British imperial
units of mass

Great Britain, the British Empire, and the Commonwealth, 1824 to various dates.

The units of mass were based upon a prototype, a particular brass weight called the Imperial Standard Troy Pound, in the custody of the Clerk of the House of Commons.  If that prototype was lost or compromised, the standard was to be restored by taking the mass of a cubic inch of water weighed under specified conditions as equal to 256.458 grains.

   

imperial troy pound┬╣

 

[troy] ounce┬▓

12

 

pennyweight

20

240

grain

24

480

5760

0.06479892
g

1.5551740
g

31.103482
g

373.24177
g

1. Abolished 1878.

2. In 1878 restricted to trade in gold and silver and precious stones, and retail sale of drugs.

The grain found as 1/256.458 of the mass of a cubic inch of water, is then used to define a second set of units, by defining the pound avoirdupois as 7000 grains.

   

pound avoirdupois

 

ounce avoirdupois

16

 

dram

16

256

grain

 27.34375

437½

7000

by 1883 comparison with
Int'l Prototype Kilogram

64.798918
mg

1.7718454
g

28.3495267
g

453.5924277
g

1889 official rounding off
 by British*

64.79892
mg

1.7718454
g

28.349527
g

453.59243
g

by the 1963 Weights
and Measures Act

64.79891
mg

1.7718452
g

28.349523
g

= 453.59237
g

*The 1883 equivalence continued to be used in the United States.  

In the United Kingdom, the Weights and Measures Act of 1963 based British units of mass on the Prototype Kilogram (for the first time) and adopted the international pound.

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