Inches to Millimeters conversion

Converting inches to millimeters

Whenever a specification written in inches has to be built, cut or printed with metric tools, the inches must become millimeters. Because the inch has been defined as exactly 25.4 mm since 1959, this is one of the few conversions with no rounding error in the factor itself: inches × 25.4 = millimeters.

Working with fractions of an inch

Imperial dimensions are often given as fractions: 3/8", 1/2", 3 1/4". Convert the fraction to a decimal first (3/8 = 0.375), add the whole inches, and only then multiply by 25.4. So 3 1/4 in = 3.25 × 25.4 = 82.55 mm. A short table of the common fractions helps:

• 1/16 in = 1.59 mm
• 1/8 in = 3.18 mm
• 1/4 in = 6.35 mm
• 3/8 in = 9.53 mm
• 1/2 in = 12.7 mm
• 3/4 in = 19.05 mm
• 1 in = 25.4 mm

Whole inches

• 2 in = 50.8 mm
• 4 in = 101.6 mm
• 6 in = 152.4 mm
• 10 in = 254 mm
• 12 in = 304.8 mm (one foot)

Practical uses

Woodworkers following American plans in a metric workshop, 3D-printing enthusiasts downloading models designed in inches, cyclists comparing tyre widths, and anyone ordering a picture frame or a sink from a foreign supplier will run into this conversion. Plumbing is a classic case: pipe sizes such as 1/2" and 3/4" are nominal, so check the actual outside diameter rather than converting the name of the size.

Precision tip

Keep the full 25.4 in the calculation and round only the final answer. Multiplying by 25 instead of 25.4 introduces a 1.6 % error — harmless for a bookshelf, unacceptable for a bearing seat.