Tolerance Reference for CNC Machining
The numbers engineers actually look up: IT tolerance grades, a hole-basis fit calculator, general (untoleranced) dimensions, GD&T symbols with untoleranced geometric values, and preferred numbers. Values come from the Chinese national standards (GB/T) that align with ISO; compiled and verified by the KGL engineering desk. Materials and surface treatment now live on their own pages, one click away.
Standard tolerance grades IT1 to IT18
Total tolerance band in micrometers by nominal size range, per GB/T 1800.1-2009 (identical to ISO 286-1). A band is the full spread: IT7 at 100 mm is 35 um wide, so the process must land inside roughly plus or minus 0.018 mm around nominal. Pick your size and read across; click a value to copy it.
| Nominal size (mm) | IT1 | IT2 | IT3 | IT4 | IT5 | IT6 | IT7 | IT8 | IT9 | IT10 | IT11 | IT12 | IT13 | IT14 | IT15 | IT16 | IT17 | IT18 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| >0 - 3 | 0.8 | 1.2 | 2 | 3 | 4 | 6 | 10 | 14 | 25 | 40 | 60 | 0.1 | 0.14 | 0.25 | 0.4 | 0.6 | 1 | 1.4 |
| >3 - 6 | 1 | 1.5 | 2.5 | 4 | 5 | 8 | 12 | 18 | 30 | 48 | 75 | 0.12 | 0.18 | 0.3 | 0.48 | 0.75 | 1.2 | 1.8 |
| >6 - 10 | 1 | 1.5 | 2.5 | 4 | 6 | 9 | 15 | 22 | 36 | 58 | 90 | 0.15 | 0.22 | 0.36 | 0.58 | 0.9 | 1.5 | 2.2 |
| >10 - 18 | 1.2 | 2 | 3 | 5 | 8 | 11 | 18 | 27 | 43 | 70 | 110 | 0.18 | 0.27 | 0.43 | 0.7 | 1.1 | 1.8 | 2.7 |
| >18 - 30 | 1.5 | 2.5 | 4 | 6 | 9 | 13 | 21 | 33 | 52 | 84 | 130 | 0.21 | 0.33 | 0.52 | 0.84 | 1.3 | 2.1 | 3.3 |
| >30 - 50 | 1.5 | 2.5 | 4 | 7 | 11 | 16 | 25 | 39 | 62 | 100 | 160 | 0.25 | 0.39 | 0.62 | 1 | 1.6 | 2.5 | 3.9 |
| >50 - 80 | 2 | 3 | 5 | 8 | 13 | 19 | 30 | 46 | 74 | 120 | 190 | 0.3 | 0.46 | 0.74 | 1.2 | 1.9 | 3 | 4.6 |
| >80 - 120 | 2.5 | 4 | 6 | 10 | 15 | 22 | 35 | 54 | 87 | 140 | 220 | 0.35 | 0.54 | 0.87 | 1.4 | 2.2 | 3.5 | 5.4 |
| >120 - 180 | 3.5 | 5 | 8 | 12 | 18 | 25 | 40 | 63 | 100 | 160 | 250 | 0.4 | 0.63 | 1 | 1.6 | 2.5 | 4 | 6.3 |
| >180 - 250 | 4.5 | 7 | 10 | 14 | 20 | 29 | 46 | 72 | 115 | 185 | 290 | 0.46 | 0.72 | 1.15 | 1.85 | 2.9 | 4.6 | 7.2 |
| >250 - 315 | 6 | 8 | 12 | 16 | 23 | 32 | 52 | 81 | 130 | 210 | 320 | 0.52 | 0.81 | 1.3 | 2.1 | 3.2 | 5.2 | 8.1 |
| >315 - 400 | 7 | 9 | 13 | 18 | 25 | 36 | 57 | 89 | 140 | 230 | 360 | 0.57 | 0.89 | 1.4 | 2.3 | 3.6 | 5.7 | 8.9 |
| >400 - 500 | 8 | 10 | 15 | 20 | 27 | 40 | 63 | 97 | 155 | 250 | 400 | 0.63 | 0.97 | 1.55 | 2.5 | 4 | 6.3 | 9.7 |
IT1 to IT11 in micrometers (um); IT12 to IT18 in millimeters (mm). Total band width, not plus or minus. IT01 and IT0 exist for gauge work and are rarely quoted.
Hole-basis fit calculator
Enter a nominal size and pick zones. The calculator returns hole and shaft limits, fit type, and the clearance or interference extremes. Common starting points: H7/g6 easy sliding, H7/h6 locational clearance, H7/k6 transition, H7/p6 light press, H7/s6 medium press.
Computed per GB/T 1800.1-2009 (ISO 286-1) from the verified grade and deviation tables, including the delta rule for hole zones J to ZC (K/M/N up to IT8, P to ZC up to IT7 use the tabulated delta; coarser grades use the base value). Hole-basis; inch mode converts the size and limits. Deviations stay in micrometers (1 um = 0.001 mm). Untoleranced-dimension tolerances live in the section above.
General tolerances for untoleranced dimensions
When a drawing gives no individual tolerance, these values apply per GB/T 1804-2000 (equivalent to ISO 2768-1). State the grade in the title block: for example "GB/T 1804-m". KGL inspects to the drawing grade; if the title block is silent we quote and build to m (medium).
Linear dimensions (mm, ±)
| Grade | 0.5 - 3 | >3 - 6 | >6 - 30 | >30 - 120 | >120 - 400 | >400 - 1000 | >1000 - 2000 | >2000 - 4000 |
|---|---|---|---|---|---|---|---|---|
| f | ±0.05 | ±0.05 | ±0.1 | ±0.15 | ±0.2 | ±0.3 | ±0.5 | - |
| m | ±0.1 | ±0.1 | ±0.2 | ±0.3 | ±0.5 | ±0.8 | ±1.2 | ±2 |
| c | ±0.2 | ±0.3 | ±0.5 | ±0.8 | ±1.2 | ±2 | ±3 | ±4 |
| v | - | ±0.5 | ±1 | ±1.5 | ±2.5 | ±4 | ±6 | ±8 |
External radii and chamfer heights (mm, ±)
| Grade | 0.5 - 3 | >3 - 6 | >6 - 30 | >30 |
|---|---|---|---|---|
| f | ±0.2 | ±0.5 | ±1 | ±2 |
| m | ±0.2 | ±0.5 | ±1 | ±2 |
| c | ±0.4 | ±1 | ±2 | ±4 |
| v | ±0.4 | ±1 | ±2 | ±4 |
Angular dimensions (±)
| Grade | up to 10 | >10 - 50 | >50 - 120 | >120 - 400 | >400 |
|---|---|---|---|---|---|
| f | 1° | 30′ | 20′ | 10′ | 5′ |
| m | 1° | 30′ | 20′ | 10′ | 5′ |
| c | 1° 30′ | 1° | 30′ | 15′ | 10′ |
| v | 3° | 2° | 1° | 30′ | 20′ |
GD&T: symbols, modifiers, untoleranced values
The working vocabulary of machined-part drawings per GB/T 1182-2008 (ISO 1101), with the untoleranced geometric values of GB/T 1184-1996 (ISO 2768-2). Symbol artwork is our own redraw.
Feature symbols
| Feature | Class | Datum | Typical use on machined parts |
|---|---|---|---|
| Straightness 直线度 | form | no | two parallel lines or a cylinder; long shafts |
| Flatness 平面度 | form | no | two parallel planes; sealing and mounting faces |
| Circularity 圆度 | form | no | two concentric circles; bearing seats |
| Cylindricity 圆柱度 | form | no | two coaxial cylinders; circularity plus straightness |
| Line profile 线轮廓度 | profile | no or yes | band around a true curve |
| Surface profile 面轮廓度 | profile | no or yes | band around a true surface; five-axis contours |
| Parallelism 平行度 | orientation | yes | band parallel to datum; rails and mating faces |
| Perpendicularity 垂直度 | orientation | yes | square to datum; mount faces and bores |
| Angularity 倾斜度 | orientation | yes | any angle other than 0 or 90 deg to datum |
| Position 位置度 | location | yes or no | round zone at true position; the strongest callout |
| Concentricity 同心度 | location | yes | median point within a zone to datum point |
| Coaxiality 同轴度 | location | yes | axis within a cylinder to datum axis |
| Symmetry 对称度 | location | yes | median plane within a zone to datum plane |
| Circular runout 圆跳动 | runout | yes | single cross-section, indicator while rotating |
| Total runout 全跳动 | runout | yes | whole surface while rotating; coaxial cylinders |
Modifiers
| Modifier | Meaning |
|---|---|
| MMC 最大实体要求 | bonus tolerance as material leaves max material; position threads and pins |
| LMC 最小实体要求 | protects minimum wall or edge distance |
| Enveloping requirement 包容要求 | perfect form at MMC, Taylor principle |
| TED 理论正确尺寸 | exact location boxed on the drawing; starting point of position |
| Free state 自由状态 | non-rigid parts, thin walls, gaskets |
| CZ 公共公差带 | combined zone, keeps several features in one pattern |
Untoleranced geometric values (mm), grades H / K / L
Straightness and flatness, by basic length
| Grade | up to 10 | >10 - 30 | >30 - 100 | >100 - 300 | >300 - 1000 | >1000 - 3000 |
|---|---|---|---|---|---|---|
| H | 0.02 | 0.05 | 0.1 | 0.2 | 0.3 | 0.4 |
| K | 0.05 | 0.1 | 0.2 | 0.4 | 0.6 | 0.8 |
| L | 0.1 | 0.2 | 0.4 | 0.8 | 1.2 | 1.6 |
Perpendicularity, by shorter leg
| Grade | up to 100 | >100 - 300 | >300 - 1000 | >1000 - 3000 |
|---|---|---|---|---|
| H | 0.2 | 0.3 | 0.4 | 0.5 |
| K | 0.4 | 0.6 | 0.8 | 1 |
| L | 0.6 | 1 | 1.5 | 2 |
Symmetry, by longer feature
| Grade | up to 100 | >100 - 300 | >300 - 1000 | >1000 - 3000 |
|---|---|---|---|---|
| H | 0.5 | 0.5 | 0.5 | 0.5 |
| K | 0.6 | 0.6 | 0.8 | 1 |
| L | 0.6 | 1 | 1.5 | 2 |
Circular runout (any measuring direction)
| Grade | Runout value (mm) |
|---|---|
| H | 0.1 |
| K | 0.2 |
| L | 0.5 |
Declare on the drawing: "GB/T 1184-H" (or K / L). Roundness untoleranced value equals the diameter tolerance; cylindricality and position have no untoleranced value.
Preferred numbers
Preferred number series (GB/T 321 / ISO 3) keep nominal sizes, speeds and ratings on a geometric ladder, which is why stock sizes and machine ranges cluster where they do.
| Series | Values (1 to 10, multiply by 1/10/100...) |
|---|---|
| R5 | 1, 1.6, 2.5, 4, 6.3, 10 |
| R10 | 1, 1.25, 1.6, 2, 2.5, 3.15, 4, 5, 6.3, 8 |
| R20 | 1, 1.12, 1.25, 1.4, 1.6, 1.8, 2, 2.24, 2.5, 2.8, 3.15, 3.55, 4, 4.5, 5, 5.6, 6.3, 7.1, 8, 9 |
| R40 | 1, 1.06, 1.12, 1.18, 1.25, 1.32, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.12, 2.24, 2.36, 2.5, 2.65, 2.8, 3, 3.15, 3.35, 3.55, 3.75, 4, 4.25, 4.5, 4.75, 5, 5.3, 5.6, 6, 6.3, 6.7, 7.1, 7.5, 8, 8.5, 9, 9.5 |
What KGL holds in practice
Chart values describe what a maintained process holds in production, not a best-ever first article. Bands below are our own floor data across 100+ CNC machines including 11 five-axis centers.
| Process | Practical tolerance | Typical Ra |
|---|---|---|
| 3-axis milling, finished pass | 0.010 - 0.030 mm | 1.6 - 3.2 um |
| 5-axis milling, single setup | 0.005 - 0.025 mm | 0.8 - 1.6 um |
| CNC turning, finished diameter | 0.008 - 0.025 mm | 0.8 - 1.6 um |
| Surface grinding | 0.002 - 0.005 mm | 0.2 - 0.8 um |
| Wire EDM | 0.003 - 0.010 mm | 0.8 - 1.6 um |
| Verification | Capability |
|---|---|
| Standard quoted tolerance | ±0.005 mm on metals, plastics, composites |
| Specific 5-axis features | ±0.002 mm with full CMM verification |
| CMM class | Hexagon and Zeiss CMMs, 0.0015 mm accuracy |
| Certifications | ISO 9001:2015, ISO 13485:2016, AS9100D |
| First article | FAI per AS9102 on request |
Where these tolerances are held and measured
The tables above say what the standard allows. These pages are the ones that have to hold, measure and report it on a real part.
Machining services that hold these bands
- CNC millingPosition, profile and flatness callouts on milled features, holes and slots.
- CNC turningDiameter fits, boring and reaming, where the IT grade sets the quoted tolerance.
- 5-axis CNC machiningOne-setup work where stacked datums are the reason for the tolerance.
How the tolerance is accepted
- Tolerance standardsThe quoted standard tolerance and the condition under which a tighter one applies.
- Inspection and QCCMM, optical and hard gauging used to verify the callout on the drawing.
- AQL samplingThe sampling plan that follows the first article on a production batch.
Where the tight end of the table matters
- AerospaceAS9100D flow-down, first article to AS9102 and lot traceability.
- Medical devicesISO 13485 process control and documented measurement on every lot.
- Semiconductor equipmentFlatness, sealing faces and cleanliness on vacuum-facing parts.
Have a feature that sits at the edge of one of these bands? Send the drawing and the tolerance question with the drawing and we will answer against it.
Sources and how to use this page
Values are the underlying national standard numbers (GB/T) aligned with their ISO counterparts, cross-checked against the Mechanical Design Handbook, 6th Edition, Volume 1, during compilation. This page is a convenience compilation for engineering work: the drawing always governs, and acceptance is decided by the specified standard and the measurement plan, not by this page.
| Standard | Covers |
|---|---|
| GB/T 1800.1-2009 / ISO 286-1 | standard tolerance grades IT, fundamental deviations |
| GB/T 1800.2-2020 / ISO 286-2 | limit deviations for tolerance classes |
| GB/T 1804-2000 / ISO 2768-1 | general tolerances for linear and angular dimensions |
| GB/T 1184-1996 / ISO 2768-2 | geometric tolerances without individual indications |
| GB/T 1182-2008 / ISO 1101 | geometric tolerancing symbols and rules |
Compiled by the KGL engineering desk. Reviewed by Qiaosheng Hu, Founder and General Manager. Questions or a value you cannot find: kgl@kglmfc.com. Last updated 2026-09-29.
Sending drawings that lean on these numbers?
Send 2D and 3D files and we return a quote with engineering feedback within 24 hours, including the honest word when a tolerance fights the material or the process. No MOQ, single prototypes supported.