Convert Pounds to Kilograms

About Pounds to Kilograms Conversion

Pounds (lb) and kilograms (kg) are perhaps the most common units of mass measure in their respective systems of measure. An accurate conversion between the two is therefore important for accurate trade, engineering, and science. Recognizing this fact, six nations convened in 1959 to agree upon an exact conversion between the pound and the kilogram.

The United States, the United Kingdom, Canada, Australia, New Zealand, and South Africa signed an international yard and pound agreement on July 1, 1959. This agreement established the common pound from the avoirdupois system as exactly 0.453 592 37 kilograms. It can also be referred to as the international pound.

International Pound

Adoption of the international conversion became effective in the United States on July 1, 1959. Other countries that signed the agreement passed laws at various times to make the precise conversion legal. Most commonwealth nations having already agreed to the conversion in 1951.

The agreement between these six nations established not only a link between the pound and the kilogram but a link between the metric system and all mass in the avoirdupois system. All other weights in the avoirdupois system can be expressed in a ratio to the pound and therefore in a ratio to the kilogram.

For instance there are 16 ounces in a pound or put another way an ounce is ¹/₁₆ a pound. Therefore, an ounce is ^0.45359237/₁₆ a kilogram.

Cautions

In common speech pounds may refer to a mass or a force. It is important to recognize that this particular conversion is only applicable to mass, or when speaking of pounds the term weight may be used. Using the term pounds-mass helps avoid confusion. The abbreviation lbm or lb_m may be used; the m indicated this is a mass.

This conversion only applies to pounds originating from the avoirdupois system. This is by far the most common pound in use, however it is worth noting some precious metals may be weighed in pounds from the troy system. This is especially true of silver. Troy pounds are often identified by the symbol lb t.

Lastly, it is important to note that this conversion employes a large number of significant figures (decimal places). Simplified conversion tables may cite a conversion factor or 2.2 pounds to a kilogram. This conversion factor makes calculations simple and is fine for some applications, especially body mass of adults where daily fluctuations are likely to exceed any errors in calculations. Please be mindful of your significant figures and use a more accurate calculation depending on your application.

Conversion calculation hacks

If you can forgo some accuracy a rounded conversion factor can be used to do a calculation mentally. A simplified version of pounds to kilograms calculation uses a conversion factor of 2.2 pounds in 1 kilogram. To use this shortcut you only need to be able to calculate a half and do a little subtraction.

Note the precise value of 120 pounds in kilograms is 54.4310844. Fairly close for a calculation that can be done mentally in a lot of cases.

This back-of-the-envelope type calculation gives a rough estimate that is pretty close. Depending on how much error you can accept in your calculation this simplified method may or may not be accurate enough. Use caution as noted above.

Reference

• Canadian Journal of Physics, 1959, 37(1): 84, 10.1139/p59-014