A1, A2, A3, and A4 are standard international paper sizes in the “A-series” defined by the ISO 216 paper size standard, used almost everywhere in the world except the US and Canada.
What is the A‑series?
The A‑series starts at A0, which has an area of exactly 1 square metre and an aspect ratio of 1 to √2. Each subsequent size is created by halving the previous sheet on its longest side, so A1 is half of A0, A2 is half of A1, A3 is half of A2, and so on. This system ensures that all sizes have the same proportions, making it easy to scale documents up or down without changing their layout. These sizes are widely used for printing, office documents, books, posters, and more across Europe, Asia, and many other regions.
Exact sizes of A1, A2, A3, A4
Here are the dimensions in millimetres and inches:
| Size | Dimensions (mm) | Dimensions (inches) | Typical orientation |
|---|---|---|---|
| A1 | 594 × 841 mm | 23.4 × 33.1 in | Portrait or landscape |
| A2 | 420 × 594 mm | 16.5 × 23.4 in | Portrait or landscape |
| A3 | 297 × 420 mm | 11.7 × 16.5 in | Often landscape |
| A4 | 210 × 297 mm | 8.3 × 11.7 in | Usually portrait |
You can see the pattern: each step down (A1 → A2 → A3 → A4) halves the area but keeps the same proportions.
How the sizes relate to each other
The relationship between them is simple and very practical.
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A1 is half the size of A0 and twice the size of A2.
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A2 is half the size of A1 and twice the size of A3.
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A3 is half the size of A2 and twice the size of A4.
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A4 is half the size of A3 and twice the size of A5.
For example, four A3 sheets put together equal one A1 sheet, and eight A4 sheets equal one A1 sheet. This makes it easy to design a page in A4 and print it as a poster in A3 or A2 by scaling.
Common uses of A1, A2, A3, A4
Each size tends to be used for different kinds of documents:
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A1
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Posters for indoor displays, event signage, large presentation charts.
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Technical drawings and large architectural plans when big detail is needed.
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A2
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Medium‑sized posters, presentation boards, and graphics for walls or exhibition panels.
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Some artwork prints and diagrams that need to be readable from a short distance.
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A3
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Smaller posters, menus on walls, classroom charts, and magazine spreads for proofs.
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Office uses like large tables, diagrams, and folded brochures (e.g., an A3 sheet folded to A4).
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A4
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Everyday office paper for letters, reports, invoices, school assignments, and forms.
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Standard home printer paper, many magazines, catalogues, and simple flyers.
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Simple way to remember them
One easy mental picture is to start from A4, since that’s the everyday “printer sheet” size.
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A3 is like two A4 sheets placed side by side on the long edge.
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A2 is like two A3 sheets; A1 is like two A2 sheets.
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So, moving “up” (A4 → A3 → A2 → A1) doubles the area each step, and moving “down” halves it.
Final Thoughts
Understanding A1, A2, A3, and A4 paper sizes makes planning any print project far more intentional and efficient. Instead of guessing which sheet will work, you can now choose sizes that fit your content, your design, and how your audience will interact with the final piece. From everyday A4 documents to eye‑catching A1 posters, each step up or down in the A‑series has a clear purpose and place. With this knowledge, you can brief designers and printers more confidently, avoid costly sizing mistakes, and ensure your work always looks professional on the page.
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