Choose a working system
Use the unit system that best matches your plan, measuring tools, and hardware. A project specified in millimeters is usually easier to follow in millimeters. A plan built around fractional-inch hardware may be simpler to keep in inches. There is no need to convert merely to make numbers familiar.
Write the unit at the top of every working list and beside any exception. A number such as 18 is ambiguous without context. Avoid writing a metric thickness beside an inch length in an unlabeled dimension string. Consistent notation prevents errors before precision becomes relevant.
Convert once and preserve the original
One inch is exactly 25.4 millimeters. Use that relationship for conversions and keep the original dimension beside the converted value during planning. Convert related dimensions together so you can inspect their relationships instead of converting isolated values at the bench.
For example, 3½ inches equals 88.9 millimeters. Rounding to 89 millimeters may be acceptable for one purpose and inappropriate for another. The required fit determines the decision. Do not round every part independently and assume the combined assembly will retain its intended overall size.
Handle fractions deliberately
When adding inch fractions, use a common denominator before combining them. One half plus three eighths equals four eighths plus three eighths, or seven eighths. Write the intermediate step if mental arithmetic is unreliable under workshop conditions.
Keep decimal inches visibly different from fractions. A measurement of 0.125 inch equals one eighth inch; it is not 0.125 millimeter. If a digital measuring tool changes units, verify the display before recording a value. A familiar-looking number can conceal an accidental mode change.
Use physical relationships when possible
Not every fit needs a conversion. Directly transferring a dimension from a verified part or using a story stick can preserve a relationship without translating it into numbers. This helps with repeated spacing or fitting a piece to an existing opening.
Numbers still matter for communicating the design, checking overall size, and ordering material. Combine physical transfer with clear records rather than treating them as competing methods. Stop an active operation before consulting a conversion chart or calculator; arithmetic deserves attention separate from tool handling and material control.
What to take with you
- Use one working unit system.
- Preserve original values during conversion.
- Round according to the required fit.
A few good questions
Can I replace every inch with 25 millimeters?
That is an approximation, not an exact conversion. Errors grow with length and can affect matching parts. Use 25.4 for conversion and make any rounding decision explicitly.