Custom Envelope Size Calculator

How envelope sizing works:

An envelope has to be bigger than the card, and the amount is smaller than most guides claim. Measure the real thing and the answer is consistent: add 1/4 inch, about 6.5 mm, to each dimension in total. That is roughly 3 mm of slack per side, which is just enough to slide the card in without fighting it.

You can check this against the standard sizes yourself. An A7 envelope is 133 × 184 mm and the A7 card it takes is 127 × 178 mm, a difference of 6 mm and 6 mm. An A6 envelope is 121 × 165 for a 114 × 159 card: 7 mm and 6 mm. The pattern holds right across the range.

Watch out for the widely repeated rule that says to add 1/4 inch per side, giving 12 mm overall. That is double the real figure, and it has a tell: apply it and no standard envelope fits the card it was designed for. If a sizing rule rejects the actual products on the shelf, the rule is wrong, not the products.

Envelope dimensions formula:

• Envelope width = Card width + 6.5 mm (1/4 in)
• Envelope height = Card height + 6.5 mm (1/4 in)
• Flap depth = Envelope height × 0.62 for a pointed flap, × 0.48 for a square one
• Seam allowance = 10–15 mm on each side for folding and gluing

A pointed flap has to be deeper because it comes to a point: the depth quoted is to the tip, and the flap only actually covers the envelope for about half that. A square flap covers its full depth, so it needs less.

To cut a flat envelope template from one sheet:

• Total sheet width = Envelope width + (2 × seam allowance)
• Total sheet height = Envelope height + Flap depth + Bottom flap (usually 25–30 mm)

Standard envelope sizes (for reference):

Worked example: Your card is 5 × 7 inches (127 × 178 mm).

• Envelope interior: 133.4 × 184.2 mm (5.25 × 7.25 in)
• Closest standard: A7, at 133 × 184 mm. That is the size the A7 envelope has always been, which is the point: the rule reproduces the standard rather than arguing with it.
• Flap depth: 184.2 × 0.62 = 114 mm to the tip of a pointed flap
• Seam allowance: 12 mm each side
• Template sheet: 157 × 326 mm minimum

If you would rather buy than cut, that result tells you to buy A7 and stop. The calculator only sends you to a custom template when nothing standard is close.

Paper weight for envelopes: Most handmade envelopes use 100–130 GSM paper. Heavier stock (160+ GSM) is harder to fold neatly and adds postage weight. Lightweight 80 GSM can work for decorative liners but tears easily.

Postal regulations: Standard letter mail (US): minimum 3.5 × 5 in, maximum 6.125 × 11.5 in, thickness up to 0.25 in. Anything outside these dimensions requires extra postage.