Envelope Template Maker

Pick up any envelope and unfold it carefully along its seams. What lies on the table is not a pouch at all. It is one flat sheet with a rectangle in the middle and four flaps around it, cut to a shape that looks nothing like the envelope it became.

That shape is the whole trick. Get it right and the flaps fold in, overlap where they should, and close over the card with nothing poking out. Get it wrong and you learn about it with glue on your fingers.

Three shapes and a rectangle

Traditional envelopes are cut from one of three shapes: a rhombus, a short-arm cross or a kite. In each case the sheet’s sides fold around a central rectangle, and that rectangle becomes the face of the envelope, the side that carries the address. The four flaps end up on the back. Which flap closes last gives the style its name. When the closing flap sits on a short side the envelope is a pocket, the shape long used for packets of seeds; when it sits on a long side it is the more common wallet, sometimes called standard.

The rhombus is the oldest and the most elegant. Folded diamond-shaped sheets whose four corners met under a single seal were already being called “new-fashioned envelopes” by the late 1830s. When Britain launched its reformed postal service in May 1840, the prepaid Mulready wrappers were printed and sold in sheets of twelve, and cutting them out was the buyer’s job. In 1845 Edwin Hill and Warren de la Rue patented a steam-driven machine that cut the shapes and also creased and folded them. Today’s envelope machines still produce blanks in all three shapes.

The geometry of the diamond is pleasingly exact. A rhombus has perpendicular diagonals that cut each other in half, and its area is half the product of the diagonals. Put a face of width W and height H at the centre and choose diagonals of 2W and 2H. Each corner of the face then lands exactly on an edge of the diamond, and each tip, folded over its face edge, lands exactly on the centre. The blank’s area is 2WH, twice the face, because the four flaps together cover the back once. A square face is the neat special case: the blank is a square of side W times the square root of two, turned 45 degrees.

Tips that only meet at the centre leave nothing to glue, so most diamond envelopes reach further. Grow both half-diagonals by an overlap and each tip passes the centre by that much. Now the corners need a small notch, the little triangle beyond both face lines cut away, or a folded flap would stick out past the edge of the face. The kite is the same diamond with the closing tip pulled longer, a kite in the strict sense: one diagonal is the perpendicular bisector of the other.

Sizing from what goes inside

An envelope is a container, so its size starts with its contents. The face must be larger than the card by a clearance on every side, or the card jams on the way in. It must also allow for the card’s thickness, because the flaps wrap around it rather than lying flat against the face. A simple allowance covers both: add twice the clearance and the thickness to each dimension.

Here is a worked example you can repeat. Take a 5 by 7 inch card, allow 1/16 inch of clearance on each side, and call the card 1/32 inch thick. Each dimension grows by 1/8 plus 1/32, which is 5/32 inch, so the face is 5.15625 x 7.15625 in. That number matters more than it looks. With the tips meeting at the centre, the diamond for this face has diagonals of 10.3125 and 14.3125 inches, more than a Letter sheet can hold inside quarter-inch margins at any angle.

The cross behaves differently. Its blank is the face plus a strip on each side, so its bounding box is easy to read. A wallet for a 9.5 by 4.125 inch face, with side flaps of 0.75 inch, a bottom flap of 2.5 inches and a closing flap of 1.75 inches, comes to 11 by 8.375 inches. Inside quarter-inch printer margins that fits Tabloid but not Letter, Legal or A4. The flaps close because 2.5 plus 1.75 is 4.25 inches, an eighth more than the face height.

The cross also shows why flaps are tapered. Each flap’s side edges leave the fold line at an angle, and when the bottom flap folds up over a side flap the two overlap only if their taper angles add up to more than 90 degrees. That overlap, a narrow wedge running in from each lower corner, is exactly where the glue goes. At 45 and 45 degrees the flaps meet edge to edge and there is nothing to stick.

Standard sizes save the arithmetic when the contents are standard too. ISO 269 defined the C series for envelopes, and although the standard was withdrawn in 2009 without a replacement, it lives on in national standards. Its sizes are built so that a C4 envelope holds an A4 sheet flat and a B4 envelope holds both. North America has its own list, from the A2 at 4 3/8 by 5 3/4 inches to the No. 10 business envelope at 4 1/8 by 9 1/2 inches, which holds a Letter sheet folded in thirds.

Paper, folding and true size

A blank is only as good as the sheet it is cut from. Paper is sold by grammage, grams per square metre, but grammage says nothing exact about thickness. Caliper has to be measured or taken from the paper’s own specification; typical 20 pound bond runs roughly 97 to 114 micrometres, about four thousandths of an inch. That is why the thickness is typed in rather than guessed from the weight.

Printing the template is where many home projects quietly fail. A template is only useful at true size, and a web page cannot guarantee that. The browser’s print method simply opens the print dialog; it takes no settings at all, so nothing on the page can stop a printer driver from shrinking the sheet to fit its margins. The visitor has to choose actual size, 100 percent, by hand.

The defence is a measurement printed on the page itself. Since 1 July 1959 the inch used in the United States has been exactly 25.4 millimetres, and the desktop publishing point, the unit of PostScript and of the PDF pages built here, is exactly 1/72 inch. So a 50 millimetre bar is 141.732 pt long and a 2 inch bar is 144 pt. Print the sheet, lay a ruler on both bars, and you know at once whether the scale survived the trip. The bars cannot tell you whether the printer fed the sheet askew. Check that against the paper’s edge.

The limits that decide a letter

A handmade envelope that is going in the mail meets a second set of rules: the size limits that decide how a sorting machine treats it. Postal handling has become steadily more automated, and machines are fussy about shape. The Mail Classification Schedule maintained by the Postal Regulatory Commission lists them. A letter is 5 to 11.5 inches long, 3.5 to 6.125 inches high and 0.007 to 0.25 inch thick. A flat meets those minimums but exceeds at least one letter maximum, up to 15 by 12 inches and 0.75 inch thick.

Shape matters as well as size. The schedule defines aspect ratio as length divided by height, and a letter whose ratio does not fall between 1.3 and 2.5 inclusive is placed in a nonmachinable category. So the machinable range runs from 1.3 to 2.5, and the No. 10 envelope sits comfortably inside it at 76/33 = 2.303.

A square envelope is the classic surprise. At 6 by 6 inches it is a letter by size, but its aspect ratio is 1, so it fails the machinable test. An ISO C5 envelope, 162 by 229 millimetres, is 6.378 inches high, above the letter limit, which makes it a flat. And a snug envelope around a quarter-folded Letter card, 5 1/2 by 4 1/4 inches with the same clearance as before, has a face of 5.65625 by 4.40625 inches: a ratio of 181/141, about 1.284, just short of 1.3.

Getting the ratio upside down is the classic slip. Divide height by length and every envelope looks hopelessly squat: the No. 10 comes out at 0.434 and would be wrongly flagged.

None of this decides whether a particular envelope can be mailed or what it costs; the postal service’s own rules do that, and they include things a template cannot measure, such as weight and the schedule’s other machinability requirements. What the template can do is put the shape in the right box before any paper is cut.

That is the real argument for sizing an envelope from its blank rather than from a guess. Every number is connected. The card sets the face, the face sets the flaps, the flaps set the sheet, and the face sets the mail shape. A blank worked out to the point, printed at true size and checked with a ruler, is the difference between an envelope that closes and one that almost does.