Machining Grade Converter
Cross-standard surface finish, IT tolerance and general tolerance
Convert between ISO / JIS / ASME / DIN / GB machining grades — with honest confidence marks so approximations are never dressed up as exact.
Pick a grade or enter Ra
Ra and Rz have NO fixed mathematical relationship. Rz ≈ 4×Ra is a rough rule of thumb only — never treat it as an exact conversion.
Full equivalence table
ISO N-grade was withdrawn (removed 2002, whole ISO 1302 withdrawn 2021, replaced by ISO 21920). Kept as a legacy cross-reference key. JIS triangle ↔ N-grade is the weakest link (unofficial folk convention, sources conflict).
Cross-standard adoption matrix (ISO / JIS / DIN / GB)
About the Machining Grade Converter
Machinists and mechanical engineers constantly cross-reference "grades" between standards — an ISO drawing calls out N7 or Ra 1.6, a US shop reads 63 µin, a fits table wants H7/g6, and a title block lists ISO 2768-m. This tool converts across ISO, JIS, ASME, DIN and GB in one place, and — crucially — marks every cell with its confidence so approximations are never presented as exact equivalences.
The one thing to understand: ISO family vs the ASME inch system
This is not "five different national systems." Modern JIS, DIN and GB surface-texture and tolerance standards are largely word-for-word adoptions of ISO (DIN ISO 2768 is identical to ISO 2768; JIS B0405 matches ISO 2768-1 cell-for-cell; GB/T 131 = IDT ISO 1302). The genuine second paradigm is the American inch system — ASME B46.1 does not natively use N-grades, ASME B4.1 fits (RC/LC/LT/LN/FN) are an independent inch system, and US general tolerances live in the title block by decimal place rather than a national table. So most "conversions" are really "the same ISO number in different clothes," except where the US inch world genuinely diverges — and there we mark it as an approximate analog, not an equivalence.
How to use
- Surface finish: pick an ISO N-grade or type an Ra value to see Ra (µm/µin), the empirical Rz, the old JIS triangle and typical process.
- IT tolerance & fits: enter a nominal size and IT grade to get the exact ISO 286-1 tolerance width (it is size-dependent — IT7 at Ø10 is not IT7 at Ø100), plus common fits mapped to their nearest ASME B4.1 analog.
- General tolerances: enter a size and ISO 2768 class (f/m/c/v) for the ± linear tolerance, with the adoption notes for DIN/JIS/GB.
Frequently Asked Questions
Q: Can I convert Ra to Rz exactly?
No. Ra (amplitude average) and Rz (peak-to-valley statistic) have no fixed mathematical relationship — the ratio depends entirely on whether the surface is periodic (turning) or random (grinding/EDM). The common Rz ≈ 4×Ra is a rough rule of thumb, not an ISO conversion. This tool always marks any Ra→Rz value as approximate.
Q: Why does the same IT grade give different tolerances?
Because IT tolerances are size-dependent by design. ISO 286-1 defines a tolerance unit i = 0.45·∛D + 0.001·D and multiplies it per grade, so IT7 is 15 µm at 6–10 mm but 21 µm at 18–30 mm. Never hardcode a single micron value for an IT grade — always read it against the nominal size, which this tool does from the published ISO 286-1 table.
Q: Is the ASME B4.1 fit shown equivalent to the ISO fit?
No — it is an approximate functional analog, not a dimensional equivalence. ASME B4.1 is an independent inch-based system (RC/LC/LT/LN/FN classes). The mapping tells you which US class plays a similar role (e.g. H7/g6 sliding ≈ RC1–RC2), but the actual limits differ. Tighter interference analogs (FN2/FN3–FN4) are the weakest and are flagged unofficial.
Q: What is the ASME equivalent of ISO 2768 general tolerances?
There isn't a direct one. US drawings usually define default tolerances in the title block by decimal place (.X = ±0.1″, .XX = ±0.01″, .XXX = ±0.005″), which is a per-company convention, not a national standard like ISO 2768. So we deliberately do not fabricate a fifth "ASME" column for general tolerances.
Last updated: July 2026 | Reference tool for planning. Verify critical callouts against the current standard (ISO 21920, ISO 286-1:2010, ISO 2768).
