Letter and A4 are close enough to cause trouble
US Letter is 8.5 by 11 inches, which is 215.9 by 279.4 millimetres exactly, since an inch is defined as 25.4 mm. A4 is 210 by 297 millimetres, which is 8.268 by 11.693 inches. A4 is 5.9 mm narrower and 17.6 mm taller — about a quarter inch and two thirds of an inch. By area A4 is 3.4 percent larger.
Those differences are small enough that a page looks right at a glance and large enough that a layout breaks. Print an A4 document on Letter at full size and 0.69 in disappears off the bottom, which usually takes the last line or the page number. Let the driver scale it to fit instead and everything shrinks to 94.1 percent, so a form designed with a 40 mm signature box now has a 37.6 mm one, and anything measured off the printed page is wrong. Going the other way, Letter onto A4, loses 0.23 in of width and leaves a band of blank paper along the bottom.
For a document that will genuinely be printed in both regions, the reliable approach is to keep all live content inside the intersection of the two sheets: 8.27 in wide by 11 in tall, or 210 by 279.4 mm. Set the page to whichever size dominates and pull the margins in so nothing important sits in the disputed strip. That is why a great many international forms have margins that look oddly generous.
Why the A series halves so neatly
ISO 216 fixes two things and derives everything else. The sides are in the ratio 1 to the square root of 2, and A0 has an area of one square metre. Solve those together and A0 comes out at 841 by 1189 mm, since 841 times 1189 is 999,949 square millimetres, a rounding whisker under a square metre.
Halving the long side of any A size gives the next one down, and because the ratio is root 2 the shape is preserved exactly. A4 is 210 by 297; half of 297 is 148.5, rounded down to whole millimetres as the standard requires, giving A5 at 148 by 210. Halve again for A6 at 105 by 148. Two A4 sheets side by side make an A3.
| Size | mm | Inches | Sheets per A0 |
|---|---|---|---|
| A0 | 841 x 1189 | 33.11 x 46.81 | 1 |
| A1 | 594 x 841 | 23.39 x 33.11 | 2 |
| A2 | 420 x 594 | 16.54 x 23.39 | 4 |
| A3 | 297 x 420 | 11.69 x 16.54 | 8 |
| A4 | 210 x 297 | 8.27 x 11.69 | 16 |
| A5 | 148 x 210 | 5.83 x 8.27 | 32 |
| A6 | 105 x 148 | 4.13 x 5.83 | 64 |
The practical consequence is the pair of numbers on every photocopier in the world outside North America: 141% to go up a size and 71% to go down, being root 2 and its reciprocal. Nothing is clipped and nothing is left over at either setting. US sizes have no equivalent because Letter at 1.294:1 and Tabloid at 1.545:1 are different shapes, so Letter onto Tabloid is 129.4% with a strip of waste rather than a clean doubling.
Fixing A0 at one square metre also makes paper weight trivial. Basis weight in gsm is grams per square metre, so one A0 sheet of 80 gsm paper weighs 80 grams and an A4 sheet weighs one sixteenth of that, 5 grams.
The two ways paper weight is quoted
Outside the US, paper weight is gsm — grams per square metre — and it means the same thing regardless of the sheet size. In the US, paper is sold by basis weight in pounds, which is the weight of 500 sheets of that grade in its own basic size. The catch is that the basic size differs by grade. Bond and writing paper use 17 by 22 in, cover stock uses 20 by 26 in, index uses 25.5 by 30.5 in. This is why 20 lb bond and 50 lb offset are the same thickness of paper, and why 65 lb cover is much heavier than 65 lb text.
| US designation | gsm | What it is |
|---|---|---|
| 20 lb bond | 75 | Standard copier and printer paper |
| 24 lb bond | 90 | Letterhead, resumes, double-sided work |
| 28 lb bond | 105 | Heavy letterhead, presentation |
| 32 lb bond | 120 | Brochure text weight |
| 65 lb cover | 176 | Light card, postcards, tabbed dividers |
| 80 lb cover | 216 | Business cards, covers |
| 110 lb index | 199 | Stiff card, filing |
Converting a bond figure to gsm is a multiplication by roughly 3.76. Because gsm is unambiguous and basis weight is not, printers increasingly quote both, and specifying gsm on an order removes an entire category of misunderstanding.
A ream of 500 Letter sheets at 20 lb bond weighs about 5 pounds. A single sheet is about 4.5 grams, or 0.16 ounces, which is where the postal rule of thumb comes from: four sheets and a #10 envelope sit inside the first-class one-ounce step, five will often push past it.
Margins, and the border you cannot print in
Almost every inkjet and laser printer has an unprintable border where the feed rollers grip the sheet, typically 0.12 to 0.2 in on the sides and sometimes more at the trailing edge. Setting margins to zero does not defeat it; the content simply gets cut. Borderless printing is a feature of specific photo printers that deliberately overspray the edge, and it is not available on ordinary office machines.
The one-inch default in Word and Google Docs is not only about looks. On Letter, one-inch side margins leave a 6.5 in measure. At 11 point body text that is roughly 90 characters per line, which is already above the 45 to 75 range typographers generally recommend, because a long measure makes the eye lose its place returning to the start of the next line. Documents with wide margins are easier to read, not just prettier. For anything that will be bound, add 0.25 to 0.5 in to the inside margin so the gutter does not swallow the text.
Folding and envelope fit
A Letter sheet folded in thirds along the long side gives 8.5 by 3.67 in. Both the #9 reply envelope at 8.875 by 3.875 in and the #10 business envelope at 9.5 by 4.125 in were dimensioned around that fold, the #9 being deliberately small enough to sit inside a #10. Folded in half it becomes 8.5 by 5.5 and needs a 6 by 9 booklet envelope. Unfolded it needs a 9 by 12 catalog envelope.
A4 folded in thirds is 8.27 by 3.9 in, or 210 by 99 mm, which is the DL envelope at 220 by 110 mm. A DL and a #10 are close but not identical, and A4 in thirds is a little taller than Letter in thirds, so it goes into a #10 with under 6 mm to spare. It fits, but not comfortably with several sheets.
The clearance used in the calculator is about 4 mm per dimension, which suits one to three sheets. A thick stack needs more, and anything with a clip or a folded insert needs the next envelope up.
Sizes not listed here
The ISO B series sits between the A sizes, defined as the geometric mean of two consecutive A sizes, giving B5 at 176 by 250 mm. The JIS B series used in Japan is defined by area instead and gives a different B5 at 182 by 257 mm. Neither appears above because neither is common in US work, but if a supplier quotes a B size it is worth asking which of the two they mean, since the difference is around 6 mm.
Book and photo trim sizes are their own world, driven by how many pages fit on a press sheet rather than by any series. If a measured sheet does not match anything here, a trim size is the most likely explanation after a plain miscut.
Also worth a look
Print resolution applied to photographs rather than documents, including how much a 4:3 source loses on a 4 by 6 sheet, is in the photo print calculator. The same pixels-per-inch division applied to screens is the PPI calculator, and screen size calculator converts a diagonal measurement into width and height. To change an image's actual pixel dimensions, image resizer does it in the browser without uploading. General length, area and weight conversions are in the unit converter.
Questions people ask
What is A4 in inches?
8.268 by 11.693 inches, from 210 and 297 millimetres divided by 25.4. Rounded for practical use it is 8.27 by 11.69, and rounding it further to 8.5 by 11 turns it into Letter, which is a different sheet. The reverse conversion is exact in the other direction: Letter at 8.5 by 11 inches is precisely 215.9 by 279.4 millimetres, since the inch has been defined as exactly 25.4 mm since 1959.
Can I just print an A4 document on Letter paper?
Only if you accept one of two outcomes. At 100 percent, A4 is 0.69 in taller than Letter, so that much falls off the bottom — usually the footer, the page number, or the last line of a paragraph. Set to fit-to-page and the printer scales everything to 94.1 percent, which keeps the whole page but shrinks every measured element, so signature boxes, cut lines and anything sized for a physical purpose come out wrong. For documents that circulate in both regions, keep the content inside the 8.27 by 11 in overlap and neither problem arises.
Why do photocopiers have 141% and 71% buttons?
Because the ISO A series has sides in the ratio 1 to root 2, roughly 1.4142. Going up a size multiplies both sides by root 2, which is 141.4 percent, and going down divides by it, which is 70.7 percent. Nothing is clipped and no paper is wasted at either setting, because the shape is preserved. US sizes have no such factor: Letter is 1.294 to 1 and Tabloid is 1.545 to 1, different shapes, so scaling between them always leaves a strip of blank paper or crops something.
How much does a ream of paper weigh?
A ream of 500 US Letter sheets in 20 lb bond, which is 75 gsm, weighs about 5 pounds or 2.26 kg. The arithmetic is the sheet area in square metres times the gsm figure: Letter is 0.0603 square metres, so one sheet is 4.52 grams and 500 sheets are 2262 grams. Twenty-four pound bond at 90 gsm pushes the same ream to about 6 pounds. A4 at 75 gsm is slightly heavier per sheet at 4.68 grams because A4 has 3.4 percent more area, giving a ream of about 2.34 kg.
What is the difference between gsm and lb for paper?
Gsm is grams per square metre and describes the paper itself regardless of sheet size, so it is directly comparable across every grade. US basis weight in pounds is the weight of 500 sheets in that grade's basic size, and the basic size differs by grade — 17 by 22 in for bond, 20 by 26 in for cover, 25.5 by 30.5 in for index. That is why 20 lb bond and 50 lb offset are the same paper thickness while 65 lb cover is far heavier than 65 lb text. Multiply a bond figure by about 3.76 to get gsm. When ordering, quoting gsm removes the ambiguity entirely.