Engineering Reference

Ra to Rz Calculator

Convert Ra between µm and µin, look up a legacy N grade, and compare explicitly assumed Rz and Rq scenarios.

Page last changed 2026-10-19 · based on n/a — standard engineering relationships, no single governing revision

Quick Answer

Ra 1.6 µm ≈ 63 µin = legacy N7. Unit conversion is exact before rounding. Ra alone cannot determine Rz or Rq; the outputs below illustrate stated profile assumptions.

Roughness Parameter Converter

The Formulas Used

Ra (µin) = Ra (µm) / 0.0254. The N grade uses the discrete lookup below. Rz = 5 × Ra is an assumed scenario, not a general conversion. Rq = π × Ra / (2√2) applies to a sinusoidal profile only.

Ra, Rz and RMS

Ra is the mean absolute profile deviation; Rq (RMS roughness) is the root mean square deviation. Rz is a height parameter whose definition depends on the measurement standard. The same Ra can occur on different profiles with different Rz and Rq.

The 5:1 Rz output and sine-profile Rq output are examples of chosen assumptions. For inspection, measure the parameter required by the drawing and use its specified filters and evaluation length. See a worked profile example.

Legacy N grades and Ra

N1–N12 is a legacy shorthand for the Ra values below. Each successive value is approximately twice the previous one. Use the published rounded values when reading a legacy N-grade callout.

Legacy roughness grades — Ra values
GradeRa (µm)Ra (µin, rounded)
N10.0250.984
N20.051.97
N30.13.94
N40.27.87
N50.415.7
N60.831.5
N71.663
N83.2126
N96.3248
N1012.5492
N1125984
N12501.97e+03

Source: MISUMI’s Ra comparison-specimen reference table, checked 2026-10-16. A value below 0.025 µm is finer than N1; a value above 50 µm is coarser than N12. For an intermediate input, the calculator shows the next nominal grade value at or above it, explicitly as a ceiling.

ISO 4287:1997 is a withdrawn legacy parameter definition. ISO’s catalogue identifies ISO 21920-2:2021 as its replacement. Follow the edition and measurement conditions stated on the drawing.

What was checked

2026-10-16: corrected the reversed N-grade calculation and checked all twelve published N/Ra pairs. For example, 12.5–50 µm corresponds to N10–N12, and 0.4–6.3 µm to N5–N9. This check does not establish that a particular machining process will achieve a stated finish. Read the recorded check scope or report a correction.

Frequently Asked Questions

What is the difference between Ra and Rz?
Ra is the average deviation from the mean line; Rz is the average peak-to-valley height. Ra is more commonly specified because it is stable and easy to measure, while Rz is more sensitive to individual peaks and better at predicting sealing and fatigue performance.
How do I convert Ra to Rz?
Ra does not uniquely determine Rz. This calculator illustrates Rz = 5 × Ra as an assumed scenario only. For acceptance work, measure Rz directly using the definition and settings required by the drawing.
What is the ISO grade N number?
Legacy N1–N12 labels map to the published Ra sequence from 0.025 to 50 µm. N7 is 1.6 µm. This calculator uses a lookup and labels an intermediate value as a ceiling; it does not assign grades outside the sequence.
What is RMS roughness?
RMS roughness is Rq, the root mean square of profile heights about the mean line. A sine profile gives Rq/Ra = π/(2√2), about 1.111. Other profiles have different ratios, so this output is conditional.
Is a lower Ra always better?
Not necessarily. Smoother costs more to produce, and very smooth surfaces can have higher friction because they adhere over a larger real contact area. Specify the roughness the function requires, not the smoothest achievable.

Related

Value Sources

References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.

#SourceTypeRevision / method
[1]Values computed in your browserderivedEvaluated locally from the formulas shown on the page. No data leaves the device.
[2]ISO 4287 — Surface texture: Profile methodreferenceISO 4287:1997 — source
[3]MISUMI / Nihon Kinzoku Denchu — Ra comparison specimens, N-grade reference tablereferenceJIS B 0659-1:2002 reference table; accessed 2026-10-16 — source

Data Sources

StandardRevisionWhat it covers on this page
Formulas as shown on this pagen/a — standard engineering relationships, no single governing revisionevery value this calculator produces
MISUMI / Nihon Kinzoku Denchu — Ra comparison specimens, N-grade reference tableJIS B 0659-1:2002 reference table; accessed 2026-10-16the twelve legacy N-grade / Ra pairs only

Verification scope

Derived values — the following values on this page are calculated, not taken directly from the standard:

ValueHow it is derived
All outputsComputed in the browser from the formulas above. No data leaves the device.

The 2026-10-16 check covers the N-grade lookup and unit arithmetic only. Process ranges and Rz/Rq scenarios are reference estimates, not measured results or acceptance limits.

Accuracy and use. Use the source notes and recorded verification scope above to assess each value. Calculated values follow the stated derivations; compilation values may lack a confirmed original document or product condition. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Read the source notes, calculation methods and verification scope above before reusing a value. To cite this page: