Custom Envelope Size Calculator
Find the right envelope size for any card, invitation, or letter.
Get exact dimensions, flap depth, and standard size codes like A2, A7, and number 10.
How envelope sizing works:
An envelope must be larger than the card it holds. The standard rule is to add 6 mm (1/4 inch) of clearance on each side — so the envelope interior is 12 mm (1/2 inch) wider and 12 mm taller than the card.
Envelope dimensions formula:
- Envelope width = Card width + 12 mm (0.5 in)
- Envelope height = Card height + 12 mm (0.5 in)
- Flap depth = Envelope height × 0.55 to 0.65 (pointed flap is deeper, square flap is shallower)
- Seam allowance = 10–15 mm on each side for folding and gluing
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):
| Name | Size (mm) | Size (in) | Fits |
|---|---|---|---|
| A2 | 111 × 146 | 4.375 × 5.75 | A2 card (4.25 × 5.5 in) |
| A6 | 121 × 165 | 4.75 × 6.5 | A6 card (4.5 × 6.25 in) |
| A7 | 133 × 184 | 5.25 × 7.25 | 5 × 7 in photo/invite |
| A8 | 140 × 206 | 5.5 × 8.125 | A5 folded card |
| A10 | 152 × 241 | 6 × 9.5 | 8.5 × 11 tri-fold |
| #10 | 105 × 241 | 4.125 × 9.5 | Standard business letter |
| C5 | 162 × 229 | 6.38 × 9.02 | A5 sheet unfolded |
| C6 | 114 × 162 | 4.49 × 6.38 | A5 folded to A6 |
| DL | 110 × 220 | 4.33 × 8.66 | A4 tri-fold |
Worked example: Your card is 5 × 7 inches (127 × 178 mm).
- Envelope interior: 5.5 × 7.5 in (140 × 190 mm)
- Closest standard: A7 envelope (133 × 184 mm) — slightly tight. Use A8 or make custom.
- Flap depth: 190 × 0.6 = 114 mm pointed flap
- Seam allowance: 12 mm each side
- Template sheet: 164 × 329 mm minimum
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.