Controlled dimensional conversion
Surface-Parameter Unit Converter
Convert an already determined or specified height-parameter value between length units while preserving its parameter identifier. This worksheet does not manufacture Rz, a grade number or a process recommendation from Ra.
Exact length-unit factorsNo Ra ↔ Rz inferenceNo grade/process lookup
Decision boundary. A unit conversion preserves one physical length; it does not convert one surface parameter into another. Parameter evaluation, filters, nesting indices, section lengths, measurement uncertainty and conformity remain governed by the referenced measurement or specification record.
4. Unit-conversion result
Parameter preserved
Nanometres
Micrometres
Millimetres
Microinches
Inches
Method and applicability
Let L be the recorded physical length in µm.
L = input × unit factor
1 nm = 0.001 µm | 1 mm = 1,000 µm
1 µin = 0.0254 µm exactly | 1 in = 25,400 µm exactly
L = input × unit factor
1 nm = 0.001 µm | 1 mm = 1,000 µm
1 µin = 0.0254 µm exactly | 1 in = 25,400 µm exactly
What is calculated
- Dimensional normalization to µm.
- Equivalent nm, mm, µin and inch values.
- The parameter label is preserved unchanged.
What is not calculated
- Any different surface parameter.
- Historical N1–N12 grades or process capability.
- Filters, uncertainty or conformance.
Input requirements
- One non-negative height value.
- One explicit original unit.
- A controlled source and operator record.
Why Ra cannot be converted to Rz. The centered profiles [−1, +1] and [−2, 0, 0, +2] have the same mean absolute deviation, 1. Their ranges are 2 and 4. One average-height value therefore cannot uniquely determine an extreme-height parameter. Multiplying every Ra by four is an estimate, not a dimensional conversion or ISO parameter evaluation.
Sources and standards boundary
- ISO 21920-1:2021, edition 1, published 2021-12: rules for indicating profile-method surface texture in technical product documentation. As of 2026-07-15 it is at stage 90.92 (to be revised); the published edition remains ISO 21920-1:2021.
- ISO 1302:2002, edition 4, was withdrawn on 2021-12-20 and revised by ISO 21920-1:2021. Its official scope concerns indication by graphical symbols and text; it is not authority for a universal Ra-to-Rz multiplier.
- ISO 4287:1997, including its corrigenda/amendment lifecycle, was withdrawn on 2021-12-20 and revised by ISO 21920-2:2021.
- PTB surface-metrology standards overview confirms that ISO 21920-1 replaces ISO 1302, ISO 21920-2 replaces the parameter-definition standards, and ISO 21920-3 supplies specification operators.
- NIST Special Publication 811, Appendix B.8: 1 microinch = 2.54 × 10−2 µm; the factor is marked exact. SI prefixes give the nm/µm/mm factors.
- ISO 468:1982, “Surface roughness — Parameters, their values and general rules for specifying requirements,” was withdrawn on 1998-04-10. The removed N1–N12/Rz/process table cannot be presented as current ISO 1302 data; its exact provenance and any contractual legacy use are NEEDS_LICENSED_SOURCE.
Classification: this is an exact general length-unit conversion, not an “ISO formula calculator.” Exact ISO parameter definitions, operator tables, legacy grade-number clauses and process-capability ranges are not reproduced or guessed.
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