Conversion from Cubic inches per minute to US Gallons per minute

Information about the Cubic inch per minute (in³/min)

The cubic inch per minute is a unit of volumetric flow rate equal to one cubic inch passing a point every minute. Its symbol is in³/min. In the machine shop it is better known by its initials: MRR, the material removal rate, the single figure that says how fast a cutting process is turning solid metal into chips.

The calculation behind it is simple multiplication. Depth of cut times width of cut times feed rate gives the volume removed per minute, so a 0.2-inch depth at 1 inch wide with a feed of 30 inches per minute removes 6 cubic inches per minute. Every variable a machinist can change appears in that product, which is why it is the number used to compare one strategy against another.

Material decides what is achievable. Roughing aluminium on a rigid machine can exceed 100 cubic inches per minute, mild steel manages perhaps 10 to 20, stainless steel less, and nickel alloys used in turbine parts only a few. The limit is set by spindle power, by the rigidity of the setup and by the heat the tool can survive, in whichever order runs out first.

Spindle power connects directly to it. Each material has a specific cutting energy, roughly 1 horsepower per cubic inch per minute for steel and a third of that for aluminium, so a 20-horsepower spindle can sustain about 20 cubic inches per minute in steel. A quoted removal rate that exceeds the machine's power is arithmetic that the machine will settle in its own way.

Beyond cutting, the unit describes small imperial flows generally. Hydraulic pilot lines, lubrication systems, fuel injection test benches and adhesive dispensers on American equipment are all rated in cubic inches per minute, where the metric world would say millilitres per minute.

For scale, 61 cubic inches per minute is about a litre per minute, and 231 cubic inches per minute is exactly one US gallon per minute — the gallon being defined as 231 cubic inches. That exact relation is why hydraulic engineers move between the two units without a conversion table.

One cubic inch per minute equals about 16.39 cubic centimetres per minute, about 0.01667 cubic inches per second, or about 0.004329 US gallons per minute.

Information about the US Gallon per minute (gal US/min)

The US gallon per minute is a unit of volumetric flow rate equal to one United States gallon passing a point every minute. Its symbol is gal US/min, and it is universally abbreviated GPM. It is the single most widely used flow unit in North America, appearing on pumps, wells, plumbing fixtures, irrigation designs and fire codes alike.

Plumbing fixtures are rated in it by federal standard. A showerhead in the United States is limited to 2.5 gallons per minute, a kitchen faucet to 2.2 and a lavatory faucet to 1.5, with stricter figures in some states. Because the flow rate multiplied by the minutes of use gives both the water and the energy to heat it, these limits do more for household energy than their modest numbers suggest.

Pumps are sold on curves drawn against this unit. A shallow-well jet pump delivers 5 to 15 gallons per minute, a submersible well pump 10 to 30, a swimming-pool pump 40 to 80, and a municipal pump thousands. Reading a curve at the required flow gives the head available and the horsepower drawn, which is how a pump is matched to a system.

Irrigation design is built on it. A sprinkler head is rated in gallons per minute, a zone is the sum of its heads, and the zone must not exceed what the supply can deliver — which is itself a figure in gallons per minute measured at the point of connection. Splitting a system into zones is entirely an exercise in staying within that number.

Fire protection uses it as its principal unit. Hydrant flow tests, standpipe requirements, sprinkler design densities and pumper truck ratings are all stated in gallons per minute, from a 1.5-inch hose line at 100 to an engine's 1,500 or more, and the national fire codes are written throughout in these terms.

For scale, one US gallon per minute is about 3.79 litres per minute, and 4.4 gallons per minute is about a cubic metre per hour. The imperial gallon is about 20 per cent larger, so a British pump described as 10 gallons per minute delivers about 12 US gallons per minute — a distinction worth checking on any imported equipment.

One US gallon per minute equals about 3.785 litres per minute, about 0.2271 cubic metres per hour, or about 0.8327 imperial gallons per minute.