Post Office

Post Office

METRICATION: 100% complete
Metric Non-metric
Letter and parcel weights in grams and kilograms
Letter and parcel dimensions in millimetres


In 1862, the Select Committee appointed to consider the practicability of adopting a Simple and Uniform System of Weights and Measures, included the following recommendation in their report:

  • The gramme should be used as a weight for foreign letters and books at the Post-office.

Report from the Select Committee on Weights and Measures – 1862

Paper and envelopes

On 15 July 1959, the Post Office became one of the first organisations in the country to announce that it would be adopting standard international paper sizes for all its paper work and publications. At the time of the announcement, the nationwide adoption of the “A” series of paper sizes was being supported by the British Standards Institution, which had already started the process of switching all of its own paper work and publications to the metric-based “A” sizes.

In the Post Office, the adoption of metric-based paper sizes began with the switch to A4 for headed note paper in 1959. Post Office forms changed more gradually – whenever updates were needed, new forms were re-designed using an “A” paper size.

The “B” and “C” series of paper sizes, derived from the “A” series, were also adopted for new standard envelope sizes. In 1959, international standard sized stamped stationery was made available for sale in Post Offices for the first time. This was in the form of a buff coloured “C6” (114 mm x 162 mm) envelope, embossed with a 3d stamp. The new “C6” stamped envelope replaced the smaller “commercial” stamped envelope.

Following the adoption of international standard paper sizes by Her Majesty’s Stationery Office, the “B” and “C” series of envelope sizes, which were designed specifically to take “A” series paper, were subsequently adopted for all OHMS envelopes.

Of the many advantages gained by adopting standard international paper sizes, the main ones that benefitted the Post Office, included a reduction in the variety of sizes and shapes of envelopes used for letters. This in turn eased the introduction of automatic letter sorting equipment.

A reduction in the variety of paper sizes also allowed a reduction in the amount of storage space needed for stocks of paper.

Postal tariffs

In the early 1970s, the Post Office would have been aware of the increasing number of countries committing to the use of metric units for postal tariffs. So, in 1972, when the Government published its official policy on Metrication in a White Paper, it was an opportune time for the Post Office to examine the issue.

On 6 July 1972, it was confirmed in Parliament that the Post Office was considering how best to meet the Government’s aims.

The White Paper included the following points:

  • Government policy – “The Government consider it desirable that British industries on a broadening front should adopt metric units, sector by sector, until that system can become in time the primary system of weights and measures for the country as a whole.”
  • Overseas freight – On 1 January 1972, UK port authorities had switched to metric units as the basis for freight charging. Major UK shipping conferences were also scheduled to complete their adoption of metric tariff structures in 1972.
  • Inland freight – The Freight Transport Association were in the process of planning an industry-wide switch to metric units for all inland freight tariffs.
  • Target dates – “… the end of 1975 should be accepted as the general target date for all provisional metrication programmes.”

In August 1974, following consultation with the Post Office Users’ National Council, the Post Office finally announced its plans to switch postal tariffs to the metric system for all letters and parcels.

Post Office metrication poster – 1975

On 29 September 1975, all weight-based postal tariffs switched to the metric system. The date of the switch, or M-day, was announced well in advance to allow customers that weighed their own letters and parcels plenty of time to switch to metric postal scales. Leaflets and posters were produced to inform businesses and regular customers.

Post Office metrication leaflet – 1975

Inland postal rates leaflet – 1975

For many customers, the switch to the metric system could easily have gone unnoticed. The procedure for letters and parcels to be weighed at the post office sales counter, before purchasing appropriate postage, remained unchanged.

Postal Rates – Inland

Postal Rates leaflets continued to show equivalent imperial weights in parentheses next to the new metric weights for a year or so after metrication, but by 1977, all tariff weights were given in grams and kilograms only.

Letter post – First and second class

Prior to metrication, first and second class weight scales were in steps of 2 oz. Following metrication, steps were in 50 g units, except for the initial value which was set at 60 g.

 17 March 1975 29 September 1975 13 June 1977 
 2 oz 60 g (2.1 oz) 60 g 
 4 oz 100 g (3.5 oz) 100 g 
 6 oz 150 g (5.3 oz) 150 g 
 8 oz 200 g (7.1 oz) 200 g 
 10 oz 250 g (8.8 oz) 250 g 
 12 oz 300 g (10.6 oz) 300 g 
 14 oz 350 g (12.3 oz) 350 g 
 1 lb 400 g (14.1 oz) 400 g 
 1½ lb 500 g (1.1 lb) 500 g 
 2 lb 750 g (1.7 lb) 750 g 
1000 g (2.2 lb)

Post Office metrication leaflet – 1975

Parcel post

Prior to metrication, parcel weight scales were in steps of 2 lb. Following metrication, steps were in 1 kg units.

 17 March 1975 29 September 1975 13 June 1977 
 2 lb 1 kg (2.2 lb) 1 kg 
 4 lb 2 kg (4.4 lb) 2 kg 
 6 lb 3 kg (5.3 lb) 3 kg 
 8 lb 4 kg (7.1 lb) 4 kg 
 10 lb 5 kg (11.0 lb) 5 kg 
 12 lb 6 kg (13.2 lb) 6 kg 
 14 lb 7 kg (15.4 lb) 7 kg 
 16 lb 8 kg (17.6 lb) 8 kg 
 18 lb 9 kg (19.8 lb) 9 kg 
 20 lb 10 kg (22.0 lb) 10 kg 

Post Office metrication poster for businesses – 1975

Stamped stationery

Prior to metrication, the Royal Mail leaflet – Postal Rates Inland specified stamped stationery sizes in inches and fractions. Following metrication, all sizes were in millimetres only. Dual units were used for less than 2 years.

17 March 1975 
 Envelope ‘A’ 5¾” × 3¾” 
 Envelope ‘B’ 9¼” × 4¾” 
29 September 1975 
 A Size 146 mm × 95 mm (5¾” × 3¾”) 
 B Size 235 mm × 120 mm (9¼” × 4¾”) 
13 June 1977 
 Size 1 146 mm × 95 mm 
 Size 2 235 mm × 120 mm 

Letter sizes

In 2006, Royal Mail introduced a new pricing structure for inland letters based on size as well as weight. Letters were split into three types according to size: letters, large letters and packets. At the time, the Post Office had already been fully metric for 30 years, so it was only natural that the new size categories would be specified in millimetres only.

A Post Office letter size guide – 2006

In the UK, paper and envelopes are produced in international standard (ISO) sizes, which are specified in millimetres. Post Office letter size steps were chosen according to common standard envelope sizes and the specifications of automatic letter sorting machines.

Letter type Maximum dimensions
Letters 240 mm × 165 mm × 5 mm
Large Letters 353 mm × 250 mm × 25 mm
Packets any dimension exceeding that allowed for large letters

The new pricing structure simplified the weight categories, and saw the maximum weight allowed for standard letters rise from 60 g to 100 g.