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ASME Guide SI-1-1982 — Orientation and Guide for Use of SI (Metric) Units
📏 ASME Guide SI-1, the Orientation and Guide for Use of SI (Metric) Units, was prepared by the ASME Board on Metrication; the Ninth Edition was published on March 24, 1982. Under ASME’s “dual capability” policy, the Guide is an essential tool for the mechanical engineering community: it helps engineers accustomed to U.S. Customary units correctly understand, convert, and use SI units, and it establishes uniform rules for expressing SI units in ASME codes, standards, and publications. Its sixteen sections cover background and policy, the history of the International System, base units, supplementary units, prefixes, coherence of derived units, rules for SI use in ASME publications, conversion and rounding, dimensioning, units outside the International System, temporarily accepted units, units not to be used in ASME documents, SI units for ASME use, and methods of introducing SI into publications and tables.
💡 Policy background: as early as 1970 ASME anticipated that SI would displace U.S. Customary usage in many fields, while recognizing that both systems would coexist for the foreseeable future. ASME therefore set out to build “dual capability” in both systems among engineers, and required that SI units be included (in addition to any other units) in ASME papers and in revised, reaffirmed, and new engineering standards as of July 1, 1974.
1. SI Base Units and Prefixes 📌
SI is built on seven base units, complemented by two supplementary units (radian and steradian). The Guide gives precise definitions and symbol rules: unit symbols are printed in upright (Roman) type, generally lowercase (symbols named after persons use an initial capital), plurals take no added “s,” and no period is used at the end of a symbol. Prefixes combine with units to form convenient decimal multiples and submultiples for engineering use.
| Base Unit | Symbol | Quantity | Prefix Examples |
|---|---|---|---|
| metre / meter | m | Length | km, mm, μm, nm |
| kilogram | kg | Mass | Mg, g, mg |
| second | s | Time | ms, μs, ns |
| ampere | A | Electric current | kA, mA |
| kelvin | K | Thermodynamic temperature | Prefixes generally not used with K |
| mole | mol | Amount of substance | mmol, μmol |
| candela | cd | Luminous intensity | — |
⚠️ Common symbol errors: writing “sec” for the second (the symbol is s), capitalizing the prefix “k” (it should be lowercase k, so km not Km), or misusing M versus m (uppercase M means mega, lowercase m means milli). Also note that degrees Celsius (°C) is not an SI base unit, although it is a named derived unit for temperature.
2. Derived Units, Conversion, and Rounding 🔧
Derived units are formed from base units by multiplication and division. The Guide lists named derived units (newton N, pascal Pa, joule J, watt W, hertz Hz, degree Celsius °C) and derived units without special names (e.g., m/s, kg/m³). For conversion, the Guide stresses a complete “convert-and-round” discipline: converted values should retain a limited number of significant figures consistent with the precision of the original value, and rounding conventions should be agreed in advance in engineering specifications to avoid false precision.
| Derived Unit | Symbol | In Base Units | Typical Mechanical Application |
|---|---|---|---|
| newton | N | kg·m/s² | Force, load |
| pascal | Pa | N/m² | Pressure (MPa common in engineering) |
| joule | J | N·m | Work, energy |
| watt | W | J/s | Power |
| hertz | Hz | 1/s | Frequency, rotational-speed expression |
🚨 The most common conversion trap is confusing “exact unit conversion” with “engineering conversion.” 1 in = 25.4 mm is exact, but not every engineering dimension needs four decimal places. Choose the number of significant figures according to tolerance and manufacturing precision, and follow the Guide’s rounding convention consistently, or errors will accumulate across multi-part fits.
3. Rules for SI Use in ASME Publications 📖
The Guide sets uniform rules for SI expression in ASME codes, standards, papers, and handbooks. Core principles include a hierarchy of preferred and acceptable units; the manner of presenting SI alongside U.S. Customary units when revising standards; how to introduce SI in tables; dimensioning rules (single-unit dimensioning, tolerance expression); and a list of units not to be used in ASME documents. The Guide also identifies units accepted for use with SI (minute, hour, day, degree, litre, tonne), units to be used for a limited time (e.g., bar, ångström, are, hectare), and CGS units with special names to be phased out (dyne, erg, gauss, oersted).
- Unit symbols: upright type; a space or centered dot separates compound units (e.g., N·m); the slash denotes division (m/s).
- Spacing between value and unit: leave a space between the numeric value and the unit symbol (e.g., 25 mm, not 25mm).
- Tables: give the SI column first with U.S. Customary equivalents alongside, or state the conversion at the table head.
- Standard body text: when revising standards, present SI as the primary unit or list both, per ASME policy and industry needs.
✅ These rules may seem trivial, but they are the key to making “dual capability” work: consistent symbols, spacing, capitalization, and rounding conventions allow documents written by engineers of different manufacturers and countries to be read directly, avoiding engineering errors caused by ambiguous unit expression.
4. Engineering Practice Essentials 🛠️
- 🔍 Build a conversion table: project teams should prepare SI equivalents for common quantities (length, force, pressure, stress, power, flow) in advance, noting exact versus engineering conversion factors.
- 📊 Standards revision: when revising standards or procurement documents, present SI and U.S. Customary units together and state which is the governing system and which is for reference.
- 📋 Drawing dimensioning: follow the Guide’s dimensioning rules with a single unit system per drawing; do not intermix units in tolerances and datum expressions.
- 🔄 Training: include SI fundamentals (prefixes, symbols, conversion and rounding) in engineer orientation and continuing education to reduce unit errors.
⚠️ This Guide was issued decades ago, and SI rules have since evolved (e.g., the ISO 80000 series). Apply the framework and principles of the Guide, but verify specific lists against current international standards and current ASME policy.
5. Frequently Asked Questions ❓
❓ Q: Why does ASME require “dual units” instead of full conversion to SI?
A: Because U.S. engineering and manufacturing continued to use U.S. Customary units extensively for the foreseeable future, while international business required SI. The “dual capability” policy lets the profession master both systems and transition smoothly, avoiding the disruption and cost of an abrupt changeover.
A: Because U.S. engineering and manufacturing continued to use U.S. Customary units extensively for the foreseeable future, while international business required SI. The “dual capability” policy lets the profession master both systems and transition smoothly, avoiding the disruption and cost of an abrupt changeover.
❓ Q: How many decimal places should a conversion keep?
A: It depends on the tolerance and manufacturing precision of the original dimension. Coarse-tolerance dimensions need few significant figures; precision dimensions need more. The key is consistent rounding within a document and no error beyond the original tolerance.
A: It depends on the tolerance and manufacturing precision of the original dimension. Coarse-tolerance dimensions need few significant figures; precision dimensions need more. The key is consistent rounding within a document and no error beyond the original tolerance.
❓ Q: Which units are prohibited in ASME documents?
A: Chiefly CGS units with special names (dyne, erg, gauss, oersted, etc.) and older non-SI engineering units (e.g., kgf, kgf/cm², to be replaced by N and Pa); see Section 12 and the associated tables of the Guide.
A: Chiefly CGS units with special names (dyne, erg, gauss, oersted, etc.) and older non-SI engineering units (e.g., kgf, kgf/cm², to be replaced by N and Pa); see Section 12 and the associated tables of the Guide.
🔍 Q: When SI and U.S. Customary are presented together, which governs?
A: The governing system is set by the standard or contract. The Guide supports a gradual direction toward SI as the primary unit but allows industry to determine the governing system; either way, the other system must be converted accurately with the rules stated to prevent mixing.
A: The governing system is set by the standard or contract. The Guide supports a gradual direction toward SI as the primary unit but allows industry to determine the governing system; either way, the other system must be converted accurately with the rules stated to prevent mixing.
