Remember back in grade school when they told us that every other country in the world used the metric system and that America would be left behind if we didn’t change, pronto?

Well, they were half right.  I still pay for gas by the gallon and I still keep my speed below 55 mph to avoid a ticket but we also happen to be the most powerful country in the history of the world, so maybe the rest of the world needs to switch back to the U.S. standard.

But I’m not here to express my opinion on the metric system.  The purpose of this article is to identify the unit of measure used in the Hazardous Material Regulations (HMR) of PHMSA/USDOT and how to convert from metric units to the U.S. standard.

The use of units of measure in the HMR and some conversion factors from the International System of Units (SI or metric) to the U.S. standard are found at 49 CFR 171.10.  This section of the HMR and its requirements are summarized below.

Most units of measure used in the HMR are expressed as SI or metric.  This is done to make the domestic regulations of PHMSA/USDOT compatible with those of the rest of the world where metric units are common (see my rant above).

Where SI units are used in the HMR, they are the regulatory standard. U.S. standard or customary units, which appear in parentheses following the SI units, are for information only and are not intended to be the regulatory standard.

A good example of this can be found at 49 CFR 173.120 which defines a class 3 Flammable Liquid:

(a) Flammable liquid. For the purpose of this subchapter, a flammable liquid (Class 3) means a liquid having a flash point of not more than 60 °C (140 °F), or any material in a liquid phase with a flash point at or above 37.8 °C (100 °F) that is intentionally heated and offered for transportation or transported at or above its flash point in a bulk packaging, with the following exceptions:

In the above definition, it is the metric units that determine compliance and not the U.S. standard units in (parenthesis) so be certain your lab results indicate the metric units or you must make the conversion for your records.

Interestingly, while abbreviations of metric units are commonly used in the HMR they are not for the U.S standard units.  Abbreviations of both metric units and U.S. standard used in the HMR are as follows:

Metric or International System of Units (SI):

• millimeter – mm
• milliliter – mL
• cubic meter – m³
• Terabecquerel – TBq
• Gigabecquerel – GBq
• millisievert – mSv
• Newton – N

U.S. standard or customary units:

• inch – in
• foot – ft
• ounce – oz
• pound – lb
• psig – psi
• gallon – gal
• cubic feet – ft³
• Curie – Ci
• millicurie – mCi
• millirem – mrem

If a conversion from U.S. standard to metric, or vice versa, is necessary a table is provided at §171.10(c) and below.  It’s values are based on those provided in ASTM E 380, “Standard for Metric Practice”.

Table of Conversion Factors for SI Units

Even if the U.S. doesn’t exclusively use the metric system, compliance with the HMR depends on it.  Be sure to refer to the conversion table to ensure your U.S. standard measurements are in compliance with the PHMSA/USDOT regulations when converted to SI Units.