Reference
Envelope sizes
The envelope you pick determines how the letter is folded. Choosing the wrong size means an extra fold, an awkward fit, or a letter that arrives creased. The reference below covers the envelopes used for most business and personal mail.
International (ISO)
C-series and DL
C-series envelopes are designed to hold the corresponding A-series sheet. C4 holds an A4 unfolded; C5 holds A4 folded once; C6 holds it folded twice. DL holds A4 folded into thirds.
DL
220 × 110 mm · 8.66 × 4.33 in
The standard business envelope across Europe and many other markets.
Fits
A4 folded into thirds
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C4
324 × 229 mm · 12.76 × 9.02 in
Holds A4 documents flat, without folding.
Fits
Unfolded A4
Reference only
C5
229 × 162 mm · 9.02 × 6.38 in
A common size for letters, statements and small booklets.
Fits
A4 folded once or unfolded A5
Read page →
C6
162 × 114 mm · 6.38 × 4.49 in
Postcard-style envelope. Suits greeting cards and short letters.
Fits
A4 folded twice or unfolded A6
Read page →
C6/C5
229 × 114 mm · 9.02 × 4.49 in
An elongated envelope, similar in role to DL but with C-series proportions.
Fits
A4 folded into thirds
Reference only
US sizes
Business and reply envelopes
US #9
98.4 × 225.4 mm · 3.87 × 8.87 in
A common reply envelope used inside US #10 envelopes.
Fits
Letter folded into thirds
Reference only
US #10
104.8 × 241.3 mm · 4.13 × 9.5 in
The standard business envelope in the United States.
Fits
US Letter folded into thirds
Read page →
US Monarch (No. 7 ¾)
98.4 × 190.5 mm · 3.87 × 7.5 in
A slightly smaller and more formal US business envelope.
Fits
Monarch / executive stationery
Reference only