Common Paper Sizes used in the UK

The ISO system is based on three series:-

A sizes

for general printing and stationery. The A series is based on A0. The size, 841 x 1189, is one square metre in area. The weight of the paper is also based on the weight of one square metre in grammes, i.e. 'gsm'.

A0 841 x 1189
A1 594 x 841
A2 420 x 594
A3 297 x 420
A4 210 x 297 (A4 is usually used for business stationery, brochures, booklets etc.)
A5 148 x 210 (A5 is commonly used for smaller items such as leaflets/flyers etc.)
A6 105 x 148 (A6 is the international postcard size.)
A7 74 x 105 (A7 for labels etc.)

B sizes

for posters and some larger envelopes

C sizes

for general envelopes or folders to contain A sizes.
The full sheet in each series is designated by the number 0. The longer length is then halved to give 1, and halved again to give 2 and halved again to give 3 etc. So, A4 is half of A3, A5 is half of A4 etc.

RA sizes

are slightly larger than A sizes, allowing the printer extra space on a sheet for grip and trim.

SRA sizes

are slightly larger than RA sizes, and allow for printed work which includes a 'bleed'.
When expressing the measurements, care should be taken as traditionally the upright measure was given first and the horizontal measurement second. The British Standards Institute recommends giving the shorter measurement first and following the measurement with the letter L on a landscape format. A4 landscape would then be: 210 x 297 L mm.