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BS ISO 80000 — Quantities and Units Standard Series (SI Fundamentals)
Quantities and units (BS ISO 80000, the UK edition of the ISO 80000 series)
📐 BS ISO 80000 is the British Standards Institution’s adoption of the ISO 80000 series “Quantities and units” — the authoritative baseline standard for expressing quantities and units within the International System of Units (SI). It defines the names, symbols, definitions, dimensions, and writing rules for unit symbols of physical quantities, and gives discipline-specific definitions and symbol conventions across physics and engineering. It is the “common grammar” for expressing quantities in science, engineering, metrology, and trade documents. In the UK the series is maintained by Technical Committee SS/7 (General metrology, quantities, units and symbols) and replaces and consolidates the old ISO 31 series and ISO 1000.
💡 Correct expression of quantities is the “spelling rule” of engineering documents. BS ISO 80000 ensures that symbols and units in a technical report, a drawing, or a calibration certificate are read consistently worldwide, preventing serious engineering failures caused by wrong symbols or wrongly written units.
1. Series Overview and Rules for Expressing Quantities 📌
Part 1 (General) is the umbrella part and defines the basic principles for expressing quantities and units, mainly superseding BS ISO 1000:1992 and BS ISO 31-0:1992. Its core content includes:
- Distinction between quantity and numerical value: a value = numerical value × unit; the quantity itself is independent of the unit chosen, but its numerical value changes with the unit.
- Symbols for quantities and units: symbols for quantities are generally italic (e.g. m, t, V), while unit symbols are upright (e.g. kg, s, V); the two must not be mixed.
- Dimensions: the dimension of a derived quantity expressed as powers of base quantities (e.g. force has dimension [M][L][T]−2).
- SI and permitted non-SI units: SI units, SI prefixes, units permitted for use with SI (e.g. min, h, L, t), and units not recommended for use.
Part 1 is kept current through revisions — including the October 2011 ISO technical corrigendum (Example 2 in Clause 7.4 was amended) — reflecting the continuous evolution of quantity and unit standards alongside international metrology developments such as the 2019 redefinition of the SI base units.
⚠️ Confusion between quantities and units is a systemic risk: writing a quantity symbol upright, a unit symbol italic, or mixing upper/lower case in one document (e.g. m vs M, Pa vs pa) creates interpretation ambiguity. BS ISO 80000 establishes explicit conventions for these writing rules.
2. Series Composition and Key Points 🔧
| Part | Subject | Key Content | Readability |
|---|---|---|---|
| BS ISO 80000-1 | General | General rules: values, symbols, dimensions, SI units and permitted non-SI units | Readable |
| BS ISO 80000-2 | Mathematical signs and symbols | Mathematical signs used in natural science and technology (public-domain knowledge) | Scanned |
| BS ISO 80000-3 | Space and time | Quantities of space and time (length, area, volume, time, velocity, etc.) (public-domain knowledge) | Scanned |
| BS ISO 80000-5 | Thermodynamics | Thermodynamic quantities (temperature, heat, entropy, enthalpy, etc.) (public-domain knowledge) | Scanned |
| BS ISO 80000-7 | Light | Light and optical quantities (public-domain knowledge) | Scanned |
| BS ISO 80000-10 | Atomic and nuclear physics | Atomic and nuclear physics quantities (supersedes ISO 31-10, 31-9) | Readable |
| BS ISO 80000-11 | Characteristic numbers | Characteristic numbers (Reynolds, Nusselt, Prandtl, etc.) | Readable |
| BS ISO 80000-12 | Solid state physics | Solid-state physics quantities (public-domain knowledge) | Scanned |
🚨 Scanned-version note: Parts 2, 3, 5, 7, and 12 are derived from scanned originals. The descriptions in this article are written from public-domain knowledge; the official BSI published version takes precedence for symbol tables and clauses.
3. Key Technical Requirements: Discipline Symbols 🔥
Part 10 (atomic and nuclear physics) defines the quantities and symbols for atomic mass, nuclides, radioactivity, and radiation dose, superseding ISO 31-10 and ISO 31-9. Part 11 (characteristic numbers) is an important chapter for engineering and heat transfer, defining dimensionless characteristic numbers for momentum transfer, heat transfer, and mass transfer in binary mixtures, for example:
- Momentum transfer: Reynolds number (Re), Euler number, Froude number, Mach number, etc.;
- Heat transfer: Nusselt number (Nu), Prandtl number (Pr), Grashof number, Péclet number, etc.;
- Mass transfer: Schmidt number, Lewis number, Sherwood number, etc.
These characteristic numbers are widely used in fluid mechanics, heat transfer, and chemical engineering; the consistency of their symbols and definitions directly affects the comparability of experimental data and empirical correlations. Parts 3, 5, 7, and 12 cover the base quantities of space and time, thermodynamics, light, and condensed-matter physics respectively.
✅ For engineering practice, the “value” of a characteristic number often matters more than its dimension: the same Reynolds number means similar flow regimes, which is why BS ISO 80000-11 is the basis for similarity analysis and model testing in heat-exchanger, pipe, and pump design.
4. Engineering Practice 🛠️
- 🔍 Reports and drawings: values, symbols, and units in technical documents should follow this series, avoiding the same symbol representing different physical quantities in different sections.
- 🔧 Metrology and calibration: units and expressions of values in calibration certificates and measurement-uncertainty reports should follow this series so that measurement results are traceable and comparable.
- 📊 Conversion and software: unit-conversion programs and the input/output unit conventions of numerical calculation software should follow this series, preventing order-of-magnitude errors from unit-system confusion.
- 📋 Education and standard references: as the expression baseline of the SI system, this series is normatively referenced by many product and test-method standards to specify how test results are expressed.
5. Frequently Asked Questions ❓
❓ Q: What is the relationship between BS ISO 80000 and ISO 31?
A: BS ISO 80000 succeeds the ISO 31 series; the UK foreword states that it supersedes the corresponding BS ISO 31-x:1992 parts. The new series unifies the expression of quantities and units more closely with the SI system.
A: BS ISO 80000 succeeds the ISO 31 series; the UK foreword states that it supersedes the corresponding BS ISO 31-x:1992 parts. The new series unifies the expression of quantities and units more closely with the SI system.
❓ Q: What is the typographical difference between quantity symbols and unit symbols?
A: Quantity symbols are generally italic (e.g. mass m, temperature T), while unit symbols are upright (e.g. kg, K); vectors and tensors may use bold or specific markings. This is an explicit convention of the series.
A: Quantity symbols are generally italic (e.g. mass m, temperature T), while unit symbols are upright (e.g. kg, K); vectors and tensors may use bold or specific markings. This is an explicit convention of the series.
❓ Q: Why do characteristic numbers such as the Reynolds number have no unit?
A: Characteristic numbers are dimensionless quantities formed by specific combinations of dimensional quantities. Non-dimensionalisation allows flow and heat-transfer phenomena to be classified by similarity, which is precisely why model testing and empirical correlations work.
A: Characteristic numbers are dimensionless quantities formed by specific combinations of dimensional quantities. Non-dimensionalisation allows flow and heat-transfer phenomena to be classified by similarity, which is precisely why model testing and empirical correlations work.
🔍 Q: Does this series affect the redefinition of SI base units after 2019?
A: BS ISO 80000 mainly governs the expression rules of quantities and units rather than the SI definition itself; its revisions (such as the 2011 corrigendum to Part 1) reflect the requirement to stay in step with developments in metrology.
A: BS ISO 80000 mainly governs the expression rules of quantities and units rather than the SI definition itself; its revisions (such as the 2011 corrigendum to Part 1) reflect the requirement to stay in step with developments in metrology.