Guest Post
By James W. Way
Besides our slow adoption of the metric system, the United States differs from the rest of the industrialized world in another way. We use US Letter (8½ × 11 inch paper), while the vast majority of other nations use A4, a paper size created in Germany in the 1920’s. Converting to inches, the dimensions for A4 are approximately 8¼ × 1111/16 (US Letter is slightly wider, but not as tall). A4 is officially defined as 210 mm × 297 mm; after converting to millimeters, US Letter is 216 mm × 279 mm.
The U. S. has a national standard for metric paper, ANSI/ASME Y14.1M, which gives identical dimensions for the A sizes, but does not go smaller than A4. There exists an A5 size, common for notepads, with A6 used for postcards (as the numbers get larger the sizes become smaller). A4 is part of a whole series of A and B paper sizes defined by ISO 216 (the International Organization for Standardization). I will summarize the advantages of these metric sizes, even though the Metric Maven has written on this topic before (see The Metric Paper Tiger from 2014-02-10).
Two sheets of 8½ × 11 inch paper equal one 11 × 17. Enlarging an image from the former onto the latter, however, results in different margins.
Combining two sheets of A4 side by side equals one A3. Also, the margins will be correct when enlarging an image from A4 onto A3. Why is this so? Because metric sizes use the only height to width ratio where this will work: H = W × √2 (height = width × the square root of two, or 1.4142).
After the French Revolution, some sizes with this aspect ratio were created, but never became widely known. In 1911, an institute called Die Brücke (The Bridge) was founded in Munich, which attempted to standardize paper formats. Sixteen sizes were created, for everything from postage stamps to books: size I – 1 cm × 1.41 cm, size II – 1.41 cm × 2 cm, etc.
Die Brücke only lasted a few years before going bankrupt. After World War I, a former associate named Dr. Walter Porstmann improved on the original concept, numbering the sizes in the opposite direction. A0 is a sheet with an area of one square meter (but a 1: √2 aspect ratio). Dividing this in half results in two A1 sheets, and so on. Thus, A4 is one sixteenth of a square meter; if the listed weight on a ream of paper is 80 g/m2, one A4 sheet is 5 g.
In 1922, these sizes became a DIN standard (Deutsches Institut für Normung, or the German Institute for Standardization) and gradually spread throughout the world.
An unfolded A4 sheet fits in a corresponding C4 envelope. The B sizes are mostly used for books, and don’t have their own envelopes, as shown below.
Another resource for information on ISO paper and envelopes is Markus Kuhn’s excellent webpage, International Standard Paper Sizes. He writes:
The ISO paper sizes are based on the metric system. The square-root-of-two ratio does not permit both the height and width of the pages to be nicely rounded metric lengths.
This can be seen in the figure above. With A and B paper, at least one side is a nice metric length, but this is not the case with C envelopes.
Envelope Name Common Use Dimensions (mm)
| C4 | A4 unfolded | 229 × 324 |
| C5 | A4 folded in half | 162 × 229 |
| C6 | A5 folded in half | 114 × 162 |
However, in a strange coincidence, the three most common C sizes do convert nicely to inches:
Name Inches Exact Conversion Rounded to mm
| C4 | 9 × 12 ¾ | 228.6 × 323.85 | 229 × 324 |
| C5 | 6 ⅜ × 9 | 161.925 × 228.6 | 162 × 229 |
| C6 | 4 ½ × 6 ⅜ | 114.3 × 161.925 | 114 × 162 |
A manufacturer would be justified in labeling these by their correct metric dimensions even if they used inches in-house. The US already has 9 × 12 and 6 × 9 envelopes; making C4 and C5 would not pose much of a problem.
Of course, a letter is most commonly folded in thirds and sent in a business envelope. How big is an A4 sheet when folded in exact thirds? The height is 297 mm, so 297 ÷ 3 = 99, with a width of 210 mm.
The most common metric commercial envelope is DL (110 mm × 220 mm); there is 11 mm of room above if folded in perfect thirds. DL originally stood for DIN Lang (DIN length); it is separate from the A, B, and C sizes.
Now, how much extra room is there for US Letter in a #10 envelope? First, let’s divide 11 inches by 3 and …if you’re a teacher, watch your students fumble around with this one! Of course, things become much easier when inches are converted to millimeters.
US Letter is 279 mm high, so 279 ÷ 3 = 93. The height of #10 is 4⅛ inches: 4.125 × 25.4 = 104.775 mm. After rounding this up: 105 – 93 = 12 mm.
The two sizes leave a similar amount of extra space relative to the size of paper that is being used.
When sending a business letter, however, most people do not fold it in exact thirds. There will be some space between the top (folded down) edge of the letter and the bottom crease. If you have difficulty estimating this by sight, the following method is a good option.
Put the bottom of an A4 letter against the inside flap of a DL envelope, as shown in the photo below.
The section not resting on the envelope is 187 mm high (297 – 110 = 187). Fold the top third of the page (at the bottom in the photo) up to the lower edge of the envelope. To calculate the size of this fold: 187 ÷ 2 = 93.5 mm.
What is the height of the remaining two thirds of the paper when we rotate it right side up? 297 – 93.5 = 203.5 mm. Now, fold the bottom of the page up to a few millimeters below the first fold, and the letter will fit nicely. A similar method will work for US Letter inside #10.
DL leaves only 5 mm of room on either side for an A4 sheet – quite narrow for automatic insertion. Since US Letter is 216 mm wide, DL (at 220 mm) cannot be used as an envelope for both.
ISO 269 (Correspondence envelopes – Designation and sizes) contains the following note about Universal Postal Union regulations:
When processing size A4 documents in inserting machines, the size of DL envelopes may be insufficient. To satisfy the needs for automatic insertion, an envelope size larger than DL may be used as long as the size can be considered standardized according to UPU regulations. (Upper limit is at present 120 mm × 235 mm.)
In Australia, the upper limit above corresponds exactly to the DLX size, allowing DL to fit inside as a reply envelope. The “X” probably stands for maximum, but also brings to mind “XL” as an abbreviation for extra-large, (even though the two letters are reversed).
Between the two is an intermediate DLE size (114 mm × 225 mm) that also fits inside DLX, though the smaller DL is used with automatic machines. DLE converts to inches quite easily.
Inches Exact Conversion Rounded to mm
| 4 ½ × 8 ⅞ | 114.3 × 225.425 | 114 × 225 |
The above dimensions, when compared with American envelopes, are equal to the short side of #11 and the long side of #9. A DLE envelope can be used for both US Letter and A4, being slightly wider than DL.
Yes, these conversion tables conflict with Pat Naughtin’s philosophy of “don’t duel with dual.” But it helps U.S. manufacturers to know that some international sizes have similar dimensions to what they already produce.
Here are some other sizes worth mentioning.
In Germany, C6/5 is popular, using the short side of C6 (114 mm) and the long side of C5 (229 mm). The U.K. prefers to name this size DL+. Italians use an envelope 10 mm wider than DL (110 mm × 230 mm). These are each fine by themselves, but can’t work together as a reply/outer envelope like the three Australian sizes.
Statistics compiled by the Envelope Manufacturers Association (EMA) show that U. S. sales peaked in 2005. In our electronic age, this market has declined, with total sales now similar to the mid 1980’s. Here are some places to buy in the U. S., if you are so inclined.
ISO envelopes are sold by amazon.com, but only the most common sizes: C4, C5, C6, DL, as well as a variety of metric paper.
Another excellent resource is Empire Imports. While they do not sell envelopes, they specialize in metric paper and related products, stocking items such as hole punches, folders, binders, etc.
Finally, some fountain pen dealers stock a limited number of metric sizes, since these types of pens work best with high quality European and Japanese stationery. A good example is The Goulet Pen Company.
These are my personal observations; I have no financial relationship with any of these sellers.
Related Essay:
If you liked this essay and wish to support the work of The Metric Maven, please visit his Patreon Page and contribute. Also purchase his books about the metric system:
The first book is titled: Our Crumbling Invisible Infrastructure. It is a succinct set of essays that explain why the absence of the metric system in the US is detrimental to our personal heath and our economy. These essays are separately available for free on my website, but the book has them all in one place in print. The book may be purchased from Amazon here.
The second book is titled The Dimensions of the Cosmos. It takes the metric prefixes from yotta to Yocto and uses each metric prefix to describe a metric world. The book has a considerable number of color images to compliment the prose. It has been receiving good reviews. I think would be a great reference for US science teachers. It has a considerable number of scientific factoids and anecdotes that I believe would be of considerable educational use. It is available from Amazon here.
The third book is called Death By A Thousand Cuts, A Secret History of the Metric System in The United States. This monograph explains how we have been unable to legally deal with weights and measures in the United States from George Washington, to our current day. This book is also available on Amazon here.
You seem to have missed the the C6/C5 size it has the width of C5 and the height of C6, which is optimal for A4 folded in thirds.
Actually, I mention C6/5 toward the end of the article (the paragraph that begins with “In Germany…”). I know C6/C5 was the original name, but it seems more common nowadays to drop the second “C”. Thanks for your interest.
Good article. A couple of points.
ANSI Y14.1 and 14.1M define Customary and metric drafting sheet sizes. In drafting, paper sizes smaller than US letter and A4 are rarely, if ever, used. I don’t know if there are standards for smaller sizes, half-letter, notepads, etc or if they just arose by convention.
C size envelopes don’t fit too gracefully into the USPS postal system. C4 is OK and similar to 9″x12″ envelope, both are “flats,” known as “large letter” in some other postal systems. However C5 is also a “flat” (pays double the postage for first ounce) because it is over the max. height 6 1/8″ for “letter” rate. Note that a 6″ x 9″ envelope squeaks by at “letter” rate. C6 is OK at letter rate. C7 is not mailable as its height is less than 3.5″ minimum height.
If you want a common envelope for letter and A4, it probably needs to be DL-derived. The DLX length of 235 is probably needed for machine insertion of letter paper, although it might be possible at 229-230 mm (DL+, C6/5). Any of the heights from 110 mm to 120 mm would work. 110 x 235 mm would be a good replacement for the ubiquitous #10 envelope.
As cardstock, A6 would be mailable at postcard rates, A5 at letter rates.
It must be a real hoot when tonnes of letters arrive in the US daily from around the world in one of the C sizes and the USPS has a problem handling them. One would think the USPS would update their systems to handle the sizes the world uses. Or do they deliberately want to keep the machines they way they are as a means to complain?
They just handle it (as a flat) but the postage paid is based on foreign rate and whether it qualifies as flat or letter in its home market.
Our #10 envelope counts as a letter in the US, but would be a flat in most any other postal system in the world; the rate is still based on US overseas rate for a letter, and they get delivered.
And if you want to mail a flat in the US, they don’t complain, they are completely happy charging you twice as much for the 1st ounce (additional ounces are the same rate as a letter), so basically, it is an “oversize” charge.
The limits are just for postal rates; they can handle flats up to the max and packages to 72 lb.
The problem probably isn’t the physical handling but that the price list is in inches (and grams, IIRC!) and the band boundaries are set just below the common metric sizes. AIUI, for cross-border mail, there is some sort of international treaty—or lots of bilateral ones—whereby the sender pays in the originating country, against their metric-optimised price list, and the recipient country just delivers the incoming letters without seeing any of the money. This is a quid pro quo so the rest of the world gets to process inch sized and priced letters outgoing from USPS. In the UK as children, we used to derive much amusement from writing addresses and sticking stamps on weird shaped/ sized objects such as pencils and posting them to our friends 😈. The post office never used to bat an eyelid at this, so I don’t suppose envelopes a few tens of millimetres different hold any fear for them or their sorting machines!
Regarding The Metric Maven’s list of stationery suppliers, metric paper sizes are often associated with [metric] hole punching to ISO 838, much to the annoyance of people with USA ring binders…
ISO 5457 stipulates a left margin of 20 mm and right margin of 10 mm for drawing frames and ISO 7200 [since 2004] says the title block is always 180 mm wide regardless of sheet size. So A5 landscape would be the theoretical smallest, possibly for 1:1 drawings of a pencil in the mail? A4 is stated as being the only permitted size for portrait orientation and was not permitted in landscape until 2010, implying that was considered the smallest useful size. BS 8888 slightly relaxes some of that, but in practice larger sheets are invariably folded down to A4 and not A5 for filing/ mailing according to the old DIN/ BIS standard—so presumably larger portrait sheets have to be treated as a [contrived] ‘“short” series’?
Coincidentally, the real ‘long series’ is designated (ISO 5457) with a dot. So a width of C5 long edge and height of C6 short edge would logically be designated C6.5 while ‘long sizes’ <sic> are designated (ISO 216) with slashes and always parallel to the short edge, e.g. 1/3 A4 for letters—although I have also seen that written as A4/3, so C6/5 might be misconstrued as 114 mm × 32 mm 😬… ISO 216 itself only states the dimensions (in millimetres, of course) for 4A0 down to A10 and B0 down to B10, but also gives the formulae for extending the range upwards or downwards and ISO 3272-4 cheerfully explains how to microfilm much larger drawings than those!