How do you convert Gallon (US) to Cubic foot?

Multiply the value in Gallon (US) by the conversion factor 0.13368055555555555 to obtain the result in Cubic foot.

Convert Gallon (US) to Cubic foot (gal to ft³)

Reference ID: EC-VOLU-GAFT | EngConverter Calculation Guide

Convert From (Gallon (US)):

gal

Result To (Cubic foot):

ft³

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In process engineering, hydraulics, and piping design, converting volumetric measurements between different systems of units is a daily necessity. Two of the most common units of volume used in United States industrial applications are the US Gallon (gal) and the Cubic Foot (ft³). Understanding the precise mathematical relationship and physical definitions of these units is critical for maintaining accuracy in mass balances, fluid dynamics calculations, and equipment specifications.

Physical Definitions and Historical Origins

The US Gallon is historically derived from the British "wine gallon" (standardized in 1707 during the reign of Queen Anne), which was defined as exactly 231 cubic inches. Under the Mendenhall Order of 1893, the United States formally defined its customary units in terms of metric standards. Today, the US liquid gallon is defined as exactly 3.785411784 liters (or \$ 3.785411784 \\times 10^{-3} \\text{ m}^3 \$).

The Cubic Foot is a coherent unit of volume within both the US Customary and Imperial measurement systems. It is defined as the volume of a cube with sides exactly one foot (0.3048 meters) in length. Consequently, one cubic foot is exactly equal to \$ 0.3048^3 \\text{ m}^3 = 0.028316846592 \\text{ m}^3 \$.

To establish the exact conversion factor between these two units, we can use their shared relationship to the cubic inch (\$ \\text{in}^3 \$):

\$ 1 \\text{ US Gallon} = 231 \\text{ in}^3 \$
\$ 1 \\text{ Foot} = 12 \\text{ inches} \\implies 1 \\text{ ft}^3 = 12^3 \\text{ in}^3 = 1728 \\text{ in}^3 \$

Dividing the volume of a gallon by the volume of a cubic foot yields the exact conversion factor:

\$ \\text{Factor} = \\frac{231}{1728} = \\frac{77}{576} \\approx 0.13368055555555555 \\text{ ft}^3/\\text{gal} \$

Engineering Applications & Technical Considerations

In industrial process plants, fluid transport systems, and municipal water treatment facilities, engineers frequently transition between gallons and cubic feet. Key applications include:

Piping and Hydraulics: Volumetric flow rates are often measured in Gallons Per Minute (GPM) for liquid pumps, but modeled in Cubic Feet per Second (CFS) or Cubic Feet per Minute (CFM) for hydraulic calculations. Converting GPM to CFS requires the application of this fundamental ratio: \$ 1 \\text{ GPM} \\approx 0.002228 \\text{ CFS} \$.
Vessel and Tank Sizing: While commercial storage tanks are purchased and inventoried in gallons, structural engineers and piping designers require the physical dimensions in cubic feet to calculate spatial footprints, structural loading, and foundation requirements.
Instrumentation and Control: Flowmeters (such as magnetic, vortex, or differential pressure meters) must be calibrated to the correct engineering units. A mismatch in the conversion factor can lead to systematic errors in DCS (Distributed Control System) integration.

Critical Pitfalls to Avoid:

Temperature and Pressure Dependencies: Volume is a temperature-dependent property. A gallon of water at \$ 40^\\circ\\text{F} \$ has a different mass than a gallon at \$ 180^\\circ\\text{F} \$ due to density changes. When performing high-precision mass balance calculations, engineers must convert volumetric flow to mass flow (e.g., using density at operating conditions) rather than relying solely on nominal volumetric conversions.
US vs. Imperial Gallons: A common and costly error in international projects is confusing the US gallon (\$ 231 \\text{ in}^3 \$) with the Imperial (UK) gallon (\$ 277.42 \\text{ in}^3 \\approx 4.54609 \\text{ L} \$). The Imperial gallon is approximately 20% larger than the US gallon.
Rounding Standards: In custody transfer and fiscal metering, rounding the conversion factor to \$ 0.134 \$ instead of using the high-precision value \$ 0.13368056 \$ can introduce a \$ 0.24\\% \$ systematic error. Over millions of barrels of product, this translates to significant financial discrepancies.

Gallon (US) to Cubic foot Conversion Reference Table

Gallon (US) (gal)	Cubic foot (ft³)
0.1	0.0134
0.5	0.0668
1.0	0.1337
2.0	0.2674
5.0	0.6684
10.0	1.3368
20.0	2.6736
50.0	6.684
100.0	13.3681
500.0	66.8403
1000.0	133.6806

To convert a volumetric measurement from US Gallons (gal) to Cubic Feet (ft³), multiply the volume by the exact conversion factor \$ 0.13368055555555555 \$ (or the fraction \$ \\frac{77}{576} \$).

Example: Convert 10 US Gallons to Cubic Feet.

\\$\$ V_{\\text{ft}^3} = 10 \\text{ gal} \\times 0.13368055555555555 \\frac{\\text{ft}^3}{\\text{gal}} \\$\$\\$\$ V_{\\text{ft}^3} = 1.3368055555555555 \\text{ ft}^3 \\$\$

For standard engineering design documents, this value is typically rounded to four significant figures:

\\$\$ V_{\\text{ft}^3} \\approx 1.337 \\text{ ft}^3 \\$\$

In gas processing, volumes are highly dependent on temperature and pressure. While liquids are relatively incompressible, gases are typically measured in Standard Cubic Feet (SCF) at defined reference conditions (e.g., \$ 60^\\circ\\text{F} \$ and \$ 14.696 \\text{ psia} \$). Liquefied gases (like LPG or LNG) are stored as liquids and measured in gallons. Converting liquefied gallons to gaseous cubic feet requires using the fluid's liquid density, molecular weight, and the Ideal Gas Law, rather than the simple geometric conversion factor of \$ 0.13368 \$.

The factor \$ 7.48051948 \$ (often rounded to \$ 7.48 \$) is the reciprocal of \$ 0.13368056 \$. It represents the number of US gallons in one cubic foot (\$ 1 \\text{ ft}^3 \\approx 7.48052 \\text{ gal} \$). Engineers use \$ 7.48 \$ when converting from cubic feet to gallons (multiplication), whereas they use \$ 0.13368 \$ when converting from gallons to cubic feet.

"On fait la science avec des faits, comme on fait une maison avec des pierres ; mais une accumulation de faits n'est pas plus une science qu'un tas de pierres n'est une maison."
"Science is built up of facts, as a house is built of stones; but an accumulation of facts is no more a science than a heap of stones is a house."
— Henri Poincaré (French Mathematician, Theoretical Physicist & Mining Engineer)