Quick answer: The Weights and Measures Act 1985 is the main law on units and quantities in UK trade. It defines the yard and pound against the metre and kilogram, lists which units traders may use, requires trade scales to be tested and stamped, makes selling short weight an offence and gives Trading Standards inspectors their powers.
Most people meet this Act unknowingly: in the stamp on a market trader’s scales, the “℮” on a packet of crisps or the line on a pint glass. This guide walks through what each part of the Act does, which parts have since been repealed or replaced, and what the Product Regulation and Metrology Act 2025 will change. It is part of our wider explainer on how the UK measurement system works in law and everyday life.
What is the Weights and Measures Act 1985?
It is a consolidating Act that brought decades of earlier weights and measures law into a single statute. Its long title is simply “An Act to consolidate certain enactments relating to weights and measures”. It was passed on 30 October 1985 and came into force three months later, at the end of January 1986.
The Act applies in Great Britain. Northern Ireland has parallel legislation, the Weights and Measures (Northern Ireland) Order 1981, which follows the same broad scheme. The 1985 Act has been amended many times since, most significantly by the metric changeover regulations of the 1990s and the packaged goods rules of 2006, so always check the revised text on legislation.gov.uk.
How is the Act organised?
The Act has seven parts and thirteen schedules, but the parts that matter most day to day are Parts II, IV and VI. Part II controls units and equipment, Part IV deals with selling goods by quantity, and Part VI sets up the inspectors who enforce it all.
| Part | Sections | What it covers | Status (October 2026) |
|---|---|---|---|
| I | 1 to 6 | Units of measurement and national standards | In force; section 1 and Schedule 1 to be omitted by the 2025 Act when commenced |
| II | 7 to 17 | Units, weights and measures lawful for trade; testing and stamping of equipment | In force; section 8(1)(a) to be omitted by the 2025 Act |
| III | 18 to 20 | Public weighing or measuring equipment | In force |
| IV | 21 to 46 | Transactions in goods: quantity rules, short weight, defences, inspectors’ powers | In force; section 29 repealed in 2008 |
| V | 47 to 68 | Packaged goods | Repealed on 6 April 2006 |
| VI | 69 to 78 | Local weights and measures authorities, inspectors, fees | In force |
| VII | 79 to 99 | Offences, penalties, Northern Ireland and general provisions | In force |
Which units does the Act allow for trade?
Section 8 allows only the units listed in Parts I to V of Schedule 1, which are almost all metric. Using any other unit for trade is an offence. The imperial survivors in those parts come with conditions attached:
- The pint may be used only for draught beer or cider, or for milk in returnable containers.
- The troy ounce may be used only for transactions in gold, silver and other precious metals.
- The metric carat may be used only for precious stones and pearls.
Part VI of Schedule 1 defines a long list of other imperial units, from the mile and acre to the stone, gallon and grain, under a heading that says they “may not be used for trade except as supplementary indications”. Section 8 allows such a supplementary imperial figure next to a metric one, provided the metric indication is the more prominent. That is why a packet can say 500 g (1 lb 1.6 oz) but not the other way round. The story of how this came about is told in our guide to when the UK went metric.
Section 1 also matters, even though few people read it. It defines the yard as exactly 0.9144 metres and the pound as exactly 0.45359237 kilograms, which is the legal basis for every exact conversion between the two systems, from pounds to kilograms to inches to centimetres.
What does the Act say about scales and measuring equipment?
Equipment used for trade must be passed as fit for use by an inspector or approved verifier and stamped to show it. Section 11 makes it an offence to use unstamped equipment for trade, and the equipment can be forfeited. If a stamped instrument is dismantled and reinstalled, it must be retested before it goes back into use.
In practice, much new weighing equipment now reaches the market already assessed under separate product regulations, such as the Non-automatic Weighing Instruments Regulations 2016, and carries conformity markings rather than a local inspector’s stamp. Either way, trade equipment must be of an approved type and accurate within prescribed limits.
What counts as short weight under the Act?
Section 28 makes it an offence to deliver less than the quantity a customer paid for, when goods are sold by weight, measure or number. A greengrocer who charges for 1 kg of apples but hands over 950 g commits the offence, as does a pub that serves a pint well short of the line. Section 30 creates a parallel offence where goods in a marked container hold less than the quantity stated on it.
The Act gives traders several defences:
- Due diligence (section 34): the trader took all reasonable precautions and exercised all due diligence to avoid the offence.
- Warranty (section 33): the trader bought the goods with a written warranty about their quantity and had no reason to doubt it.
- Subsequent deficiency (section 35): the shortfall arose afterwards, for example through evaporation, and reasonable allowance had been made for it.
Section 37 tells courts to look at the average quantity across all the items tested and to disregard any inconsiderable deficiency or excess. Section 29, which covered misrepresenting quantities, was repealed in 2008; misleading claims are now dealt with under general consumer protection law, currently Part 4 of the Digital Markets, Competition and Consumers Act 2024.
What happened to the packaged goods rules?
Part V of the Act was repealed on 6 April 2006 and replaced by the Weights and Measures (Packaged Goods) Regulations 2006. These regulations run the “average system” for prepacked goods between 5 g and 25 kg (or 5 ml and 25 litres), and packers must meet three rules:
- The actual contents of the packages must not be less, on average, than the stated (nominal) quantity.
- Only a small proportion of packages, in general no more than 2.5%, may fall short by more than the tolerable negative error (TNE).
- No individual package may be short by more than twice the TNE.
| Stated quantity | Tolerable negative error | No pack may contain less than |
|---|---|---|
| 100 g | 4.5 g (4.5%) | 91 g |
| 250 g | 9 g (3.6%) | 232 g |
| 500 g | 15 g (3%) | 470 g |
| 1 kg | 15 g (1.5%) | 970 g |
| 2 kg | 30 g (1.5%) | 1,940 g |
Packers using the average system can mark their packs with the “℮” symbol, which must be at least 3 mm high and appear in the same field of vision as the quantity.
Who enforces the Act and what are the penalties?
Local councils enforce the Act through their Trading Standards services, acting as local weights and measures authorities. Section 69 names the councils responsible, section 72 requires each to appoint a chief inspector and qualified inspectors, and section 70 requires annual reports on enforcement work. Nationally, the Office for Product Safety and Standards handles market surveillance of weighing and measuring instruments.
Penalties vary by offence. Under section 84, offences not given a specific penalty carry a fine of up to level 5 on the standard scale, which has been unlimited in England and Wales for offences committed since 12 March 2015. Courts can also order the forfeiture of unlawful weighing or measuring equipment.
Is the Weights and Measures Act 1985 being replaced?
Parts of it will be, under the Product Regulation and Metrology Act 2025, but the key changes are not yet in force. The 2025 Act received Royal Assent on 21 July 2025. Section 5 gives the Secretary of State power to make “metrology regulations” covering units of measurement, the quantities in which goods are sold, marking and packaging. Section 11(3) will omit sections 1, 8(1)(a) and 25 and Schedule 1 of the 1985 Act, so that units are defined in the new regulations instead.
As of October 2026, legislation.gov.uk shows those repeals as not yet commenced, so the 1985 Act still governs which units are lawful for trade. One protection is already written into the new Act: section 5(4) says ministers may not use the new powers to prevent or restrict the use of the pint for draught beer, cider or milk in returnable containers.
If you suspect a short measure, contact your local Trading Standards service, usually through the Citizens Advice consumer helpline. For definitions of the units themselves, metric and imperial, see the units of measurement guide.
Frequently asked questions
Does the Weights and Measures Act 1985 apply in Northern Ireland?
Mostly not. The Act applies in Great Britain, and Northern Ireland has its own parallel law, the Weights and Measures (Northern Ireland) Order 1981, which follows the same broad approach.
Is it illegal to sell in pounds and ounces?
It is illegal to sell in pounds and ounces alone. Under section 8 a trader may show pounds and ounces as a supplementary indication, provided the metric quantity is shown and is the more prominent of the two.
What is the penalty for short weight?
Short weight under section 28 is a criminal offence. Many offences under the Act carry a fine of up to level 5 on the standard scale, which is unlimited in England and Wales, and unlawful equipment can be forfeited.
Who enforces weights and measures law?
Local authority Trading Standards services, acting as local weights and measures authorities with qualified inspectors. The Office for Product Safety and Standards oversees national metrology and market surveillance of instruments.
What does the ℮ mark on packaging mean?
It shows the pack was filled under the average system in the Weights and Measures (Packaged Goods) Regulations 2006. Packs must contain at least the stated quantity on average, with strict limits on how short any single pack can be.
Sources
- Weights and Measures Act 1985 (revised text), legislation.gov.uk
- Product Regulation and Metrology Act 2025, legislation.gov.uk
- Packaged goods: average quantity, Business Companion (Chartered Trading Standards Institute)
- Weights and measures: business guidance, GOV.UK
- Weights and measures: the law, GOV.UK