There are rules which regulate the way businesses weigh, measure and label packaged goods.
Businesses must comply with three rules:
the average contents of packages should not be less than the nominal quantity
the proportion of packages that are short of the stated quantity by a defined amount - the tolerable negative error (TNE) - should be less than a specified level
no package should be short by more than twice the TNE
Businesses must ensure packages meet the three rules - either by using prescribed equipment to make up each package or checking sample packages. Businesses that check each package are not obliged to keep records, but if no records are kept each package must contain at least the stated minimum.
Businesses that check the contents by sampling must use a system that ensures packages meet the three rules. They must keep records of their checks for at least one year from the date the packages leave the premises, or until the date by which the contents must be consumed if indicated.
Approved equipment should be marked 'CE', with a black 'M' on a green background and a four-digit number. A black ‘M’ with the year of approval on a white background is also permitted. Your equipment supplier should be able to give you more information.
The weight of any package must not include the weight of the packaging. In most circumstances you must sell by metric weights and measures.
Northern Ireland Trading Standards officers can advise you. They also make occasional checks to ensure you're following the rules. If you break the rules, you could face a large fine or even a prison sentence.
All pre-packed goods (with the exception of wines and spirits) can now be packaged in any size.