C3 paper size
324 × 458 mm
12.76 × 18.03 inches · 32.4 × 45.8 cm · aspect ratio 1:1.41
What C3 is for
C3 at 324 × 458 mm is the largest commonly stocked C envelope, sized to take an unfolded A3 sheet — or a C4 envelope inside it, which is how a set of documents already in envelopes gets posted as one item. Anything this size travels as a parcel rather than a letter in most postal systems.
C3 in every unit
| Unit | Width | Height |
|---|---|---|
| Millimetres | 324 mm | 458 mm |
| Centimetres | 32.4 cm | 45.8 cm |
| Inches | 12.76 in | 18.03 in |
| Points (PostScript) | 918 pt | 1298 pt |
C3 in pixels
Pixel dimensions depend entirely on the resolution you are working at — there is no single "pixel size" for a sheet of paper.
| Resolution | Pixels | Use |
|---|---|---|
| 72 DPI | 918 × 1298 | Screen preview only — far too coarse to print |
| 150 DPI | 1913 × 2705 | Draft printing, internal documents |
| 300 DPI | 3827 × 5409 | The standard for commercial print |
| 600 DPI | 7654 × 10819 | Fine detail, line art, archival scanning |
Setting up a print document? Use 300 DPI and add 3 mm of bleed on every edge unless your printer specifies otherwise.
How C3 relates to the rest of the series
C3 is C0 halved 3 times. Halving a sheet across its longer side is what generates the next size down, and because the sides are in a 1:√2 ratio the proportions survive every fold — which is the entire reason the series exists.
That makes 8 sheets of C3 exactly one sheet of C0, with nothing left over. The area works out at 0.125 m², since C0 is defined as one square metre.
Scaling to the next size up or down is a factor of √2 ≈ 1.414 on each edge, so enlarging C3 to the next size is 141% on a photocopier and reducing it is 71%. Those two numbers are on the preset buttons of most copiers for exactly this reason, and they are the same for every size in the series.
About C3
Takes an unfolded A3 sheet, or a C4 envelope inside it.
Other ISO C series sizes
| Size | Millimetres | Inches |
|---|---|---|
| C4 | 229 × 324 mm | 9.02 × 12.76 in |
| C5 | 162 × 229 mm | 6.38 × 9.02 in |
| C6 | 114 × 162 mm | 4.49 × 6.38 in |
| DL | 110 × 220 mm | 4.33 × 8.66 in |
| C7 | 81 × 114 mm | 3.19 × 4.49 in |
| Envelope No. 10 | 104.775 × 241.3 mm | 4.13 × 9.5 in |
| A7 Envelope (US) | 133.35 × 184.15 mm | 5.25 × 7.25 in |
Frequently asked questions
What size is C3 in mm?
324 × 458 mm.
What size is C3 in inches?
12.76 × 18.03 inches.
What size is C3 in pixels?
There is no single answer — it depends on resolution. At the print standard of 300 DPI it is 3827 × 5409 pixels; at 72 DPI it is only 918 × 1298.