Gauge Blocks (Gage Blocks): Grades K, 0, 1 and 2 in Numbers, How to Wring and Stack Them, and Which Grade You Need

Quick answer: gauge blocks (gage blocks)

  • What they are: precision blocks with very flat, smooth faces (Mitutoyo) that Starrett calls primary standards of length. Regional Tech lists their uses as inspecting parts, checking gauges and calibrating instruments.
  • Grades: JIS B 7506-1997 (and the matching DIN 861 table) uses grades K, 0, 1 and 2. For a 10–25 mm block the length tolerance is ±0.14 µm for grade 0, ±0.30 µm for grade 1 and ±0.60 µm for grade 2 (Engineers Edge table). Regional Tech describes K for calibration labs, 0 for inspection and toolmaking, 1 for general engineering and 2 for general workshop tasks.
  • Wringing: wipe the faces with a trace of oil, wipe off the excess, cross the blocks at 90°, then slide and rotate them into line with steady pressure (Mitutoyo, Starrett, Great Gages).
  • Building a size: use as few blocks as possible and start with the block that sets the last decimal place (Mitutoyo).
  • Heat matters: by Starrett's formula, warming a 100 mm steel block by 1 °C makes it about 1.15 µm longer (our calculation), nearly four times the grade 0 tolerance for that length (±0.30 µm).

This guide explains what the gauge block grades mean in numbers, how to wring and stack blocks, how temperature changes the result, and how to pick a grade from the instrument you need to check. It uses Mitutoyo's and Starrett's published guides, a JIS B 7506 tolerance table and questions from user forums (r/Metrology and the AccurateShooter forum). The last part covers the Japanese-market Mitutoyo sets we stock. Sources are listed at the end.

What gauge blocks are for

Gauge blocks (also spelled gage blocks) are length standards. Mitutoyo describes them as "a premier measurement standard" whose faces are very flat and very smooth, and Starrett's tutorial calls them primary standards of length that must be handled carefully to keep their accuracy and surface finish.

Regional Tech lists their uses by grade: inspecting mechanical parts and tools, checking gauges and calibrating instruments, and, for grade 2, general workshop tasks such as mounting tools and cutters and making gauges. Blocks can also be wrung together into a stack to make a reference length for inspection (Regional Tech).

Gauge block grades in numbers

The table gives two numbers per grade: the tolerance on length (how far the block's length may differ from its nominal size, ±te) and the permissible variation (tv; our reading: how much the length may vary across the face). It is the JIS B 7506-1997 table published by Engineers Edge, which notes it is the same as DIN 861-1980, at 20 °C. Engineers Edge does not print the unit. Our reading is micrometres (µm), because MSC's grade 0 example for a 1" (25.4 mm) block, ±6 millionths of an inch (about ±0.15 µm), sits close to the ±0.14 grade 0 value for 10–25 mm, and Mitutoyo describes block accuracy in "a few millionths of an inch".

Nominal length (mm) Grade K ±te / tv Grade 0 ±te / tv Grade 1 ±te / tv Grade 2 ±te / tv
0.5 to 10 0.20 / 0.05 0.12 / 0.10 0.20 / 0.16 0.45 / 0.30
over 10 to 25 0.30 / 0.05 0.14 / 0.10 0.30 / 0.16 0.60 / 0.30
over 25 to 50 0.40 / 0.06 0.20 / 0.10 0.40 / 0.18 0.80 / 0.30
over 50 to 75 0.50 / 0.06 0.25 / 0.12 0.50 / 0.18 1.00 / 0.35
over 75 to 100 0.60 / 0.07 0.30 / 0.12 0.60 / 0.20 1.20 / 0.35

Two things stand out in the table (our reading). First, each step from grade 0 to 1 to 2 roughly doubles the length tolerance. Second, grade K does not have the tightest length tolerance: its ±te equals grade 1's, while its variation in length (tv) is the tightest of all. Starrett notes that a block's grade depends on its flatness and parallelism, and that calibration cannot upgrade a block's grade. Our reading of the table: K blocks are chosen for flatness and parallelism and are used with their measured (certified) lengths, which fits Regional Tech's description of K as the grade for checking other blocks in calibration labs.

What each grade is used for

  • K (Regional Tech groups it as "00/K"; MSC lists K and 00 separately): checking the accuracy of lower-grade blocks, mainly in calibration labs, and it needs a well-controlled environment (Regional Tech).
  • 0: quality-control labs for toolmaking, inspection and calibrating instruments; Regional Tech says it gives adequate accuracy for most workshop applications.
  • 1: general engineering: inspecting parts and tools, checking gauges and calibrating instruments that need less precision than grade 0 (Regional Tech).
  • 2: general workshop tasks such as mounting tools and cutters and making gauges (Regional Tech).

US blocks use different names. Starrett notes that newer blocks are calibrated to ASME B89.1.9 (older ones to GGG-G-15C). MSC lists five grades: K, 00, 0, AS-1 and AS-2, and gives a grade 0 example of ±6 millionths of an inch for a 1" block. Higher Precision lists US Federal Specification grades 0.5, 1, 2 and 3 (AAA, AA, A+) and calls the letter names outdated. Our advice: grade names do not map one to one between systems, so compare the tolerance numbers, not the labels.

Which grade do you need? A 10:1 check

Starrett's tutorial suggests the tool-maker's 10:1 rule: the reference should be about ten times finer than what you are checking. Its example: for micrometers that read to .0001", a block tolerance tighter than .000010" is probably unnecessary. Applying that rule to the table above (our calculation, reference ≤ resolution ÷ 10):

Instrument resolution 10:1 target for the block Grade 0 meets it up to Grade 1 meets it up to Grade 2 meets it up to
0.01 mm (calipers, 0.01 mm dial indicators) ±1.0 µm Up to 400 mm (±0.90 µm); ±0.30 µm at 100 mm Up to 200 mm (±1.00 µm); ±0.60 µm at 100 mm Up to 75 mm (±1.00 µm); not over 75 mm (±1.20 µm)
0.001 mm (digital micrometers, fine indicators) ±0.1 µm Not met at any length (±0.12 µm from 0.5 mm) Not met Not met

What this means in practice (our calculation): grades 0 and 1 meet 10:1 for 0.01 mm calipers across a 100 mm set; grade 2 meets it up to 75 mm. For 0.001 mm instruments no grade meets a strict 10:1 on nominal size alone; grade 0 comes closest (±0.12 to ±0.14 µm up to 25 mm). Our reading: using each block's measured (certified) length instead of its nominal size is the way to get closer. The figures over 100 mm are from the full Engineers Edge table, which continues to 1,000 mm. The check uses ±te only, not tv.

How to wring gauge blocks

  1. Clean. Starrett: clean blocks before and after use with a soft cloth lightly dampened with mineral spirits, and do not use chlorinated solvents.
  2. Check for burrs. Mitutoyo's quick guide uses an optical flat: place it gently on the face, slide until fringes appear and press lightly. Fringes that vanish mean no burr; fringes that stay in one spot mean a burr on the block; fringes that move with the flat mean a burr on the flat.
  3. Oil and wipe off. Apply a very small amount of oil and wipe until the film is almost gone (Mitutoyo). Starrett uses a few drops of light machinery oil on one cloth and a second dry cloth to remove the excess.
  4. Cross and slide. For two thick blocks, cross them at 90° at the middle of the faces, then rotate them into line with slight pressure (Mitutoyo; Great Gages describes the same cruciform move). Starrett: slide them together with pressure to force out trapped air.
  5. Thin blocks go on a thick one first. Mitutoyo: wring a thin block onto a thick block first to prevent bending, then wring the next thin block onto it; overlap one side of the thin block on the thick block.
  6. Do not wring above an open set, since a slipped block can damage itself and others, and do not leave steel blocks wrung together for long, because trapped moisture can cause corrosion (Starrett).

Why it works: Mitutoyo says properly wrung blocks cling without adhesive, with a gap of around one millionth of an inch. Great Gages gives the surface needed for wringing as a finish of 1 microinch (0.025 µm) or better and flatness of at least 5 µin (0.13 µm). A block that no longer wrings should be replaced (Starrett).

How to build a size from a set

Mitutoyo's rule: use the fewest blocks possible, prefer thick blocks, and start with the block that sets the least significant digit, then work back. Great Gages notes that three blocks from a set of 30 can make any of the 1,000 lengths from 3.000 to 3.999 mm in 0.001 mm steps.

Worked example (our calculation): MQS lists the 103-piece set 516-942-11 as 1.005 mm; 1.01 to 1.49 mm in 0.01 mm steps; 0.5 to 24.5 mm in 0.5 mm steps; and 25, 50, 75 and 100 mm. To make 41.255 mm:

  • Last digit (0.005): take 1.005 → remaining 40.250
  • Next digits (0.25): take 1.25 → remaining 39.000
  • Half-millimetres and units: take 14.0 → remaining 25.000
  • Remainder: take 25 → done. 1.005 + 1.25 + 14.0 + 25 = 41.255 mm with four blocks.

Each block's tolerance adds up in a stack. With grade 0 blocks the four tolerances above sum to at most ±0.12 + ±0.12 + ±0.14 + ±0.14 = ±0.52 µm (our worst-case sum from the table), which is another reason to use as few blocks as possible.

Temperature: an error larger than the grade tolerance

Gauge block lengths are defined at 20 °C (68 °F). Starrett gives the formula: change = length × (block temperature − 68 °F) × expansion coefficient, with .0000064 per °F for steel. Converted to metric (1 °C = 1.8 °F), that is about 11.5 µm per metre per °C. Our calculation from that formula:

Steel block length Growth per +1 °C Grade 0 tolerance (table) Warming that uses up the grade 0 tolerance
10 mm 0.12 µm ±0.12 µm about 1 °C
25 mm 0.29 µm ±0.14 µm about 0.5 °C
50 mm 0.58 µm ±0.20 µm about 0.35 °C
100 mm 1.15 µm ±0.30 µm about 0.26 °C

Starrett notes that body heat from your hands expands blocks, so they must acclimate before use, and suggests a steel or aluminum soaking plate (not granite, which conducts heat poorly). Mitutoyo's quick guide shows a 100 mm block growing after being handled by hand and says to let it stabilize before measuring. Starrett also notes that steel blocks, steel parts and steel micrometers expand similarly, so the error shrinks when everything is at the same temperature; non-ferrous parts need their own calculation (Starrett).

Steel, ceramic or carbide?

  • Steel: the most common material, but prone to corrode over time (Regional Tech). After use, wipe steel blocks with a rust-preventive oil and handle them so your fingers never touch the block (Starrett).
  • Ceramic: Regional Tech says up to 10× the abrasion resistance of steel and dimensionally stable.
  • Tungsten carbide: described by Regional Tech as the hardest, most durable and corrosion-resistant option.

MISUMI lists the BM1 sets it carries, including the BM1-103-0 and BM1-32-1, as steel. The material of our BM1-103-1, BM1-103-2 and BM1-112-2 is Not confirmed (not listed by MISUMI or on the product pages).

Why look at Japanese-market Mitutoyo sets?

We sell Japanese tools, so the sets below are Mitutoyo BM1 sets sold in Japan. What we can state:

  • JIS grades. MISUMI lists the BM1 series by JIS grade (0, 1, 2 and K) with Mitutoyo code numbers, for example BM1-103-0 = grade 0, 103 pieces, code 516-942. Our product pages state the grade number.
  • Made in Japan per each product page.
  • Related export codes. 516-942 and 516-944 also appear in other markets with a suffix (516-942-26 in the US, 516-942-11 in the UK, 516-944-26 in the US). The US 516-944-26 is listed by Global Test Supply as grade AS-2, so export versions may be graded to a different standard. We have not confirmed block by block that the Japanese-market box matches any export version (Not confirmed).

What to check before buying: a calibration certificate is Not confirmed for our sets (the product pages do not list one), and MQS's UK listing for 516-942-11 mentions a Mitutoyo Certificate of Inspection. If you need a certificate traceable to a national standard for an audit, ask us before ordering. We do not claim these sets are more accurate than other brands of the same grade.

Mitutoyo gauge block sets we stock

Model (code) Grade Pieces Range Weight Fits (10:1, 0.01 mm tools)
BM1-103-0 (516-942) 0 (MISUMI) 103 (MISUMI) 0.5 to 100 mm (MISUMI) 2.16 kg Whole set
BM1-103-1 (516-943) 1 103 Not confirmed 2.158 kg Whole set if the largest block is 200 mm or less (range Not confirmed)
BM1-103-2 (516-944) 2 103 Not confirmed 2.158 kg Blocks up to 75 mm
BM1-112-2 (516-940) 2 112 Not confirmed 2.176 kg Blocks up to 75 mm
BM1-32-1 (516-967) 1 32 1 to 60 mm (MISUMI) 0.426 kg Whole set

Grade, piece count and weight are from each product page unless marked MISUMI. The "Fits" column applies our 10:1 calculation above (±te only): a set "fits" if every block up to its largest size is within ±1.0 µm. Grade 1 stays within ±1.0 µm up to 200 mm.

MITUTOYO BM1-103-0 Gauge block A (516-942) Mitutoyo BM1-103-0 Gauge Block A (516-942) MITUTOYO BM1-103-1 Gauge block B (516-943) Mitutoyo BM1-103-1 Gauge Block B (516-943)

Which set for which job

Inspection room or toolroom checking micrometers

BM1-103-0: grade 0, the closest of our sets to a 10:1 reference for 0.001 mm instruments (±0.12 µm up to 10 mm), though it still does not meet a strict 10:1 for them (see above). It has 103 blocks; the composition of the Japanese box is Not confirmed, and the UK 516-942-11 version builds sizes in 0.005 mm steps per MQS.

General shop QC, checking calipers and indicators

BM1-103-1: grade 1, the same 103-piece size, and within ±0.60 µm up to 100 mm. If you only need a few reference lengths, the BM1-32-1 is a 32-piece grade 1 set covering 1 to 60 mm at 0.426 kg.

Workshop setting and tool setup

BM1-103-2 or BM1-112-2: grade 2, which Regional Tech lists for mounting tools and cutters and making gauges.

Mitutoyo BM1-103-0 grade 0 gauge block set 516-942Grade 0, 103 pcsMitutoyo BM1-103-0

Inspection-room grade; closest to 10:1 for 0.001 mm tools.

View BM1-103-0
Mitutoyo BM1-103-1 grade 1 gauge block set 516-943Grade 1, 103 pcsMitutoyo BM1-103-1

General QC; ±0.60 µm up to 100 mm.

View BM1-103-1
Mitutoyo BM1-32-1 grade 1 gauge block set 516-967Grade 1, 32 pcsMitutoyo BM1-32-1

Smaller set, 1 to 60 mm, 0.426 kg.

View BM1-32-1
Mitutoyo BM1-103-2 grade 2 gauge block set 516-944Grade 2, 103 pcsMitutoyo BM1-103-2

Workshop setting and tool setup.

View BM1-103-2

FAQ: questions people actually ask on forums

When should gauge blocks be replaced instead of recalibrated?

A recent r/Metrology thread asked exactly this; the popular comment shown in Google's snippet was that if a block keeps its tolerance at calibration, it never needs to be replaced. Starrett adds the practical limits: light nicks can be stoned out, severely damaged blocks should be replaced, and so should blocks that no longer wring. Calibration cannot upgrade a block's grade (Starrett).

Do I need a full set just to check my calipers and micrometers?

Not necessarily. On the AccurateShooter forum a member asked for a single block to check micrometers and calipers; one reply pointed to a 36-piece set of "almost gage blocks" good to about a tenth or two (0.0001–0.0002 in), and the same reply said that measuring to one tenth or better often calls for better blocks. MSC's guide says a smaller set with common increments works for routine shop checks, and a full set with finer increments suits calibration work. The 32-piece BM1-32-1 is our smaller option.

Is grade K better than grade 0?

For variation in length, yes: in the JIS B 7506 table K has the tightest permissible variation (0.05 µm up to 25 mm). For length deviation, no: K's ±0.20 µm up to 10 mm equals grade 1's, and grade 0 is tighter at ±0.12 µm. K is meant for checking other blocks in calibration labs (Regional Tech).

Is it "gage block" or "gauge block"?

Both. MSC uses "gage", Mitutoyo America uses both spellings, and UK sellers such as MQS use "gauge". "Gage blocks" alone averaged about 3,600 US searches a month from October 2025 to September 2026 (Google Keyword Planner, our data).

Can I leave blocks wrung together in a stack?

Starrett says not to leave steel blocks wrung for long periods, because trapped moisture can cause corrosion. Separate, clean and oil them after use.

Why does my reading drift right after I pick the blocks up?

Heat from your hands. Mitutoyo's quick guide shows a 100 mm block growing after hand handling, and by Starrett's formula a 1 °C rise adds about 1.15 µm to a 100 mm steel block (our calculation). Use a soaking plate and let blocks stabilize (Starrett, Mitutoyo).

Related guides

Find your gauge block set

Japanese-market Mitutoyo BM1 sets in grades 0, 1 and 2, shipped from Japan.

Shop gauge blocks

Sources

Last reviewed: October 2026. Not affiliated with Mitutoyo, Starrett, MSC, MISUMI or the other sources above.

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