Paper Size Converter

Paper size is one of those details that quietly breaks projects. A printer that expects Letter chokes on A4. A PDF author designs for A4 and a US customer prints it clipped because Letter is slightly wider and noticeably shorter. A graphic designer hands a vendor artwork at 72 DPI and the banner comes out fuzzy at print scale. The Paper Size Converter exists to make these conversions fast, accurate, and obvious. It is a reference and a calculator for the ISO 216 A and B series, the ISO 269 C series of envelopes, and the North American sizes that still dominate offices in the United States, Canada, and parts of Mexico. Every size can be reported in millimetres, centimetres, inches, typographic points, or pixels at a DPI of your choice.

The tool's primary job is to take the guesswork out of "what size is that, really?" Whether you are setting up a print job, exporting a PDF, sizing a scanned image, or building CSS for an international audience, the Paper Size Converter gives you the exact numbers you need, in every unit you might be asked for.

How to Use

1. Pick a size from the selector. Choose from ISO A (A0 through A10), ISO B (B0 through B5), ISO C envelopes (C0 through C6), or a North American size (Letter, Legal, Tabloid, Ledger, Executive, Half Letter).
2. Choose a DPI. The default is 300 DPI because that is what most laser and inkjet printers expect for crisp output. Switch to 72 DPI for screen work or for the legacy PDF page size conventions.
3. Toggle portrait or landscape. The width and height swap automatically. Useful for brochures, leaflets, and folded documents.
4. Read the five unit systems. The card under the selector shows millimetres, centimetres, inches, points, and pixels at the selected DPI, plus the aspect ratio and area of the sheet.
5. Use the reference table. Scroll down for a sortable, searchable list of every supported size. Sort by series, by area, or alphabetically. Click any row to load it into the selector.
6. Match custom artwork. Open the custom-dimensions panel, enter width and height in any of the four physical units (mm, cm, in, pt), and the tool will tell you which standard size is the closest match. This is the fastest way to figure out "what paper do I print this on?".

The ISO 216 System

The A series is the international standard for paper, defined in ISO 216. The entire series is built on a single mathematical trick: every sheet has the aspect ratio 1 : √2 (approximately 1 : 1.4142). That ratio is special because √2 is irrational, which means halving a sheet along its long edge produces two smaller sheets with the identical ratio. Cut an A0 sheet in half along its long edge and you get two perfect A1 sheets. Halve an A1 and you get two A2 sheets. Continue all the way down to A10. Each size has half the area of the previous one, and any sheet folds or scales into any other sheet without distortion, without trimming, and without leftover strips.

A0 is defined to cover exactly one square metre. Because the ratio is fixed at 1 : √2, the long edge of A0 is √(2) metres, about 1189 mm, and the short edge is 1/√2 metres, about 841 mm. Multiply those two values together and you get 0.999949 m², which is as close to one square metre as the standard allows while still keeping both dimensions to whole millimetres. Every other A size is just A0 halved some number of times: A1 is half of A0 (594 × 841 mm), A2 is a quarter (420 × 594 mm), A3 is an eighth (297 × 420 mm), and A4, the size you almost certainly have in your printer right now, is one sixteenth (210 × 297 mm). The same logic gives A5 at 148 × 210 mm, A6 at 105 × 148 mm, and so on down to A10 at 26 × 37 mm.

The B series is ISO 216's intermediate option. Each B size is the geometric mean of the adjacent A sizes, so B1 sits exactly between A0 and A1 in area, B2 sits between A1 and A2, and so on. The B series is popular for posters, books, and any time you need a sheet a little bigger than A series without jumping all the way up.

The C series is defined in ISO 269 and exists almost entirely for envelopes. Each C size is the geometric mean of the corresponding A and B sizes, which means a C4 envelope fits an unfolded A4 sheet perfectly with a sliver of margin, a C5 fits an A5 sheet or an A4 folded in half, and a C6 fits an A4 folded in thirds (the classic business-envelope size). The arithmetic guarantees the fit; you never need to measure. The geometric-mean trick works for the B series in exactly the same way: a B4 sheet fits a C4 envelope with the same small margin.

North American Sizes

The United States and Canada never adopted ISO 216. Their standard sizes, defined in ANSI/ASME Y14.1, are independent and have no consistent aspect ratio. The workhorses are Letter (8.5 × 11 inches, or 215.9 × 279.4 mm) and Legal (8.5 × 14 inches, or 215.9 × 355.6 mm), with Tabloid at 11 × 17 inches (279.4 × 431.8 mm) for larger sheets. Ledger is exactly Tabloid rotated: 17 × 11 inches, used when you want the long edge horizontal. Executive at 7.25 × 10.5 inches (184.15 × 266.7 mm) was the original American business stationery size before Letter took over, and it still appears in some accounting software default reports. Half Letter at 5.5 × 8.5 inches (139.7 × 215.9 mm) is popular for booklets, planners, and small-format printing.

The lack of a consistent ratio is the practical pain point. You cannot fold a Letter sheet in half and get a usable Legal sheet, or vice versa. You cannot scale Letter to fit an A-series envelope without a custom cut. Most international print jobs therefore explicitly request ISO sizes, and most US-only jobs use Letter, the cross-over is where mistakes happen.

Pixels and DPI

For digital work, paper sizes turn into pixel counts, and pixel counts depend on DPI (dots per inch). The conversion is straightforward: pixels = inches × DPI. At 72 DPI (the long-standing convention for screen rendering and the default unit of measurement in PostScript and PDF), an A4 sheet is 595 × 842 pixels. At 150 DPI, the same sheet is 1240 × 1754 pixels, fine for office inkjet printers. At 300 DPI, A4 becomes 2480 × 3508 pixels, the workhorse resolution for laser printing and high-quality inkjet. At 600 DPI, A4 is 4961 × 7016 pixels, reserved for fine-art reproduction and high-end photo printing.

The interesting comparison is between 72 DPI and 300 DPI. The 300 DPI version has about 4.17 times as many pixels in each dimension, and roughly 17 times the total pixel count, of the 72 DPI version. That is why a 1 MB PDF designed for screen looks terrible when printed at full bleed, the printer is being asked to stretch each pixel across more than four source pixels, with predictable mushiness. Always design print artwork at the target DPI from the start; do not scale up a 72 DPI image and hope for the best.

Screen DPI in 2026 is closer to 100 to 160 DPI on most phones and laptops, but the legacy 72 DPI assumption lives on in design software and PDF standards. If you are exporting images for the web, 72 to 150 DPI is plenty. If you are exporting for print, 300 DPI is the safe minimum for text and line art; photographs can sometimes get away with 200 DPI.

Worked Examples

1. Convert A4 to inches. A4 is 210 × 297 mm. Divide by 25.4 mm per inch: 210 / 25.4 ≈ 8.2677 inches and 297 / 25.4 ≈ 11.6929 inches. Notice that A4 is slightly taller and slightly narrower than US Letter (8.5 × 11).

2. Find the pixel size of A4 at 300 DPI. A4 in inches is 8.2677 × 11.6929. Multiply by 300: width ≈ 2480 px, height ≈ 3508 px. This is the standard A4 size in image editors when you set the print resolution to 300 DPI.

3. Confirm A4's aspect ratio. 297 / 210 ≈ 1.4143, which rounds to √2 (1.41421...). Every ISO A and B size has this exact ratio, regardless of scale.

4. Check A0's area. A0 is 841 × 1189 mm. Convert to metres (0.841 × 1.189) and multiply: 0.999949 m². The ISO standard rounds this to "exactly one square metre" because the underlying definition is 1 m² rounded to the nearest whole millimetre on both edges.

5. Match a custom dimension. A poster is 400 × 600 mm. The closest standard size is A2 (420 × 594 mm), since both dimensions are within about 5% of the input. If the poster were 350 × 500 mm, the match would be A3 (297 × 420) or B3 (353 × 500), the tool picks the closest match orientation-aware.

6. Convert Letter to mm. Letter is 8.5 × 11 inches. Multiply by 25.4 mm per inch: 8.5 × 25.4 = 215.9 mm and 11 × 25.4 = 279.4 mm exactly. These are the canonical mm equivalents of US Letter.

7. Pick the right envelope. You need to mail an A4 document without folding. A C4 envelope (229 × 324 mm) fits an A4 sheet (210 × 297 mm) with about 9 mm of margin on each side, snug but correct. A C5 envelope (162 × 229 mm) fits an A5 sheet or an A4 sheet folded once in half.

Where It Shows Up

Printing and copy shops. Printers always ask "what size?" before they run a job. ISO sizes are universal in Europe, Asia, Africa, and most of South America; North American sizes dominate the US and Canada. The Paper Size Converter settles the question instantly.

Graphic design. Designers routinely produce artwork for clients in multiple regions. A brochure designed at A4 in Europe needs a Letter version for the US office. Knowing the exact dimensions prevents white margins, clipped content, and last-minute redesigns.

Scanning and archiving. When you scan a document, you need to pick a paper size so the scanner knows where to crop. Setting the wrong size either clips the content or leaves a thick white border that has to be cropped manually.

PDF page setup. Most word processors let you choose page sizes by name ("A4", "Letter", "Legal") rather than by typing dimensions. The dropdown is populated by the operating system's locale settings, so a French laptop will default to A4 and a US laptop will default to Letter. The Paper Size Converter helps you confirm which size you actually have before you send the file.

International document exchange. Contracts, invoices, certificates, and government forms often need to be valid in multiple jurisdictions. Knowing that A4 is 210 × 297 mm and Letter is 215.9 × 279.4 mm (so A4 is narrower and slightly taller) helps you explain to a client why the layout shifted when they opened your file.

Common Mistakes

Assuming Letter and A4 are interchangeable. They are not. A4 is 210 × 297 mm; Letter is 215.9 × 279.4 mm. A4 is taller (by 17.6 mm) and narrower (by 5.9 mm). A document designed on Letter will not fit cleanly on A4 and vice versa; expect either clipped edges or extra whitespace.

Using 72 DPI artwork for print. 72 DPI is a screen convention. Print jobs typically need 300 DPI for crisp text and 200 DPI or more for photographs. Scaling a 72 DPI image up to 300 DPI does not add detail; it just enlarges the existing pixels.

Rounding millimetres to inches badly. The exact inch equivalent of A4 is 8.26771654 × 11.69291339. Rounding to 8.25 × 11.75 or 8.27 × 11.69 introduces small errors that show up at the edges of printed sheets. Use at least four decimal places when precision matters.

Confusing A4 with A5. A5 is exactly half of A4 by area, but its dimensions are 148 × 210 mm, not 105 × 148.5 mm or any other plausible guess. A5 is what fits in a C5 envelope; A4 does not.

Ignoring the B and C series. The B series is useful when A series is just a touch too small or too large. The C series is the only one to use for envelopes designed to fit A series sheets. Choosing randomly leads to ill-fitting mailings and awkward layouts.

Treating "A4" as a pixel size. Different software interprets "A4" at different DPIs. InDesign defaults to A4 at 300 DPI (2480 × 3508 px). A web browser's print preview defaults to A4 at 96 DPI (794 × 1123 px). The numeric dimensions of the same "A4" file change; the physical size does not.

Frequently Asked Questions

What is the aspect ratio of A4 paper?

The aspect ratio of A4 paper, and every other A series size, is 1 : √2, which is approximately 1 : 1.4142. Concretely, an A4 sheet is 210 mm wide by 297 mm tall, and 297 / 210 ≈ 1.4143. This ratio is the entire reason the ISO 216 series works: it is preserved when you halve a sheet along its long edge, so A3 folds into two A4 sheets, A4 folds into two A5 sheets, and so on without distortion.

How many pixels is A4 at 300 DPI?

A4 at 300 DPI is 2480 × 3508 pixels. The math is straightforward: A4 is 8.2677 × 11.6929 inches, and 8.2677 × 300 ≈ 2480 and 11.6929 × 300 ≈ 3508. This is the resolution most laser printers need for crisp text output. At 600 DPI the same sheet becomes 4961 × 7016 pixels, and at 72 DPI it is only 595 × 842 pixels, the legacy PDF page size.

Is A4 the same as Letter?

No. A4 (210 × 297 mm) and Letter (8.5 × 11 in, or 215.9 × 279.4 mm) are different sizes. A4 is slightly narrower (by 5.9 mm) and noticeably taller (by 17.6 mm) than Letter. The two are not interchangeable: a document designed for Letter will leave extra white space at the top when printed on A4, and a Letter printout of an A4 design will be slightly cropped on the long edge.

What is A0 in millimetres?

A0 is 841 × 1189 mm. It is defined by ISO 216 to cover exactly one square metre: 0.841 m × 1.189 m = 0.999949 m², rounded to "one square metre" at the millimetre precision. A0 is the largest standard sheet in the A series; A1 is exactly half an A0 by area, A2 is a quarter, and so on.

What envelope fits an A4 sheet?

A C4 envelope (229 × 324 mm) fits an A4 sheet (210 × 297 mm) without folding. A C5 envelope (162 × 229 mm) fits an A5 sheet or an A4 sheet folded once in half. A C6 envelope (114 × 162 mm) fits an A4 sheet folded into thirds, the classic business envelope. The C series is the geometric mean of the A and B series, which is why each size fits the corresponding A sheet perfectly.

Why does ISO 216 use √2?

ISO 216 uses √2 because that is the unique ratio that is preserved when you scale a rectangle by a factor of two. If a sheet has aspect ratio 1 : √2 and you halve it along its long edge, the two resulting sheets have the exact same ratio. No other common ratio has this property. The result is that any A size can be scaled up or down to any other A size without distortion or cropping.

How big is A4 in centimetres?

A4 in centimetres is 21.0 × 29.7 cm. The values are simply the millimetre dimensions divided by 10. For comparison, A3 is 29.7 × 42.0 cm, A5 is 14.8 × 21.0 cm, and A6 is 10.5 × 14.8 cm. The pattern is always the same: each size's short edge equals the previous size's long edge.

References

• ISO 216:2007, Writing paper and certain classes of printed matter, Trimmed sizes, A and B series, and indication of machine direction. International Organization for Standardization, Geneva.
• ISO 269, Envelopes, Series C, Sizes and designations. Defines the C series of envelopes used to fit A series sheets.
• ANSI/ASME Y14.1, Decimal Inch Drawing Sheet Size and Format. American National Standards Institute / American Society of Mechanical Engineers. Defines Letter, Legal, Tabloid, Ledger, and the other North American sheet sizes.