Metric to Imperial Screw Size Conversion

Metric to Imperial Screw Size Conversion: How to Convert from Centimeters from Inches (And More)

Unless you’re an avid DIY-er or a professional working in the housing or similar industries, you probably have no idea how to convert from Metric to Imperial scale.

Both scales are used to measure length and are found on precision tools such as rulers and tape measures. The Metric scale is more common in Europe, the United Kingdom, and Canada, where it’s used in combination with the Imperial scale. The U.S. market still favors the Imperial scale, but Metric is starting to make its appearance.

Metric and Imperial Units of Measure for Screws

What do you even measure when measuring screws?

In both cases (when the measurements you have are in Metric or Imperial scale) you measure the length of the screw. The other measurement given is the diameter (Metric) or the gauge (Imperial).

How to Read Metric Measurements

The unit of measure used for Metric measurements is millimeters. The first number is the diameter of the screw, and the second is the length.

For example, the measurements will read 3.0 (diameter in mm) x 25 (length in mm). 

It is worth noting that the size of the diameter does not exactly correspond to the size of the gauge (or the head of the screw, for that matter).

There is no standard for providing measurements for screws. Each manufacturer does as they please: some provide conversion charts while others don’t. If you’re not sure if a certain Metric size screw will be suitable for you, it is best to contact customer service (if shopping online) or buy screws personally.

How to Read Imperial Measurements

The  Imperial unit of measure for length is inches. The gauge is expressed by a standardized number that roughly corresponds to the diameter of the same screw in millimeters (but it’s always best to check if the screws are truly identical).

A measurement expressed in Imperial Units will read 4 (gauge) x 1 (length in inches).

Keep in mind the same diameter and gauge screws come in various lengths.

Conversion Charts: Going from Metric to Imperial

No, you don’t have to whip out your ruler and measure by hand. Just use a conversion chart instead.

Here’s an example:

  • Diameter = 6 mm, length = 150 mm. Imperial equivalent = 12 (gauge) x 6 (length in inches)
  • Diameter = 6 mm, length = 130 mm. Imperial equivalent = 12 (gauge) x 5 1/8 (length in inches)
  • Diameter = 5 mm, length = 75 mm. Imperial equivalent = 10 (gauge) x 3 (length in inches)
  • Diameter = 5 mm, length = 25 mm. Imperial equivalent = 10 (gauge) x 1 (length in inches)

Other Useful Information About Screws

Screw measurements are not the only thing you can find on the packaging. You’ll often see abbreviations or acronyms that don’t tell much to the uninitiated.

Here are the most common acronyms explained:

  • ST = Self-tapping. Screws that don’t require pre-drilling a pilot hole.
  • ZP = Zinc Plating. These screws have a layer of zinc that makes them rust-repellent (very important for outdoor projects exposed to rain, humidity, or salty air)
  • ZYP = Zinc Yellow Passivated. Zinc with extra yellow coating.

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