pound per imperial gallon
Densityexact by definition
The pound per imperial gallon is exactly 99.77637266... kilograms per cubic metre, close to 99.776. It is the older British density and fuel unit, and it was chosen deliberately: the imperial gallon was originally defined as the volume of ten pounds of water, so water is very nearly exactly 10 lb/imp gal.
Watch out: The US gallon is a fifth smaller, so water is 8.34 lb/US gal against 10 lb/imp gal. Fuel density figures moved between British and American documents without checking the gallon are wrong by 20 percent.
| 1 lb/imp gal | 99.776373 kg/m³ |
The 1824 imperial gallon was defined as the volume of ten avoirdupois pounds of distilled water at 62 °F, which is the whole reason for the number ten. The 1985 redefinition made it exactly 4.54609 litres and broke the round relationship by a fraction of a percent.