A3 Millimetres to Inches Converter
Convert A3 paper size from millimetres to inches for print, poster artwork, frames, cutting guides and layout checks.
Set The A3 Sheet
The standard A3 trim size is 297 x 420 mm. Add bleed, scale or DPI only if you need a production or proofing check.
A3 Size In Inches
Standard A3 trim size, no bleed, full scale.
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A3 Size In Inches: The Short Answer
A3 paper is 297 x 420 mm. Converted to inches, that is approximately 11.69 x 16.54 inches. Those two numbers are the trimmed paper size before bleed, crop marks, frame overlap, mounting board or printer margins. If you are designing artwork for commercial print, the file size may need to be larger than the finished A3 sheet because bleed is added outside the trim edge. If you are buying a frame, the opening, mount and visible area may be slightly different from the paper size, so check the frame listing before ordering.
The converter above keeps the standard A3 dimensions visible while letting you add bleed, scale and DPI. That makes it useful for more than a simple millimetres-to-inches answer: you can check a print-ready file with 3 mm bleed, a half-scale proof, a wide poster, or the pixel dimensions needed for a 300 DPI export.
Formula And Rounding
The conversion uses the international inch definition.
inches = millimetres / 25.4
For standard A3, 297 / 25.4 = 11.6929 inches and 420 / 25.4 = 16.5354 inches. Most print and frame checks only need two decimal places, so the practical answer is 11.69 x 16.54 inches. If you are setting up technical artwork, keep the millimetre size as the master value and use the inch value as a reference for software or supplier forms that ask for inches.
A3 Quick Conversion Table
| A3 measurement | Millimetres | Inches | Use |
|---|---|---|---|
| Short side | 297 mm | 11.69 in | Portrait width or wide-format height |
| Long side | 420 mm | 16.54 in | Portrait height or wide-format width |
| 3 mm bleed each edge | 303 x 426 mm | 11.93 x 16.77 in | Common print-ready artwork size |
| 5 mm bleed each edge | 307 x 430 mm | 12.09 x 16.93 in | Use only if the printer requests it |
| Half scale | 148.5 x 210 mm | 5.85 x 8.27 in | Small proof or rough layout check |
| 300 DPI trim export | 297 x 420 mm | 3508 x 4961 px | Typical high-quality raster artwork check |
Print, Poster And Frame Checks
Set the document to the finished trim size unless your printer asks for a bleed-size document. Add bleed outside the trim and keep text, logos and important edges away from the cut line. The converter shows both trim and bleed size so you can check which value belongs in the file setup.
A frame sold as A3 is normally intended for 297 x 420 mm paper, but the visible opening may be smaller if a mount or lip covers the edge. Use the inch conversion for comparing international listings, then check the frame’s internal size in millimetres before buying.
Pixel size depends on DPI. A3 at 300 DPI is about 3508 x 4961 pixels at trim size. A bleed-size file will be larger. Vector artwork may not need a fixed pixel size until export, but photos and raster backgrounds need enough pixels for the chosen print resolution.
Why A3 Is Not Exactly 12 x 17 Inches
A3 belongs to the ISO A paper series, which is metric. Its size is defined as 297 x 420 mm, not as a rounded inch measurement. The inch values are therefore decimals. Rounding A3 to 12 x 17 inches is sometimes good enough for a casual description, but it is too loose for artwork setup, cropping, frame fit or print ordering. The difference is not tiny: 12 inches is about 7.8 mm wider than the A3 short side, and 17 inches is about 11.8 mm longer than the A3 long side.
Common Mistakes
Do not confuse A3 trim size with A3 plus bleed. Do not swap portrait and wide-format values without checking which side your software calls width. Do not assume a home printer can print borderless A3 just because the artwork is A3. Do not set a raster image to 72 DPI and expect it to print sharply at full A3 size. For commercial jobs, always use the printer’s own bleed, safe-area and file-format instructions.
FAQ
What is A3 paper size in inches?
A3 is 297 x 420 mm, which is approximately 11.69 x 16.54 inches. In wide format, the same sheet is 16.54 x 11.69 inches.
How big is A3 with 3 mm bleed in inches?
Adding 3 mm bleed to every edge makes the artwork 303 x 426 mm. That is approximately 11.93 x 16.77 inches.
What pixel size is A3 at 300 DPI?
At trim size, A3 is about 3508 x 4961 pixels at 300 DPI. Add bleed before calculating pixels if your printer asks for a bleed-size raster export.
Is A3 the same as 11 x 17 inches?
No. 11 x 17 inches is a North American tabloid or ledger-style size. A3 is 11.69 x 16.54 inches, so it is wider and slightly shorter than 11 x 17 inches.
Should I design in millimetres or inches?
For A-series paper in the UK, millimetres are usually the safest master unit. Use inches only when a supplier, frame listing or software setting asks for them.
Does printer margin change A3 size?
No. Printer margins change the printable area, not the paper size. If your printer cannot print edge-to-edge, keep important content inside the printable area or use a larger sheet and trim down.
Sources
- International Organization for Standardization. (2007). ISO 216: Writing paper and certain classes of printed matter: Trimmed sizes: A and B series, and indication of machine direction. ISO.
- Thompson, A., & Taylor, B. N. (2008). Guide for the Use of the International System of Units (SI) (NIST Special Publication 811). National Institute of Standards and Technology. https://www.nist.gov/pml/special-publication-811
- Bureau International des Poids et Mesures. (2019). The International System of Units (SI Brochure) (9th ed.). BIPM. https://www.bipm.org/en/publications/si-brochure