To fold paper into even thirds, loosely roll it into an S shape, nudge the layers until all three edges line up, then press the creases; on US Letter paper each panel is about 3 2/3 inches, and on A4 it is 99 mm.
Halves are easy because the paper checks itself. Thirds are not, unless you know the trick.
Folding a sheet in half is close to foolproof. You match two edges, and the paper tells you if you got it right. Thirds give you no such check on the first fold. You have to guess where the crease goes, and by the time you can see the error, the crease is already there.
These five methods fix that. The first takes about five seconds and is good enough for nearly every letter you will ever mail.
If you want to measure, here is where the folds go.
| Paper | Long side | Each panel |
|---|---|---|
| US Letter | 11 in | 3 2/3 in (93 mm) |
| US Legal | 14 in | 4 2/3 in (119 mm) |
| A4 | 297 mm | 99 mm |
| A5 | 210 mm | 70 mm |
A standard #10 envelope is 4 1/8 by 9 1/2 inches, so a Letter sheet folded this way slides in with room to spare.
This is the one to memorize.
It works because the roll lets you compare all three panels before anything is permanent. Mistakes stay fixable until you press.
Use the table above. Mark the fold points with a pencil at two places along the edge, line them up with a ruler, and fold. It is the slowest method but the most repeatable on thick paper, where rolling is hard.
Here you do not need any fractions. On Letter paper, 11 inches does not divide neatly by 3. So use a number that does.
The ruler is longer than the page, and tilting it makes up the difference. This works for any page and any number of equal parts, as long as you can pick a ruler length that divides cleanly. It is an old geometry trick that carpenters still use to split a board into equal widths.
This method needs no ruler, and it is mathematically exact.
Origami folders use this method when they need a true third. It does leave faint creases on the page, so save it for craft paper and not for your cover letter.
For a stack of 50 invitations, get one sheet right using any method above. Then lay it on the next sheet, line up the edges, and use it as a guide. Always copy from the original, because small slips build up if you copy from a copy.
There is one wrinkle. A perfect three-way split does not always sit perfectly flat. The inside panel has to tuck under the outer layer, and with thick paper it can buckle.
Print shops handle this on brochures by making the inner panel slightly shorter, by about 1/16 to 1/8 of an inch. On Letter paper, that gives you something like:
For ordinary office paper you can ignore this. For heavy brochures, it is why the pros do not fold perfectly equal thirds.
Try folding one sheet by eye before you use any of these methods. Then unfold it and measure the panels. If you get 3.4 inches and 3.9 inches, your two panels are off by half an inch, which looks obvious once you see it on a ruler but feels fine while you are folding.
A handy way to grade it: divide the smaller panel by the larger one and multiply by 100. Those panels score 87. That is the same formula used in Halfsies, a free daily game where you cut three hand-drawn objects exactly in half by eye, and a scale weighs the pieces. Lighter half over heavier half, times 100, with 300 points possible each day. Halves are easier than thirds, right up until the objects stop being rectangles. You can play today's three cuts in your browser with nothing to install.
Roll the sheet loosely into an S shape, slide the layers until the edges line up, then press the two creases. It takes a few seconds and needs no tools.
When folding along the 11 inch side, as you would for a letter, each panel is 3 2/3 inches, about 93 mm. Along the 8.5 inch side, each would be about 2 5/6 inches.
A common business habit is to fold the bottom third up first, then the top third down over it. The top flap then opens first, so the reader sees the start of the page right away.
Crease a diagonal from corner to corner, then a line from another corner to the midpoint of the far short edge. The two lines cross exactly one third of the way along the sheet.