Question 1

What is the exact conversion factor from pounds to kilograms?

Accepted Answer

1 pound equals exactly 0.45359237 kilograms (or equivalently, 1 kilogram = 2.2046226218... pounds). This definition comes from the International Yard and Pound Agreement of 1959, signed by the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa, which defined the international avoirdupois pound as 0.45359237 kilograms exactly. Prior to 1959 the US used a slightly larger definition (0.4535924277 kg), still retained for geodetic survey work as the 'US Survey Pound' before its retirement January 1, 2023, per joint NIST/NOAA 2020 announcement. The kilogram itself was redefined on May 20, 2019, by the 26th General Conference on Weights and Measures (CGPM, 2018) in terms of the Planck constant h fixed at exactly 6.62607015 × 10^-34 J·s, replacing the International Prototype of the Kilogram (IPK) - a physical platinum-iridium cylinder held at BIPM since 1889.

Question 2

Why does the US still use pounds instead of kilograms?

Accepted Answer

The Metric Conversion Act of 1975 (Pub. L. 94-168, codified at 15 U.S.C. §205a et seq.) declared the SI system 'the preferred system of weights and measures for U.S. trade and commerce' but stopped short of mandating conversion. The Fair Packaging and Labeling Act of 1966 (Pub. L. 89-755, codified at 15 U.S.C. §1451 et seq.) as amended in 1992 requires dual (metric and customary) labeling of most consumer commodities. USDA nutrition labeling regulations at 21 CFR §101.9 require nutrition facts in grams and milligrams. FDA drug labeling at 21 CFR §201 requires metric dosing. Nevertheless, retail meat, produce, and other consumer goods are typically priced and sold by the pound. The United States, Liberia, and Myanmar remain the only three nations that have not fully adopted SI. NIST maintains the US customary system alongside SI per NIST Handbook 44.

Question 3

What is a stone as a unit of body weight?

Accepted Answer

The stone (abbreviated 'st') is a British Imperial unit of mass equal to 14 avoirdupois pounds = 6.35029318 kilograms exactly. Its use in commerce was abolished in the UK by the Weights and Measures Act 1985 (c. 72), but the stone persists in the United Kingdom and Ireland as an informal unit for personal body weight - a 12-stone person weighs 168 lb or 76.2 kg. The stone historically had varying values by commodity and region (wool stone of 14 lb, meat stone of 8 lb, glass stone of 5 lb) before standardization at 14 lb for general purposes by British statute. It is not used in the United States or in medical, scientific, or trade contexts, though UK doctors continue to receive patient weights in stones-and-pounds format for reporting purposes.

Question 4

How do I convert body weight to kilograms for medical dosing?

Accepted Answer

Weight-based drug dosing per FDA labeling requirements at 21 CFR §201.57(c)(9) is calculated in milligrams per kilogram (mg/kg). To convert pounds to kilograms: divide by 2.2046 (approximate) or by exactly 2.2046226218 (precise). Worked example: a 150-lb adult weighs 150 / 2.2046 = 68.04 kg, so an intended dose of 10 mg/kg equals 680.4 mg. Ideal Body Weight (IBW) - used in obesity, ICU sedation, and aminoglycoside dosing - uses the Devine formula (Devine 1974): male IBW = 50 + 2.3 × (height in inches over 60); female IBW = 45.5 + 2.3 × (height in inches over 60). Adjusted Body Weight (ABW) for obese patients: IBW + 0.4 × (Total Body Weight - IBW), per ASHP consensus recommendations. Pediatric dosing typically weight-based per kg with maximum-adult-dose ceilings from AAP Red Book and Nelson's Pediatric Antimicrobial Therapy.

Question 5

What is troy weight vs avoirdupois weight?

Accepted Answer

Two distinct pound systems exist in modern commerce: (1) the avoirdupois pound - 1 lb = 16 avoirdupois ounces = 453.59237 g - used for general commerce, food, and body weight per NIST Handbook 44; and (2) the troy pound - 1 troy lb = 12 troy ounces = 373.2417216 g - used for precious metals per the Coinage Act of 1792 as codified in Title 31 of the United States Code. 1 troy ounce = 31.1034768 g, versus 1 avoirdupois ounce = 28.349523125 g - troy ounces are about 10% heavier. Gold, silver, platinum, and palladium bullion are traded in troy ounces per London Bullion Market Association (LBMA) Good Delivery Rules for Gold and Silver Bars (current edition 2021). Retail jewelry uses karat for gold purity (24k = 100%, 18k = 75%, 14k = 58.3%) and fineness for platinum (typically 950/1000 or 900/1000 marked per hallmarking standards).

Pounds to Kilograms Converter