Sanding Belt Grit to Surface Roughness (Ra) Chart & Guide

1. First, what is Ra?

Ra is the "arithmetic average roughness" — the most common surface roughness metric in engineering, usually in micrometers (μm). The lower the Ra, the smoother the surface. 1μm is about 40 microinches (μin); US drawings often use μin.

One detail that is easy to miss: "Ra≤0.5μm" (an upper limit that allows a few points to exceed) and "Ra max 0.5μm" (a maximum that no single point may exceed) are two different requirements. Always clarify which one the customer means before quoting or machining, to avoid rework.

2. Key pitfall: two grit systems you can't mix

There are actually two grit-numbering systems on the market, and mixing them causes real problems:

• FEPA-P system (P prefix, e.g. P180): the standard in Europe and China, based on ISO 6344. Tight tolerance, limits oversized particles, leaves fewer stray scratches.
• CAMI/ANSI system (no prefix, e.g. 180# or 180 grit): the US standard, allowing a wider particle-size range for the same number.

In the coarse range (P60–P220) the two numbering systems give similar particle sizes and roughly match. But from around P400 / CAMI 360 they diverge quickly — the same number means a different particle diameter, and the gap grows as grit gets finer. A frequently cited example: US 240# is about Ra 0.45μm, while UK 240# is about Ra 1.5μm — a threefold difference. So P220 ≠ 220 grit and P1200 ≠ 1200 grit; never swap between systems directly.

3. Complete grit ↔ Ra ↔ finish chart

The table below shows typical values for standard-pressure dry grinding of stainless/carbon steel in a single direction. Note that Ra is a range, not a fixed value.

4. Stainless steel finish grades (No.1–No.8)

Many customers order by "stainless steel grade." Here is how they map (per ASTM A480 / EN 10088-2):

• No.1: hot-rolled, annealed, pickled — Ra about 2.0–8.0μm, the roughest.
• 2B: the most common cold-rolled base, Ra about 0.1–0.5μm.
• No.3: mechanical coarse grind, Ra within about 1μm, roughly 80–100#.
• No.4 (standard satin, the industry workhorse): Ra generally up to 0.64μm, roughly 120–180#. Note the ASTM standard itself warns that the Ra ranges of No.3 and No.4 overlap.
• HL (hairline): continuous one-direction long lines, roughly 150–320#, Ra about 0.1–0.5μm.
• No.6: No.4 reworked with 240–320#, Ra about 0.3–0.5μm.
• No.8 (mirror): progressively ground to 600# then polished with compound, Ra about 0.02–0.1μm.

5. Why Ra is always a range, never a fixed number

With the same 180# belt, one operator gets Ra 0.6 and another gets Ra 0.9 — both normal. These variables are at work:

1. Workpiece material: stainless/carbon steel is tough, mid-range Ra; aluminum is soft and loads easily (use silicon carbide); titanium/high-nickel is hard, tough and heat-sensitive (use ceramic).
2. Grain type: aluminum oxide is versatile but dulls over time; zirconia is tough, self-sharpening and needs higher pressure, ideal for coarse/medium heavy removal; ceramic is microcrystalline, cuts sharply under light pressure, lasts longest and is friendly to heat-sensitive metals.
3. Pressure and belt speed: higher pressure raises Ra, light grinding lowers it.
4. Belt wear: a new belt cuts aggressively with higher Ra; as it wears Ra drops but heat rises and burning risk increases — replace it in time.
5. Dry vs wet: wet grinding gives a finer, more uniform Ra and better heat control.

Quick answers

Which grit gives a Ra of 0.8μm?

Typically P180 to P220. But grit-to-Ra is an empirical correlation, not an exact conversion. The real result also depends on material, pressure, belt wear and wet vs dry grinding, so confirm with a sample on your own workpiece.

What grit is used for a No.4 brushed (satin) finish on stainless steel?

A standard No.4 satin finish typically uses P120 to P180. A hairline (HL) finish commonly uses P150 to P320. Finer grit gives a finer line pattern.

Is P220 the same as 220 grit?

No. The P prefix is the FEPA/ISO system (used in Europe and China); grit without a prefix is the CAMI/ANSI system (USA). Above roughly P400 the same number means a different particle size, and the gap widens as grit gets finer, so the two systems cannot be directly swapped.

What Ra does 320 grit give on stainless steel?

Typically about Ra 0.2–0.3 μm with single-direction dry grinding at normal pressure (P320 / CAMI 280–320). That is a semi-bright finish and the usual base for going on to mirror polishing.

What Ra value can you get from 100 grit on stainless steel?

Roughly Ra 1.0–1.4 μm for P100–P120: a coarse satin or base finish (around No.3). To get under Ra 0.8 μm you need to step up to about P180–P220.

Does a 240 grit finish guarantee Ra 0.8 μm?

No. Grit sets the likely range, not a guaranteed number. P220–P240 normally lands at about Ra 0.3–0.6 μm, but pressure, belt wear, speed and the starting surface all move the result, so measure Ra on a sample panel when a spec such as hygienic Ra ≤ 0.8 μm must be met.

6. MOOSEFOS product selection

Mapped to the MOOSEFOS range:

• Coarse/medium heavy removal (P40–P120): zirconia belts PZ633+, DY528, WY1266/WY1289, or ceramic belt WY1599 for hard-to-grind metals.
• Satin/brushed and fine grinding (P120–P400): alumina-zirconia WY1513/WY1531, full grit range — grind, refine and polish in one belt.
• Fine finishing (P180–P600): aluminum oxide belt JA513 (the fullest grit range, 60–600#) and JA537 (anti-clog, for aluminum and soft metals).
• Beyond P600, toward a No.8 mirror (P800 and finer): often worked by hand or on a flat platen with waterproof silicon carbide paper before polishing — Eagle AP806 SiC waterproof paper covers P60–P10000 in one series (Namkyung, Korea; supplied by us as an authorized distributor).

Not sure which to choose? Tell us your workpiece material, target Ra or finish grade, and current equipment — we'll recommend the model and grit, and send samples to test on your own parts.