ANSI to ISO Tolerance ConverterB4.1 to ISO 286 Standards
Use this converter to map inch-based ANSI B4.1 fit classes to approximate ISO 286 metric fit designations. It is designed for engineering reviews, supplier RFQs, and CNC documentation where legacy inch drawings need a practical ISO reference.
ANSI B4.1 Specifications
ISO 286 Equivalent
B4.1 Standard
Hole Tolerance
Shaft Tolerance
Tolerance Zones (Metric)
| Feature | Nominal (mm) | Upper Limit | Lower Limit | Tolerance Zone |
|---|---|---|---|---|
| Hole (H8) | 25.4000 | 25.9588 | 25.4000 | +0.5588 / +0.0000 |
| Shaft (h7) | 25.4000 | 25.4000 | 24.8412 | +0.0000 / -0.5588 |
Manufacturing Recommendations
- Achievable with precision turning or milling
- Sharp tools and rigid setup required
- Final pass should be light (0.1-0.2mm)
- Design for lubrication channels if continuous rotation
- Consider surface finish Ra 0.8-1.6μm for bearing surfaces
- Use micrometers or CMM for inspection (±0.002mm accuracy)
- Temperature-controlled inspection room recommended
What this converter can and cannot decide
It can translate common RC, LC, LT, LN, and FN communication into approximate ISO fit language. It cannot approve a drawing change, supplier capability, inspection plan, or safety-critical substitution by itself.
Before sending a converted tolerance to a supplier
Engineering note: tolerance conversions are practical references for estimating equivalent fit behavior. They do not replace the original ISO 286 or ANSI B4.1 standards, customer specifications, inspection plans, or supplier capability reviews.
ISO tolerance grade chartUnderstanding Tolerance Systems
Tolerance systems define how precise parts must be manufactured. ANSI B4.1 (American) and ISO 286 (International) are the two dominant systems, each with different nomenclature but achieving similar results.
ANSI B4.1 (Imperial)
Used primarily in the United States. Specifies fits as letter-number combinations like RC2, LC3, FN4.
- • RC = Running/Sliding Clearance
- • LC = Locational Clearance
- • LT = Locational Transition
- • LN = Locational Interference
- • FN = Force/Shrink Fit
ISO 286 (Metric)
International standard used worldwide. Specifies hole and shaft tolerances separately like H7/g6, H8/h7.
- • H = Hole tolerance (uppercase)
- • g, h, f = Shaft tolerance (lowercase)
- • 6, 7, 8, 9 = Tolerance grade (IT)
- • Lower number = Tighter tolerance
- • H-hole basis is most common
Common Tolerance Equivalents
| ANSI B4.1 | ISO 286 | Typical Application | Tolerance Range |
|---|---|---|---|
| RC-1 | H7/g6 | Precision bearings, close-running fits | IT7 (±0.015mm @ Ø25) |
| LC-2 | H8/h7 | Slip fits for easy assembly/disassembly | IT8 (±0.022mm @ Ø25) |
| LT-2 | H8/h7 | Locating pins, light press fits | IT8 (±0.022mm @ Ø25) |
| LN-2 | H7/h6 | Press fits for gears and pulleys | IT7 (±0.015mm @ Ø25) |
| FN-2 | H7/h6 | Permanent assemblies, bearing races | IT7 (±0.015mm @ Ø25) |
ISO IT Grade Precision Levels
IT6-IT7 (Precision Grade)
High precision fits requiring grinding or honing. Typical tolerance ±0.010-0.015mm for Ø25mm parts.
- ✓ANSI Equivalent: RC1, LC1, LN1-2, FN1-2
- ✓Machining: Requires grinding or precision turning
- ✓Applications: Bearings, precision gears, gauge blocks
IT8-IT9 (Standard Grade)
General manufacturing precision achievable with quality CNC operations. Typical tolerance ±0.022-0.036mm for Ø25mm parts.
- ✓ANSI Equivalent: RC2-4, LC2-4, LT2-4
- ✓Machining: Precision turning/milling sufficient
- ✓Applications: General machine parts, shafts, housings
IT10-IT12 (Coarse Grade)
Loose fits for easy assembly, non-critical dimensions. Typical tolerance ±0.058-0.150mm for Ø25mm parts.
- ✓ANSI Equivalent: RC7-9, LC7-9
- ✓Machining: Standard milling/drilling operations
- ✓Applications: Clearance holes, non-precision assemblies
Why Convert Between ANSI and ISO?
International Manufacturing
Working with overseas suppliers or reading international drawings? ISO 286 is the global standard. Convert your ANSI specs to ensure correct communication.
Engineering Drawing Review
Received a drawing with H7/g6 callout but only familiar with RC fits? Our converter helps you understand foreign tolerance specifications instantly.
Quality Inspection
Converting tolerances ensures your inspection equipment and procedures match the design intent, whether specified in ANSI or ISO standards.
Common Tolerance Conversion Mistakes
❌ Mixing Unit Systems
- • Using inches with ISO tolerances
- • Forgetting to convert nominal size to mm
- • Applying imperial tolerance to metric parts
- • Incorrect conversion factor (using 25 instead of 25.4)
❌ Misunderstanding Fit Types
- • Confusing clearance with interference fits
- • Using wrong tolerance grade for application
- • Ignoring hole vs shaft tolerance differences
- • Not considering assembly method
❌ Ignoring Size Range Effects
- • Tolerance varies with nominal size
- • Small parts need tighter absolute tolerances
- • Large parts allow looser tolerances
- • Check tables for specific size ranges
❌ Machining Capability Mismatch
- • Specifying IT6 without grinding capability
- • Expecting precision from standard operations
- • Not validating machine capability
- • Overlooking thermal expansion effects
ANSI B4.1 to ISO 286 FAQ
Can I replace an ANSI RC fit with the suggested ISO fit directly?
No. Treat the ISO result as an engineering starting point, not an automatic replacement. Check function, load, temperature, manufacturing process, and inspection requirements.
Why does the converter show approximate ISO equivalents?
ANSI B4.1 and ISO 286 use different fit systems and naming conventions. A practical mapping can be useful for communication, but it should be validated against the drawing intent.
Should I convert tolerance values before or after converting inches to millimeters?
Convert the base dimensions exactly using 1 inch = 25.4 mm, then evaluate the tolerance class or fit system in the target standard.
Related Manufacturing Tools
Precision Inch to MM Converter
Convert design dimensions from inches to millimeters with up to 6 decimal precision. Perfect for preparing metric drawings.
Use Converter →Machining Allowance Calculator
Calculate blank, rough, and finish machining sizes based on material type and process. Essential for CNC programming.
Calculate Allowances →Fastener Matcher
Find metric equivalents for imperial threads and bolts. Includes strength grade comparison and compatibility warnings.
Match Fasteners →BOM Batch Converter
Convert entire Excel/CSV bills of materials from imperial to metric. Saves hours on international RFQ preparation.
Convert BOM →Start Converting Tolerances
Join thousands of engineers and machinists who use our converter for accurate ANSI to ISO tolerance conversion. Ensure proper communication with international suppliers and understand metric drawings with confidence.