-table declines. Low-yield wells are perfectly workable with storage, because a household needs 300–400 gallons a day but only for a few hours.

Worked example

A 6″ well (1.47 gal/ft) with static level 40 ft is pumped at 10 gpm for an hour to 180 ft; 206 gal came from casing storage, so aquifer inflow was 6.6 gpm, and recovering 136 ft in 90 minutes confirms about 2.2 gpm of pure recovery — the yield is ≈ 6.6 gpm, 4.6 gpm safe. That is 6,600 gal/day, ample for four people (300 gal), but below the 8 gpm peak, so 400 gal of storage smooths showers and laundry.

Frequently asked questions