FermUnits

FermUnits is a Pint-based Python library for units, measurement scales, and conversions used in brewing, winemaking, cider making, mead making, distilling, and related fermentation industries.

Project status: stable. Latest release: 1.0.0. FermUnits provides a tested public API covering brewing units and calculations plus shared solution-chemistry conversions for downstream fermentation-engineering work. The documented public API and semantics are intended to remain compatible throughout the 1.x series. Relationships awaiting stronger source verification remain explicitly provisional and are tracked in the project documentation.

FermUnits requires Python 3.11 or later and currently supports Pint >=0.25.3,<0.26.

Install the latest release from PyPI:

pip install ferm-units

For a development checkout:

uv sync --dev

The complete FermUnits public API, including the Pint functionality available through Q_, Quantity, UnitRegistry, and ureg, FermUnits-specific unit definitions, and all conversion functions, is documented in docs/API.md. The supported Python/Pint and downstream-contract policy is documented in docs/compatibility.md.

from fermunits import Q_, Quantity

cask: Quantity[int] = Q_(1, "firkin")
print(cask.to("liter"))

FermUnits re-exports Pint's Quantity and UnitRegistry types plus DimensionalityError as part of its public downstream contract. Applications should normally import quantity construction, typing, registry access, and dimension-error handling from FermUnits itself:

from fermunits import (
    DimensionalityError,
    Q_,
    Quantity,
    UnitRegistry,
    ureg,
)

Pint remains an implementation dependency of FermUnits and is installed transitively with FermUnits. A downstream package does not need to import or declare Pint solely to construct, annotate, convert, or perform ordinary unit operations on FermUnits quantities, annotate isolated registries, or catch dimensionally invalid conversions. A direct Pint dependency is only appropriate when that downstream package intentionally uses Pint-specific APIs that FermUnits does not expose.

The package-level ureg is a shared mutable registry and is the default choice for ordinary use. Use create_registry() only when deliberate isolation or independent registry mutation is required; quantities from different registries should not be mixed in arithmetic.

from fermunits import (
    gravity_points_to_sg,
    plato_to_sg,
    sg_to_gravity_points,
    sg_to_plato,
)

points = sg_to_gravity_points(1.050)
specific_gravity = gravity_points_to_sg(points)

plato = sg_to_plato(1.048)
estimated_sg = plato_to_sg(plato)

The SG-to-Plato polynomial remains provisional pending verification against authoritative ASBC extract tables or methods. The inverse function numerically inverts the same polynomial to preserve internal consistency.

The current numerical inversion interval is an implementation limit rather than an ASBC-approved scientific range.

These functions represent a wort-specific refractometer correction for unfermented wort only. They are not general conversions between the Brix and Plato scales, and the simple wort correction factor is not valid once alcohol is present in a fermenting or fermented sample.

The correction factor must be supplied explicitly. FermUnits does not assume a default wort correction factor. The convention used here is:

wort correction factor = apparent refractometer Brix / reference true Plato
corrected Plato = apparent refractometer Brix / wort correction factor

For example, paired readings of 12.48 apparent Brix and 12.0 reference Plato give a wort correction factor of 1.04.

from fermunits import (
    plato_to_wort_refractometer_brix,
    wort_refractometer_brix_to_plato,
)

plato = wort_refractometer_brix_to_plato(
    apparent_brix=12.48,
    wort_correction_factor=1.04,
)

apparent_brix = plato_to_wort_refractometer_brix(
    plato=12.0,
    wort_correction_factor=1.04,
)

FermUnits applies the documented scale-factor relationship between modern method-derived SRM and EBC color indices. The full analytical-method qualification remains provisional.

from fermunits import ebc_to_srm, srm_to_ebc

ebc = srm_to_ebc(10.0)
srm = ebc_to_srm(ebc)

Lovibond conversions are explicitly labeled as approximations because the older visual Lovibond scale is not equivalent to the modern spectrophotometric SRM and EBC scales.

from fermunits import (
    lovibond_to_srm_approx,
    srm_to_lovibond_approx,
)

srm = lovibond_to_srm_approx(10.0)
lovibond = srm_to_lovibond_approx(srm)

FermUnits implements the coordinated ASBC/EBC-style analytical relationship between absorbance at 275 nm and bitterness units.

from fermunits import (
    absorbance_275nm_to_bitterness_units,
    bitterness_units_to_absorbance_275nm,
)

bitterness_units = absorbance_275nm_to_bitterness_units(0.5)
absorbance = bitterness_units_to_absorbance_275nm(bitterness_units)

Bitterness units are operational analytical results. They are not represented as an exact concentration of iso-alpha-acids or as a direct measurement of perceived bitterness.

FermUnits does not currently provide a separate arithmetic conversion between IBU and EBU because those names refer to coordinated analytical methods rather than clearly distinct numerical scales.

from fermunits import (
    lintner_to_windisch_kolbach,
    windisch_kolbach_to_lintner,
)

windisch_kolbach = lintner_to_windisch_kolbach(60.0)
lintner = windisch_kolbach_to_lintner(windisch_kolbach)

The Lintner and Windisch-Kolbach relationship remains provisional pending verification against the original ASBC and EBC analytical methods.

The quantity-aware APIs keep physical CO2 mass concentration explicit for unit-aware downstream applications:

from fermunits import (
    Q_,
    co2_mass_concentration_to_volumes,
    co2_volumes_to_mass_concentration,
)

concentration = co2_volumes_to_mass_concentration(2.5)
kilograms_per_cubic_meter = concentration.to("kilogram / meter ** 3")
volumes = co2_mass_concentration_to_volumes(Q_(4.94, "gram / liter"))

The original scalar grams-per-liter APIs remain available for compatibility:

from fermunits import (
    co2_grams_per_liter_to_volumes,
    co2_volumes_to_grams_per_liter,
)

grams_per_liter = co2_volumes_to_grams_per_liter(2.5)
volumes = co2_grams_per_liter_to_volumes(grams_per_liter)

For these APIs, one volume of CO2 means one volume of CO2 gas at 273.15 K and 101.325 kPa per equal volume of beverage. Peer-reviewed analysis of the ASBC Beer-13 chart states that reference condition explicitly. FermUnits uses the sourced 506.07 mL/g volumes-to-weight factor, equivalent to approximately 1.976011 g/L per volume and consistent with independent CO2 density data at the same reference state. The direct volumes-to-mass-concentration relationship is therefore Verified.

The same factor is used in both directions to preserve round-trip consistency. This reference-state conversion is not a carbonation-equilibrium model: beer composition can affect CO2 solubility, so FermUnits does not encode a universal pressure/temperature chart or Henry coefficient for beer. Gauge versus absolute pressure, equilibrium modeling, gas blends, and draft-system balancing remain downstream engineering semantics.

FermUnits does not currently implement hydrometer temperature correction.

The provisional formula listed in the original project inventory was rejected because it omitted the hydrometer calibration temperature and produced physically implausible results.

A correction will not be added until an authoritative method or table can be implemented with:

• explicit sample temperature;
• explicit hydrometer calibration temperature;
• a defined temperature scale;
• supported temperature and specific-gravity ranges;
• a clearly identified sample matrix.

FermUnits also provides shared solution-chemistry conversions intended for water-treatment and other fermentation engineering applications. These APIs keep chemical semantics explicit rather than hiding them inside ambiguous unit labels.

Chemical-equivalent concentration uses FermUnits' separate equivalent dimension. Converting from amount concentration requires an explicit equivalence factor:

from fermunits import Q_, amount_concentration_to_equivalent_concentration

calcium = Q_(1.0, "millimole / liter")
charge_equivalents = amount_concentration_to_equivalent_concentration(
    calcium,
    equivalence_factor=2.0,
)

For conventional water-analysis reporting, FermUnits implements the relationship 50 mg/L as CaCO3 = 1 mEq/L. The as CaCO3 reporting basis remains application metadata; it is not encoded as though calcium carbonate were necessarily the dissolved analyte.

from fermunits import (
    Q_,
    caco3_basis_mass_concentration_to_equivalent_concentration,
)

alkalinity_as_caco3 = Q_(100.0, "milligram / liter")
alkalinity = caco3_basis_mass_concentration_to_equivalent_concentration(
    alkalinity_as_caco3
)

Mass concentration and mass fraction are not treated as interchangeable. A conversion such as mg/L to mg/kg requires explicit solution density:

from fermunits import Q_, mass_concentration_to_mass_fraction

concentration = Q_(100.0, "milligram / liter")
density = Q_(1.05, "kilogram / liter")
mass_fraction = mass_concentration_to_mass_fraction(concentration, density)

Likewise, conversion between mass concentration and amount concentration requires an explicit molar mass supplied as a Pint quantity:

from fermunits import Q_, mass_concentration_to_amount_concentration

sodium_chloride = Q_(58.44, "milligram / liter")
molar_mass = Q_(58.44, "gram / mole")
amount_concentration = mass_concentration_to_amount_concentration(
    sodium_chloride,
    molar_mass,
)

FermUnits preserves Pint's generic ppm unit, but canonical chemistry data should use an explicit ratio such as mg/kg or microgram / kilogram when the intended basis is mass fraction. FermUnits does not define a generic ppb alias.

FermUnits treats pH as a logarithmic semantic value rather than a Pint unit. PHValue provides a small non-Pint representation for the finite numeric scale value, while the public pH helpers convert only between that value and the dimensionless hydrogen-ion activity appearing in the IUPAC definition. They do not equate activity with hydrogen-ion concentration or infer an activity coefficient. Pint's pH spelling remains untouched because it is the standard prefixed-unit spelling for picohenry in the underlying registry.

from fermunits import PHValue, hydrogen_ion_activity_to_ph, ph_to_hydrogen_ion_activity

ph = PHValue(5.0)
activity = ph_to_hydrogen_ion_activity(ph)
restored_ph = hydrogen_ion_activity_to_ph(activity)

Reported bounds, ranges, nondetects, detection or quantitation limits, and measurement uncertainty remain downstream measurement/reporting semantics. FermUnits converts the underlying Pint quantities but does not choose how such a measurement should be resolved to a scalar.

• Pint remains the physical-unit engine.
• FermUnits adds fermentation-industry definitions and domain-specific APIs.
• Ambiguous names such as bare barrel are not defined.
• Existing Pint meanings are preserved when they are legitimate.
• Domain-qualified names distinguish conflicting industry meanings.
• Empirical scales and calculations are kept separate from physical units.
• Approximate and provisional formulas are labeled clearly.
• Input validation rejects nonfinite or physically invalid values, and validated conversions reject arithmetic results that overflow to nonfinite values.
• Every domain definition and calculation should have a documented source and tests.
• Scientific verification status is tracked separately from implementation status.
• Unsupported formulas are rejected rather than implemented merely because they appeared in an early project inventory.

Implemented physical units include:

• British brewery cask units, including current-use and historical large-cask measures;
• modern US beer barrel;
• Imperial beer barrel;
• pin cask;
• firkin;
• kilderkin;
• domain-qualified brewing hogshead;
• domain-qualified brewing puncheon, butt, and tun.

Pint's bare hogshead remains available with Pint's 63-US-liquid-gallon meaning. FermUnits intentionally does not define a generic wine_hogshead because legitimate wine-industry meanings vary by region.

Implemented brewing calculations include:

• specific gravity and gravity points;
• provisional specific gravity and degrees Plato conversion;
• explicit wort refractometer correction with a caller-supplied factor;
• SRM and EBC color-index conversion;
• approximate Lovibond and SRM conversion;
• analytical bitterness units from 275 nm absorbance;
• provisional Lintner and Windisch-Kolbach conversion;
• dissolved CO2 conversion between volumes and physical mass concentration, with scalar grams-per-liter compatibility APIs.

Not yet implemented:

• hydrometer temperature correction;
• generic Brix, Plato, and Balling scale conversion;
• recipe-estimation formulas such as Tinseth or Rager bitterness;
• calculations that require unverified assumptions or inaccessible source details.

Additional wine, distilling, sake, cider, biofuel, and fermentation-process definitions remain demand-driven. Their maintained references document regional, historical, legal, and technical meanings so new APIs can be added without speculative or ambiguous aliases when a downstream consumer actually needs them.

The architectural boundary between Pint, FermUnits, and downstream engineering applications is documented in DESIGN.md. Development priorities and milestone sequencing are tracked in ROADMAP.md.

The master source ledger, project-wide source status, and citation conventions are documented in docs/sources.md. The centralized audit of source status for relationships FermUnits already implements is docs/verification-status.md. Maintained domain inventories live under docs/reference/, and unresolved ASBC/EBC verification work is tracked in docs/asbc-verification.md.

FermUnits is distributed under the MIT License. See LICENSE.