What is the exact conversion factor?

The exact conversion factor is 1 inch = 25.4 millimeters. This was standardized internationally in 1959. For engineering applications, always use exactly 25.4 (not rounded values).

How many decimal places should I use?

For general work: 2-3 decimals (63.50mm). For precision work: 4 decimals (63.5000mm). For ultra-precision: 5-6 decimals. Our converter defaults to 4 decimals for engineering precision.

Can I use inches on a metric CNC machine?

Most modern CNC controllers support both G20 (inch) and G21 (mm) modes. However, converting to metric is recommended for consistency with machine DRO and metric tooling.

What's the difference from other converters?

Unlike generic converters, we provide manufacturing-specific tools: machining allowance calculation, tolerance conversion (ANSI to ISO), BOM batch processing, and fastener matching.

What is the exact conversion factor for inches to mm?

The exact conversion factor is 1 inch = 25.4 millimeters, standardized internationally in 1959. All tools on inches-to-mm.com use exactly 25.4 to ensure precision for manufacturing and CNC machining.

How many decimal places should I use for machining?

For general machining 2–3 decimals are typical, for precision work 4 decimals, and for ultra-precision 5–6 decimals. Inches-to-mm.com defaults to engineering-grade precision suitable for most ISO tolerance applications.

Can I use inches directly on a metric CNC machine?

Most CNC controllers support both inch (G20) and metric (G21) modes, but converting designs to metric improves consistency with tooling, fixtures, and DROs and reduces unit-conversion mistakes.

Frequently Asked QuestionsInches to MM Conversion for Manufacturing

Complete answers to common questions about unit conversion, machining allowances, tolerance grades, fastener matching, and BOM processing. Find quick solutions to your manufacturing conversion challenges.

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Can't find the answer you're looking for? Browse our technical guides, reference charts, or use our specialized manufacturing tools.

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Frequently Asked QuestionsInches to MM Conversion for Manufacturing

What is the exact conversion factor for inches to mm?

How many decimal places should I use for machining?

Can I use inches directly on a metric CNC machine?

Is the conversion the same for all types of measurements?

Why do some tools show different results?

What's the difference between this and other converters?

Do you support fractional inches?

How accurate is the conversion for precision work?

Can I bookmark specific conversions?

Is there a mobile app available?

How much machining allowance should I add for steel shafts?

Do I need different allowances for aluminum vs. steel?

What about internal features like holes?

How do I account for heat treatment distortion?

What if my part has very tight tolerances?

How much allowance for casting + machining?

Can I use the same allowance for all features on a part?

What's the minimum allowance I can use?

How do I calculate allowance for milling vs. turning?

Should I leave extra material for first article inspection?

What tolerance should I specify when converting from inches?

Can I directly replace H7 with a simple ± tolerance?

How do I convert ANSI tolerance classes to ISO?

What's the difference between IT grades?

Can I mix imperial and metric tolerances on the same drawing?

How tight can I realistically hold tolerances?

What does H7/g6 fit mean?

Should I use geometric tolerancing (GD&T) or ± tolerances?

How do I tolerance threaded holes?

What tolerance should I use for hole-shaft clearance fits?

Can I use M6 bolts instead of 1/4"-20?

What's the metric equivalent of Grade 5 bolts?

How do I find metric equivalents for pipe threads?

Can I tap an imperial hole to metric threads?

What about metric bearing replacements for inch bearings?

How do wrench sizes compare between imperial and metric?

What about set screws and socket cap screws?

Can I use metric nuts on imperial bolts?

How do I specify fasteners on drawings for international manufacturers?

Where can I buy metric fasteners in the US?

Can your tool convert my Excel BOM table?

Will my formulas be preserved in the converted Excel?

How does the tool detect which columns to convert?

Can I convert BOM tables with mixed units?

What happens to material specifications during conversion?

Can I process multiple BOM files at once?

How should I prepare my BOM for conversion?

Can the tool convert tolerance callouts?

What format should I use for output - Excel or CSV?

Is my BOM data secure? Where is it processed?

How do I copy the conversion result?

Can I link to a specific conversion?

Does the converter work offline?

How do I change between light and dark mode?

Can I print the reference charts?

How do I report a bug or suggest a feature?

Are there keyboard shortcuts available?

Can I embed the converter on my website?

What browsers are supported?