You can make your own origami envelopes for cards or letters. I like the one from this YouTube video but I don’t like having to watch the video everytime I have to remember how to fold it. So I made origami instructions.
Sizing Your Paper
The size of envelope depends on the sheet of paper you start with. To choose what paper size you need I have derived some equations for you to use.
Let $x$, be the original width of the sheet.
Let $y$, be the original height of the sheet.
Let $x’$, be the final width of the envelope.
Let $y’$, be the final heigh of the envelope.
Decide on the width and height of the envelope and then plug them into Eq. 1 and Eq. 2. To make sure it fits round up the results.
$$ \begin{equation} y = \frac{y’}{0.315} \end{equation} $$
$$ \begin{equation} x = x’+\frac{0.25}{0.315}y' \end{equation} $$
For example if you want the final envelope to be 5.5cm x 9.5cm, after calculating the height and width. Then rounding it up you would get 14cm x 18cm.
$$ y=\frac{5.5cm}{0.315}=17.8cm\approx18cm $$
$$ x=9.5cm+\frac{0.25}{0.315}5.5cm=13.84cm\approx14cm $$