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.
Not sure which one you need? The envelope finder works it out from your paper size and fold.
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
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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
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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
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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