C4 envelope size: the A4 envelope

What a C4 envelope is for

A C4 envelope measures 229 × 324 mm (9 × 12¾ in), which leaves a little room around an A4 sheet so it slides in without being folded. That is why most people call it the A4 envelope, and why our envelope size chart lists its use as A4 flat. You reach for one when a crease would spoil what you are sending: a CV, a signed contract, a certificate or a few photographs.

The name comes from the ISO C series of envelopes. The series is built around the A paper sizes so that each envelope holds the sheet with the same number: C4 for A4, C5 for A5, C6 for A6. A plain C4 takes a modest stack of paper, and for a bound report there are gusseted C4 envelopes whose sides fold out to make room. A full gusseted envelope or a bound report can, however, end up thicker than a UK Large Letter or a US flat may be, or too lumpy for a flat, and then it is posted as a parcel, so check the thickness limits in the postage section below.

C4, C5 or DL: how much to fold

The same A4 letter fits three common envelopes, and which one you use depends on how many times you fold it. In a C4 it goes in flat. A C5 envelope takes it folded once, and a DL envelope takes it folded in three, which is the usual choice for letters and invoices.

The fold also changes the postage, because postal services price by size before weight. A DL envelope with a few sheets counts as an ordinary letter in both the UK and the US, but a C4 is too big to count as a letter in either country. That means the same three pages cost noticeably more to post flat. If the reader won't mind a fold, DL is cheaper, but for a job application or a certificate the C4 is usually worth the extra.

Which postage format a C4 envelope counts as

For a standard letter, Royal Mail sets these limits: up to 24 × 16.5 cm, 5 mm thick, 100 g. An empty C4 is already bigger than that in both directions. Even a single sheet in a C4 therefore travels as a Large Letter, so it needs a Large Letter stamp rather than an ordinary 1st or 2nd Class one. A Large Letter has far more room, with these limits: up to 35.3 × 25 cm, 2.5 cm thick, 750 g. The UK stamp prices page lists what a Large Letter costs at each weight.

USPS draws the same line, with different names. A C4 is longer and taller than the largest US letter, so it mails as a large envelope, also called a flat. Flats are priced by the ounce and start well above the letter price, so don't guess how many Forever stamps to use: look up the flat price for your envelope's weight on the US stamp prices page and make up that amount, or pay at the counter. A US flat has to meet these conditions: bigger than a letter, up to 15 × 12 in, ¾ in thick, 13 oz; must be flexible and evenly thick. The last part matters if you add stiff card to protect photographs, so ask at the counter how a rigid envelope will be charged.

Here is a worked example. Ten sheets of ordinary printer paper weigh about 50 g, and a C4 envelope adds 15 to 25 g, so a typical job application comes to around 70 g, or about 2½ oz. That is well within the weight limit in both countries, but the price depends on the weight step. In the UK, 70 g is still within the first Large Letter step, so one Large Letter stamp covers it, while a heavier C4 needs more postage than a single Large Letter stamp. In the US, 2½ oz is charged at the 3 oz price for a flat, not the 1 oz price. Weigh anything you post in a C4 before you buy stamps.

Printing the address on a C4 envelope

Most home printers take paper up to 216 mm (8.5 in) wide. Even the short side of a C4 is wider than that, so turning the envelope round doesn't help. That is why the buttons on this page open a sheet of address labels: you print the address on a label and stick it on the front. The sheet has 21 labels on A4 paper, and the editor also has US Letter sheets, such as 30 labels per sheet (the Avery® 5160 size). For a bigger mailing you can import an address list from Excel, a CSV file or Google Sheets and print one label per address. Mail merge needs a free account.

If your printer takes A3 or 11 × 17 in paper, it can probably take a C4 as well, and the editor has the C4 envelope size with the address blocks already laid out. Feed the envelope short edge first, because the long side is too long even for those printers, and use the rear or manual feed tray if there is one. Printers differ, so test on a spare envelope first to see which way up it goes and which way the flap should face.