Imperial units

Use a catalogue directly in formulas. No registry setup is needed:

>>> from unitx import Quantity, Unit
>>> from unitx.imperial import imperial
>>> from unitx.si import si, SI
>>> (3 * imperial.ft).value_in(si.m)
Fraction(1143, 1250)
>>> imperial.parse('ft/s') == imperial.ft / si.s
True
>>> SI.c.value_in(imperial.parse('ft/s'))
Fraction(374740572500, 381)

The submodule exports the immutable imperial unit namespace, following from unitx.si import si. Its definitions live in unitx/imperial/units.py. Physical constants remain in SI; convert them explicitly when needed.

Imperial conventions

All scales below are exact. Decimal input in the definitions is not a binary floating-point approximation.

Attribute	Meaning
inch	International inch, 0.0254 m; parser symbol in
ft, yd, mi	International foot (12 inches), yard (3 feet), mile (1760 yards)
lb, oz, st, ton	Avoirdupois pound (0.45359237 kg), ounce (1/16 lb), stone (14 lb), long ton (2240 lb)
gal, qt, pt, fl_oz	Imperial gallon (4.54609 L), quart (1/4 gal), pint (1/8 gal), fluid ounce (1/160 gal)
lbf, psi	Pound-force (lb × 9.80665 m/s²), pound-force per square inch

imperial uses British volume and long-ton conventions. us contains the US-specific survey, liquid/dry volume and short-ton definitions; imperial_historical contains stated historical English conventions. Import them from unitx.imperial. Temperature intervals and coordinates are available as imperial.degR and imperial.degF. All three catalogues disable prefixes for new definitions; explicitly included SI units retain their prefix permissions. The exact foot convention follows NIST's international foot definition; conversion conventions are also tabulated in NIST SP 811.

imperial.parse() accepts the catalogue's units and their names, plus explicitly shared SI definitions: metre, second, gram, kelvin, newton, pascal, litre and hour, including their permitted prefixes and aliases. Products, exact rational powers, [name] references and parser precedence work as in Unit(...). It does not consult private registrations in the default table.

Use Unit objects or imperial.parse() results as conversion targets. A bare string still uses the default environment. In particular, Unit('gal') denotes SI galileo (acceleration); imperial.parse('gal') denotes Imperial gallon. Physical constants remain available from SI and can be explicitly converted to these units. This catalogue does not provide a second constant namespace.

US and historical conventions

>>> from unitx.imperial import imperial_historical, us
>>> (1 * us.us_survey_foot).value_in(si.m)
Fraction(1200, 3937)
>>> (1 * us.us_cable).value_in(si.m)
Fraction(27432, 125)
>>> (1 * us.us_gal).value_in(si.L)
Fraction(473176473, 125000000)
>>> us.parse('[inch of water]') == us.inH2O
True
>>> (1 * imperial_historical.ramsden_chain).value_in(imperial.ft)
100

The legacy survey foot is exactly 1200/3937 m; the survey mile is 5280 such feet. It is distinct from the international foot. us.us_cable instead uses 720 international feet, exactly 219.456 m, following the modern convention in NIST's revised conversion table. section and township remain nominal areas based on the international mile; they do not describe the measured area of an individual surveyed parcel.

imperial.btu_IT is exactly 1055.05585262 J. The thermochemical btu_th is exactly (453.59237 * 5/9) thermochemical calories, with 1 calorie = 4.184 J. The code derives it from the pound, Rankine interval and si.cal, retaining the rational value instead of a truncated decimal. These definitions agree with NIST REFPROP's Btu conventions.

us.circular_mil is the area of a circle one mil in diameter: pi/4 * imperial.th**2. Its factor retains symbolic pi. us.inHg and us.inH2O use conventional column densities of 13595.1 and 1000 kg/m³, respectively, and standard gravity 9.80665 m/s². Their exact scales under these conventions are 3386.388640341 and 249.08891 Pa. They represent fixed conventions, rather than temperature-dependent fluid properties; see NIST's pressure conversion table. The conventional mercury scale is distinct from converting a column through the catalogue's si.Torr scale.

The water-column attribute is spelled inH2O in Python. Its parser symbol is inH₂O with a subscript two; the ASCII name inch of water is also accepted. ASCII digits in unit tokens remain reserved for parser exponents, so the text inH2O is not a registered spelling.

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