2 Inches to Millimeters | Complete Pipe Size Conversion Guide
Unit inconsistency is a common issue in pipe and fitting industries. Imperial units including inch and foot are adopted in the US and several regions, while metric units such as millimeter and meter are applied in most global industrial standards like ISO, DIN, JIS, as well as China GB standards. When reviewing ASME standard elbow drawings or purchasing stainless steel pipe fittings, conversion demands such as “2inch to mm” frequently occur. Accurate inch-millimeter conversion ensures proper material matching, and guarantees tightness and pressure resistance of piping systems.
Core Conversion Formula
1 inch = 25.4 millimeters, an internationally defined fixed value
Multiply inch figures by 25.4 to get millimeter values, and divide millimeter values by 25.4 to convert into inches.
Necessity of Mastering Inch-Millimeter Conversion in Piping Industry
In accordance with ASME B36.10 standard for welded and seamless rolled steel pipes, nominal pipe size (NPS) ranges from 1/2 inch to 24 inches. Corresponding outer diameters do not follow simple integer multiple conversion rules.
- NPS 2 inch → Outer diameter: 60.3 mm, rather than 50.8 mm
- NPS 4 inch → Outer diameter: 114.3 mm, rather than 101.6 mm
The deviation derives from historical design based on wall thickness schedule and NPT thread taper. Basic linear length conversion still conforms to 1 in = 25.4 mm. Precise millimeter conversion is mandatory for matching dimensions of elbows, tees, flanges and other fittings, including center-to-end distance and thread length, to prevent excessive assembly clearance.
Common Inch to Millimeter Conversion List
All calculations follow the formula: Millimeter = Inch × 25.4, rounded to two decimal places. Industrial measurement adopts precision of 0.01mm.
| Inch (in) | Fraction & Decimal | Calculation | Millimeter (mm) | Application Scenario |
|---|---|---|---|---|
| 1/18 in | 0.055555… in | 0.0555556 × 25.4 | 1.41 mm | Ultra-thin adjusting gasket, instrument thread fine tuning |
| 1/8 in | 0.125 in | (1/8)×25.4 | 3.18 mm | Wall thickness marking, small-bore pipe inner diameter |
| 1/4 in | 0.25 in | (1/4)×25.4 | 6.35 mm | Regular bolt diameter, compressed air pipe connector |
| 1/4 in in mm | Same as above | Same as above | 6.35 mm | Identical to 1/4 inch to mm |
| 1/4in to mm | Same as above | Same as above | 6.35 mm | Common online search expression |
| 1.5 in | 1.5 in | 1.5 × 25.4 | 38.10 mm | Linear length reference for 1.5 inch pipe outer diameter (actual OD: 48.3 mm) |
| 1/4 inch to mm | 0.25 in | 0.25 × 25.4 | 6.35 mm | Thread bottom hole, flange bolt center distance |
| 3.75 in | 3.75 in | 3.75 × 25.4 | 95.25 mm | Flange neck length, valve structural dimension |
| 4 in | 4 in | 4 × 25.4 | 101.60 mm | Pure conversion of NPS 4 nominal size (actual OD: 114.3mm) |
| 4in to mm | Same as above | Same as above | 101.60 mm | – |
| 4.25 in | 4.25 in | 4.25 × 25.4 | 107.95 mm | Eccentricity of reducing elbow, custom flange thickness |
| 5.25 in | 5.25 in | 5.25 × 25.4 | 133.35 mm | Non-standard fitting center height, hydraulic connector length |
| 5 1/4 in | 5.25 in | 5.25 × 25.4 | 133.35 mm | Same as 5 1/4 inches to mm |
| 5.25 inches in mm | Same as above | Same as above | 133.35 mm | – |
| 6 in | 6 in | 6 × 25.4 | 152.40 mm | Contrast length of NPS 6 pipe actual OD 168.3mm |
| 6inch to mm | Same as above | Same as above | 152.40 mm | – |
| 24 in | 24 in | 24 × 25.4 | 609.60 mm | Large-size butterfly valve structural length, pipe support spacing |
| 24 inch to mm | Same as above | Same as above | 609.60 mm | – |
| 25 in | 25 in | 25 × 25.4 | 635.00 mm | Equipment interface positioning dimension, special filter length |
Note: Data listed above refers to linear length conversion. Nominal sizes such as NPS 4 inch have standard outer diameter of 114.3mm, which differs from 4×25.4=101.6mm. Do not confuse the two concepts. The conversion formula applies fully to connecting dimensions including thread length, flange bolt hole center distance and butt weld bevel length.
Efficient Methods for Pipe Inch-Millimeter Conversion
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Adopt Fixed Conversion Value
Keep in mind 1 in = 25.4 mm. Avoid approximate value 1 in ≈ 25 mm. Cumulative error of 4.8mm will occur in 12-inch measurement, resulting in misaligned flange bolt holes.
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Quick Mental Calculation for Fractional Inch
- 1/2 in = 12.7 mm
- 1/4 in = 6.35 mm
- 1/8 in = 3.175 mm
- 1/16 in = 1.5875 mm
Calculate special fractions via calculator: 1 ÷ 18 × 25.4 = 1.4111… mm, rounded to 1.41 mm for industrial use.
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Refer to Authoritative Standard Charts
Complete inch-millimeter comparison tables are provided in ASME B36.19 for stainless steel pipes and MSS SP-43 for forged fittings. In compliance with ASME B16.9 Factory-made Wrought Butt-welding Fittings, the center-to-end dimension of 2-inch long radius elbow is defined as 76 mm. The universal conversion formula is applied to non-standard design work.
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Utilize Reliable Conversion Tools
Use NIST certified online converters or inch-millimeter conversion tables from Machinery’s Handbook 31st Edition. Ordinary mobile calculators are not recommended due to inconsistent rounding rules.
