ISO Tolerance Grades Chart: IT01 to IT18 for CNC and InspectionIT7, IT8, H7 and hole-basis fit reference
ISO tolerance grades define how wide a permitted size variation is for a part feature. Lower IT numbers such as IT6 or IT7 mean tighter tolerances; higher numbers such as IT12 or IT14 allow more variation. This chart helps CNC machinists, QA inspectors, and design engineers compare ISO 286 IT grades by nominal size range before choosing a fit or converting inch-based drawings.
IT7 and IT8 tolerance examples in mm
IT7 is commonly used for accurate fits, bearing seats, and locating features. IT8 is looser and often balances manufacturing cost, assembly clearance, and repeatability. Always read the value from the correct nominal size row.
Hole-basis fits: H7, H8, H9 explained
H7 combines the H hole-basis position with IT7 tolerance width. H8 and H9 use wider tolerance zones for easier machining or rougher holes.
ISO Tolerance Grade Chart
Click grades to show/hide columns. Hover for application description.
| Size Range (mm) | IT5 Precision grinding | IT6 Precision fits | IT7 Standard fits (H7) | IT8 General machining | IT9 Coarse machining | IT10 Drilling, boring | IT11 Rough machining |
|---|---|---|---|---|---|---|---|
| Nominal Size | Tolerance (μm) | Tolerance (μm) | Tolerance (μm) | Tolerance (μm) | Tolerance (μm) | Tolerance (μm) | Tolerance (μm) |
| 0-3 | 4 | 6 | 10 | 14 | 25 | 40 | 60 |
| 3-6 | 5 | 8 | 12 | 18 | 30 | 48 | 75 |
| 6-10 | 6 | 9 | 15 | 22 | 36 | 58 | 90 |
| 10-18 | 8 | 11 | 18 | 27 | 43 | 70 | 110 |
| 18-30 | 9 | 13 | 21 | 33 | 52 | 84 | 130 |
| 30-50 | 11 | 16 | 25 | 39 | 62 | 100 | 160 |
| 50-80 | 13 | 19 | 30 | 46 | 74 | 120 | 190 |
| 80-120 | 15 | 22 | 35 | 54 | 87 | 140 | 220 |
| 120-180 | 18 | 25 | 40 | 63 | 100 | 160 | 250 |
| 180-250 | 20 | 29 | 46 | 72 | 115 | 185 | 290 |
| 250-315 | 23 | 32 | 52 | 81 | 130 | 210 | 320 |
| 315-400 | 25 | 36 | 57 | 89 | 140 | 230 | 360 |
| 400-500 | 27 | 40 | 63 | 97 | 155 | 250 | 400 |
Note: Tolerance values shown are ±(value/2) for bilateral tolerances, or full range for unilateral tolerances.
Reference: ISO 286-1:2010 - Geometrical product specifications (GPS) - ISO code system for tolerances on linear sizes
Highlighted row: 30-50 mm range shows tolerance values for nominal sizes in this range.
How to Use This Chart
Find Your Nominal Size
Locate the row corresponding to your nominal dimension. For example, if you're machining a Ø40mm shaft, use the 30-50mm row.
Select Appropriate IT Grade
Choose the IT grade based on your manufacturing process and function requirements: IT6-IT7 for precision fits, IT8-IT9 for general machining, IT10-IT11 for rough machining.
Apply Fundamental Deviation
Combine IT grade with fundamental deviation letter (H for holes, h for shafts, etc.) to create tolerance class. For example: Ø40 H7 means a hole with IT7 tolerance in 30-50mm range = ±25μm (±0.025mm).
Convert Units if Needed
Use the unit selector above to view tolerances in micrometers (μm), millimeters (mm), or inches ("). For imperial dimensions, convert nominal size to mm first, then find IT grade tolerance.
Precision Applications
Use IT5-IT7 for bearing seats, precision gears, and gauge work. Requires grinding or precision turning.
H7/g6, H7/h6, H6/k5General Machining
Use IT8-IT10 for standard fits, running assemblies, and general CNC work. Achievable with good machining practice.
H8/f7, H9/d9, H11/c11Rough Work
Use IT11-IT14 for rough machining, castings, and non-critical dimensions. Large clearances for easy assembly.
IT12, IT13, IT14What are ISO Tolerance Grades (IT Grades)?
ISO tolerance grades (International Tolerance grades) are standardized tolerance values defined by ISO 286-1 for limits and fits. The IT grade number represents the precision level, with lower numbers indicating tighter tolerances and higher precision.
Precision Grades (IT01-IT5)
Used for gauge blocks, precision measuring tools, and ultra-high precision applications. Requires grinding, lapping, or honing.
- •IT01: Gauge blocks, master standards (0.3-0.8 μm)
- •IT1-IT4: Precision gauges, measuring equipment
- •IT5: High-precision fits, fine grinding
Standard Grades (IT6-IT11)
Most common for general machining, bearings, and mechanical assemblies. Achievable with standard CNC machining processes.
- •IT6-IT7: Precision bearings, press fits, H7/g6 clearance
- •IT8-IT9: General machining, running fits
- •IT10-IT11: Loose fits, easy assembly
Coarse Grades (IT12-IT16)
For rough machining, castings, forgings, and non-critical dimensions. Large clearances and easy assembly.
- •IT12-IT13: Rough machining, drilled holes
- •IT14-IT16: Castings, flame cutting, rough surfaces
Very Coarse (IT17-IT18)
Used for sand castings, die castings, and stamped parts where dimensional accuracy is not critical.
- •IT17-IT18: Sand casting surfaces, rough stampings
- •Tolerances range from ±0.5mm to several millimeters
Understanding Fundamental Deviations (Hole and Shaft Basis)
The ISO system uses letter codes to indicate the position of tolerance zones relative to the nominal size. Capital letters (A-Z) are for holes, lowercase letters (a-z) are for shafts.
Hole Basis System (H)
The hole is the reference element with H designation (lower deviation = 0). The shaft tolerance is adjusted to achieve desired fit. Most common in manufacturing because holes are harder to machine than shafts.
- ●H7: Standard hole tolerance for general fits (reaming, boring)
- ●H8: Coarser hole tolerance for easy machining
- ●H9-H11: Drilled or rough-bored holes
H7/g6- Clearance fit (sliding, rotating)H7/k6- Transition fit (location, light press)H7/p6- Interference fit (permanent assembly)
Shaft Basis System (h)
The shaft is the reference element with h designation (upper deviation = 0). The hole tolerance is adjusted to achieve desired fit. Used when shaft is harder to machine or when using standard shaft sizes.
- ●h6: Precision ground shaft for tight fits
- ●h7: Standard turned shaft for general use
- ●h9-h11: Rough-turned or as-rolled shafts
G7/h6- Clearance fit (sliding)K7/h6- Transition fit (location)P7/h6- Interference fit (press)
Common Fit Recommendations by Application
Clearance Fits (Always have clearance)
Parts can move freely relative to each other. Used for rotating or sliding assemblies.
H7/f7- Close running fit (precise rotation)H7/g6- Precision sliding fit (common for bearings)H8/f7- Easy running fit (general rotation)
H9/d9- Large clearance (easy assembly)H11/c11- Very loose fit (castings, rough parts)
Transition Fits (May have clearance or interference)
Slight interference or clearance depending on actual manufactured sizes. Used for accurate location.
H7/k6- Location fit (precise positioning, easy assembly)H7/n6- Location fit (tighter, may need light press)
- • Gears on shafts (require exact centering)
- • Locating rings and collars
- • Coupling hubs
Interference Fits (Always have interference)
Shaft is larger than hole, requiring force or heating for assembly. Permanent or semi-permanent connection.
H7/p6- Light press fit (disassembly possible)H7/s6- Medium press fit (permanent assembly)H7/u6- Heavy press fit (high torque transmission)
- • Bearing outer rings in housings
- • Gear hubs pressed on shafts
- • Bushings in bores
- • Permanent wheel-shaft assemblies
ANSI B4.1 vs ISO 286 Tolerance Systems
Understanding the differences between American (ANSI) and International (ISO) tolerance systems is crucial for global manufacturing and engineering communication.
ANSI B4.1 (American System)
- •Units: Typically specified in inches (thousandths of an inch)
- •Classes: RC (Running), LC (Locational Clearance), LT (Locational Transition), LN (Locational Interference), FN (Force/Shrink)
- •Example: RC3 fit, Class 2A/2B threads
- •Tolerance Notation: ±0.005", +0.000/-0.002"
ISO 286 (International System)
- •Units: Always specified in millimeters (microns for tolerance)
- •Classes: IT grades (IT01-IT18) combined with deviation letters (A-Z, a-z)
- •Example: H7/g6 fit, Ø25 H7
- •Tolerance Notation: +0.021/0 mm, 25⁺⁰·⁰²¹⁄₀
⚠️ Important When Converting Between Systems
- 1.Direct equivalence is approximate: ANSI Class 2 ≈ ISO H7/g6, but not exact
- 2.Always verify fit requirements: Check clearance/interference values, not just grade names
- 3.Use our tolerance converter tool: Get exact tolerance values for your specific size
- 4.Communicate clearly with suppliers: Specify tolerance in actual numbers (±0.025mm) in addition to grade codes
FAQ about ISO tolerance grades
What does IT mean in ISO tolerance grades?
IT means International Tolerance grade. It defines the width of the allowed variation for a feature size, but not the direction of the deviation.
Is IT7 tighter than IT8?
Yes. IT7 is tighter than IT8 for the same nominal size range. The exact difference must be read from the ISO grade row for that size range.
Is H7 the same as IT7?
Not exactly. H7 combines a fundamental deviation letter H with tolerance grade IT7. IT7 defines the tolerance width, while H defines where that tolerance zone sits relative to the nominal size.
Can I convert ANSI B4.1 classes directly to ISO tolerance grades?
You can map many ANSI fit classes to approximate ISO fit designations, but it is not a one-to-one replacement. Always verify function, material, process capability, and inspection method.
Related Tools & Resources
Tolerance Converter
Convert ANSI B4.1 tolerances to ISO 286 grades. Interactive tool calculates exact tolerance values for your specific dimensions.
Use Converter →Inches to MM Converter
Convert nominal dimensions from inches to millimeters with precision. Essential before applying ISO tolerance grades.
Convert Dimensions →Fastener Tolerance Chart
Thread fit classes and tolerance grades for imperial and metric fasteners. Includes 2A/2B to 6g/6H conversion.
View Chart →Surface Roughness Chart
Convert Ra μin to μm when tolerance planning also needs CNC finish and inspection context.
View Chart →Machining Allowance Calculator
Calculate blank, rough, and finish machining sizes. Understand how much material to leave for achieving your target tolerance.
Calculate Allowances →Master Tolerance Selection for Your Projects
Understanding ISO tolerance grades is essential for quality manufacturing. Bookmark this chart for quick reference, or use our interactive tolerance converter for specific calculations.