Our Handling Charges, and your forwarding options, are based upon the very precise size and weight limits below.
Some more information: Although an item of post may be 'Letter' sized on delivery, incurring the lowest handling charge, this same letter may incur Royal Mail postage at the 'Large Letter' rate when forwarded as a single item because the tamper-proof postal bag in which it is enclosed adds overall size. This becomes much less important, of course, if we are forwarding several letters in the same bag!
a letter is... a letter!
To be classed as a Letter, an item must be under 100gm in weight, no larger than 24cm x 16.5cm and less than 5mm in thickness.
when is it a large letter?
The next step up, with a maximum size of 35.3cm x 25cm and up to 2.5cm, (1 inch) thick is the Large Letter format.
A Large Letter can weigh as much as 750gm and might contain things like A4 documents, cased DVDs or magazines.
is this a parcel?
The Small Parcel format, weighing up to 2Kg and with a maximum size of 45cm x 35cm x 16cm, can be carried by Royal Mail 1st Class post to addresses within the UK and abroad, just like the two letter formats.
You can, however, use your mailbox for items up to 20Kg in weight and as large as 61cm x 46cm x 46cm (the Royal Mail's Medium Parcel limits) at no extra charge. . . but they must be forwarded by courier to destinations outside of the UK.