Custom Envelope Size Calculator
Calculate the right envelope size for your card or invitation.
Get dimensions, flap allowance, and template measurements.
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.