The miner's inch is a unit of flow in terms of volume per unit time, usually in relation to the flow of water. The definition of a miner's inch varies by location.
In hydraulic mining and some forms of placer mining, as well as ore dressing, a large and regular supply of water is needed. The miner's inch is a method of measuring the amount of flow a particular water supply system (such as a flume or sluice) is capable of supplying.
The miner’s inch was derived from the amount of water that would flow through the hole of a given area at a given pressure (for example, 4-6 inches of water, or 1-1.5 kPa). The word 'inch' actually refers to the area of the hole in 'square inches'.
Historically, the unit lacked a firm definition or equivalent measurement, and varied by location, leading to confusion within the mining industry. In 1905, its usage in California was standardized. Today, the standards are:
- 1/60 ft³/s (472 mL/s) New Zealand
- 1/50 ft³/s (566 mL/s) southern California, Idaho, Kansas, Nebraska, New Mexico, North Dakota, South Dakota, Utah, Washington
- 1/40 ft³/s (708 mL/s) Arizona, northern California, Montana, Nevada, Oregon
- 1/38 ft³/s (745 mL/s) Colorado
- 1/36 ft³/s (787 mL/s) British Columbia
State regulations sometimes forbid the use of the unit without its being associated with a definition in the same document.