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Gage Block Grades Explained

Guide · Updated October 5, 2026

A gage block's grade is a tolerance class: how far the block's actual length may be from nominal, plus limits on flatness and on length variation across the face. Higher grades cost more and matter most when you use blocks at nominal size without applying calibration corrections.

The current US standard: ASME B89.1.9

ASME B89.1.9 defines grades K, 00, 0, AS-1, and AS-2. Grades K, 0, AS-1 and AS-2 match the same-named grades in ISO 3650 (the "AS" prefix keeps them from being confused with the old Federal grades 1 and 2). Grade 00 was added to match earlier US practice.

Length limit deviations (± microinches) for a few common size ranges:

Summary of ASME B89.1.9-2002 values. Flatness and variation-in-length limits also apply; use the standard itself or your gage maker's catalog for compliance work.

Grade K is the calibration grade. Its length tolerance is actually looser than Grade 00, but its length variation and flatness limits are tight. K blocks are meant to be used with their calibrated lengths (the actual measured size from the certificate), typically as reference masters for calibrating other blocks.

Metric: ISO 3650

ISO 3650 uses grades K, 0, 1, 2. For blocks 0.5–10 mm, the limit deviations are ±0.20 µm (K), ±0.12 µm (0), ±0.20 µm (1), and ±0.45 µm (2). Tolerances grow with length.

Old Federal grades (GGG-G-15C)

You'll still see older sets marked Grade 0.5, 1, 2, or 3 (and older still, "AA" and "A"). Under GGG-G-15C, blocks through 1" were held to ±1 µin (Grade 0.5), ±2 µin (Grade 1), +4/−2 µin (Grade 2), and +8/−4 µin (Grade 3). That spec has been replaced by ASME B89.1.9, which suggests Grade 1 → 00, Grade 2 → 0, Grade 3 → AS-1. The tolerances aren't identical, so a block meeting one spec doesn't automatically meet the other.

Which grade for which job?

• K / 00: calibration labs and reference masters used to check other gages.
• Grade 0: gage rooms and inspection, setting comparators and high-precision gages.
• AS-1: inspection and toolroom work, setting micrometers and height gages.
• AS-2: shop-floor setups, sine bar work, machine setups, and general checking.

These are typical uses, not rules. Your measurement uncertainty budget decides. As a rule of thumb, the reference should be several times more accurate than the tolerance you're checking.

The certificate matters more than the label

• A calibration certificate lists each block's measured deviation and the uncertainty. Applying those corrections can make a lower-grade set more useful than its grade suggests.
• Grade tolerances assume 20 °C (68 °F). Temperature error easily swamps grade differences. Steel grows about 6.4 µin/in/°F.
• Blocks wear. A set that met Grade 0 when new may not after years of use, so recalibrate on the interval your quality system sets.

Materials at a glance

• Steel: the most common choice. Its expansion (~6.4 µin/in/°F) is close to most steel workpieces, which reduces temperature mismatch. It can rust.
• Ceramic (zirconia): corrosion resistant and wear resistant, with expansion close to steel (Starrett lists ~5.5 µin/in/°F).
• Carbide (chrome or tungsten): very wear resistant and popular for wear blocks. Lower expansion than steel (Starrett lists chrome carbide ~4.7 µin/in/°F).

Shopping by grade

• HFS 36-piece inch set: a budget inch set; check the listing for its stated grade and certificate.
• Mitutoyo 516-938-26 112-piece metric set: listed as Grade 0.
• 81-piece inch sets (search): compare grades and certificates before you buy.
• Mitutoyo 0.050" steel block: listed as ASME Grade 0.

General shop information only — follow your gage manufacturer's instructions and your quality system. As an Amazon Associate I earn from qualifying purchases; links marked "sponsored" may earn a commission at no cost to you.