A Series Paper Size Lookup

What is A-Series Paper?

210 mm × 297 mm—this is the exact dimension of A4, the world's most commonly used paper size, and the standard we encounter every day when printing, copying, or taking notes. The A-series paper is defined by the international standard ISO 216, featuring 14 sizes ranging from the largest A0 down to the smallest A13. The most ingenious part about them is that you only need to remember one action: "folding in half." This allows you to deduce the next size down from a single sheet of paper while always maintaining the exact same aspect ratio. Our lookup tool lays out the entire A-series table, displaying both millimeters and inches at once, so you no longer need to check your phone's notes or do a quick web search.

Principles and Formulas

A-series paper is based on a simple geometric rule: the ratio of the long edge to the short edge is fixed at √2, which is approximately 1.414. This ratio ensures that every time the paper is folded in half along its long edge, the resulting new sheet maintains the exact same aspect ratio. The formula can be understood like this: if a sheet of paper has a width of W and a height of H, then W × H is its area, and H = W × √2. The area of A0 is defined as exactly 1 square meter, which gives A0 its dimensions of 841 mm × 1189 mm. Every subsequent size (A1, A2, A3...) is obtained by folding the long edge of the previous size in half. All the values displayed in our tool strictly follow this recursive relationship, which is why the area of A4 is exactly half that of A3, yet the aspect ratio remains unchanged.

How to Use This Tool

1. Open the A-series paper size lookup page, and you will immediately see a complete size chart from A0 to A13.
2. The left half of the table shows the width and height in millimeters (mm), while the right half provides the corresponding width and height in inches (in).
3. Simply scroll down to find the paper size you need, such as A4 or A3, and you can read the exact dimensions in both units on the same row.
4. If you want to quickly compare two sizes, you can look at the data in adjacent rows side by side to get a feel for the changes in area and edge length.
5. On the far right of the table, there is also a proportion diagram to help you visually understand the size relationships between different specifications.

Complete Example: Folding A4 into A5

Let's look at one of the most common everyday actions—folding an A4 sheet in half along its long edge to make an A5 booklet. In our lookup table, the dimensions of A4 are 210 mm (width) × 297 mm (height), and A5 is 148 mm × 210 mm. When folding, you fold along the 297 mm height, halving it, which theoretically should give you 148.5 mm. However, ISO 216 specifies that A5 has a height of 210 mm and a width of 148 mm, meaning the actual dimension is rounded down to 148 mm, discarding the 0.5 mm. This is exactly the rounding process permitted by the standard. Now let's look at the inch values: A4 is 8.27 in × 11.69 in, and A5 is 5.83 in × 8.27 in. You will notice that in inches, the width goes from 8.27 to 5.83, following the same folding rule. By verifying this with our tool, you'll see the margin of error is kept well within 1 millimeter.

Quick Reference Chart: Common Sizes

The table above covers the 7 most commonly used sizes, from large posters to small cards. You can view the complete data from A0 to A13 directly in the lookup table above, with all values accurate to the millimeter.

Typical Use Cases

• Office Printing and Document Binding: Whether you are purchasing A4 copy paper, determining the size of an A3 cover, or formatting an A5 flyer, you can first confirm the millimeter dimensions in this lookup table to avoid proportion mistakes that lead to trimming waste.
• Graphic Design and Printing: Designers frequently switch between A4 and A3 artboards. If you want to scale an A4 poster up to A3 proportionally, simply look up the A3 dimensions in the table and scale it up by a ratio of √2 ≈ 141%, eliminating the need for trial and error.
• Home Crafts and Frame Making: Many DIY photo frames and scrapbooks use A5 or A6 as their default baseboard size. A quick glance at the tool lets you buy the right size backing boards and photo paper directly, saving you from measuring things in the store with a ruler.

Common Mistakes and Pitfalls

• Confusing "half the area" with "half the edge length": Many people assume that the width and height of A5 are half those of A4. In reality, the edge lengths are about 0.707 times the original, while the area is 0.5 times. Simply halving the dimensions will result in a size that is about 30% too small.
• Misremembering A3 dimensions and placing A4s side-by-side incorrectly: Some think two A4 sheets placed side-by-side horizontally make an A3. Wait, A3 is 297 × 420 mm, and two A4s placed side-by-side (each 297 mm high and 210 mm wide) create a width of 420 mm, which is exactly A3! This isn't actually a mistake. The real mistake is assuming A3 is twice the length and twice the width of A4 simultaneously. Another common error is ignoring rounding when converting to inches.
• Only looking at standard sizes before printing and forgetting to add bleed: If a design file has a full background color or images that need to extend to the edge of the paper, you actually need to provide a bleed size slightly larger than A4 (usually adding 3 mm to each side). Outputting directly at the 210 × 297 mm from the table will result in white borders.
• Confusing the A-series with the B-series or envelope sizes: A-series paper does not directly correspond to C-series envelopes. For example, an A4 sheet cannot fit perfectly into a C4 envelope (C4 is slightly larger than A4). Without checking a chart, it's easy to buy the wrong envelopes.

Notes and Limitations

This tool displays the theoretical dimensions defined by the ISO 216 standard, accurate to the millimeter. During actual papermaking and cutting processes, paper of the same specification may have a tolerance of ±1 to 2 mm due to different manufacturer equipment, and cumulative deviations can occur, especially in large print runs. Additionally, a few countries or regions may use traditional sizes (such as the US Letter size), which do not fall under the A-series category. If your design must target the North American market, it is recommended to also check the ANSI/ASME Y14.1 standard. This tool focuses solely on the A-series and does not cover the B-series, C-series, or non-standard sizes. The lookup results are for design reference and preliminary planning only; you still need to confirm the final finished size with your printer before official printing.

Frequently Asked Questions (FAQ)

Q: What are the exact dimensions of A4 paper?
A: The standard A4 size is 210 mm × 297 mm, which converts to approximately 8.27 inches × 11.69 inches. You can see both of these units simultaneously in the A4 row of our table.

Q: Is A3 paper twice the size of A4?
A: The area of A3 is twice that of A4, but the edge lengths are not doubled. The length and width of A3 are approximately √2 times (≈ 1.414 times) the width and length of A4, respectively. Therefore, when scaling an A4 document up to A3, you should scale it by 141%, not 200%.

Q: Why is the aspect ratio of A-series paper 1:√2?
A: This ratio was proposed by the German scientist Georg Christoph Lichtenberg in 1786 and was later adopted as an international standard. Its charm lies in the fact that after every fold in half, the aspect ratio of the new sheet remains unchanged, making printing and scaling very convenient without causing image distortion.

Q: Can I look up B-series or C-series paper sizes in this tool?
A: This lookup tool currently focuses on the ISO 216 A-series (A0–A13) and does not include the B-series or C-series. If you also need envelope sizes like C4 or C5, you can cross-reference with other standard charts.

Q: Why doesn't the A4 paper I actually buy measure exactly 210 × 297 mm with a ruler?
A: There are physical tolerances in the papermaking and cutting processes, typically resulting in an error of about ±1 mm on each side. Furthermore, changes in humidity can cause the paper to slightly expand or shrink, so minor deviations during measurement are perfectly normal.

You can now directly look up the millimeter and inch dimensions of any A-series paper size in the calculator above.