49 CFR § 171.10 - Units of measure.

---
identifier: "/us/cfr/t49/s171.10"
source: "ecfr"
legal_status: "authoritative_unofficial"
title: "49 CFR § 171.10 - Units of measure."
title_number: 49
title_name: "Transportation"
section_number: "171.10"
section_name: "Units of measure."
chapter_name: "PIPELINE AND HAZARDOUS MATERIALS SAFETY ADMINISTRATION, DEPARTMENT OF TRANSPORTATION"
subchapter_number: "C"
subchapter_name: "HAZARDOUS MATERIALS REGULATIONS"
part_number: "171"
part_name: "GENERAL INFORMATION, REGULATIONS, AND DEFINITIONS"
positive_law: false
currency: "2026-04-05"
last_updated: "2026-04-05"
format_version: "1.1.0"
generator: "[email protected]"
authority: "49 U.S.C. 5101-5128, 44701; Pub. L. 101-410 section 4; Pub. L. 104-134, section 31001; Pub. L. 114-74 section 701 (28 U.S.C. 2461 note); 49 CFR 1.81 and 1.97."
cfr_part: "171"
---


# 171.10 Units of measure.

(a) *General.* To ensure compatibility with international transportation standards, most units of measure in this subchapter are expressed using the International System of Units (“SI” or metric). Where SI units appear, 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.

(b) Abbreviations for SI units of measure generally used throughout this subchapter are as shown in paragraph (c) of this section. Customary units shown throughout this subchapter are generally not abbreviated.

(c) *Conversion values.* (1) Conversion values are provided in the following table and are based on values provided in ASTM E 380, “Standard for Metric Practice”.

(2) If an exact conversion is needed, the following conversion table should be used.

Table of Conversion Factors for SI Units

| Measurement | SI to U.S. standard | U.S. standard to SI |
| --- | --- | --- |
| Activity | 1 TBq = 27 Ci | 1 Ci = 0.037 TBq |
| Length | 1 cm = 0.3937008 in1 m = 3.280840 ft | 1 in = 2.540000 cm1 ft = 0.3048000 m |
| Thickness | 1 mm = 0.03937008 in | 1 in = 25.40000 mm |
| Mass (weight) | 1 kg = 2.204622 lb1 g = 0.03527397 oz | 1 lb = 0.4535924 kg1 oz = 28.34952 g |
| Pressure | 1 kPa = 0.1450377 psi1 Bar = 100 kPa = 14.504 psi1 kPa = 7.5 mm Hg | 1 psi = 6.894757 kPa1 psi = 0.06895 Bar |
| Radiation level | 1 Sv/hr = 100 rem/hr | 1 rem/hr = 0.01 Sv/hr |
| Volume (liquid) | 1 L = 0.2641720 gal1 mL = 0.03381402 oz1 m = 35.31466 ft | 1 gal = 3.785412 L1 oz = 29.57353 mL1 ft = 0.02831685 m |
| Density | 1 kg/m = 0.06242797 lb/ft | 1 lb/ft = 16.01846 kg/m |
| Force | 1 Newton = 0.2248 Pound-force | 1 Pound-force = 4.483 N |
| Abbreviation for units of measure are as follows: |  |  |
| Unit of measure and abbreviation: |  |  |
| (SI): millimeter, mm; centimeter, cm; meter, m; gram, g; kilogram, kg; kiloPascal, kPa; liter, L; milliliter, mL; cubic meter, m; Terabecquerel, TBq; Gigabecquerel, GBq; millisievert, mSv; Newton, N; |  |  |
| (U.S.): Inch, in; foot, ft; ounce, oz; pound, lb; psig, psi; gallon, gal; cubic feet, ft ; Curie, Ci; millicurie, mCi; millirem, mrem. |  |  |

[Amdt. 171-111, 56 FR 66159, Dec. 20, 1991, as amended by Amdt. 171-136, 60 FR 49108, Sept. 21, 1995; Amdt. 171-135, 60 FR 50302, Sept. 28, 1995; 66 FR 33335, June 21, 2001; 66 FR 45378, Aug. 28, 2001; 68 FR 75740, Dec. 31, 2003]