Paper Size Converter: A4 vs Letter, Dimensions in mm, inches & Pixels

Paper sizes seem like a straightforward topic until you need to design a document for international distribution, calculate pixel dimensions for a print-ready design, or figure out why your carefully formatted resume looks wrong when printed on a different continent. The seemingly simple question “what size is A4?” has answers in millimeters, inches, centimeters, and pixels — with the pixel answer depending entirely on your DPI setting. Our paper size converter provides all these conversions instantly for any standard or custom paper size.

The Two Paper Size Standards: ISO vs. ANSI

The world uses two incompatible paper size systems, creating endless formatting frustrations for international communication.

ISO 216 (A-Series): Used Almost Everywhere

The ISO A-series is used by virtually every country except the United States, Canada, and parts of Mexico. It was standardized in 1975 based on a German DIN standard from 1922, though the mathematical principles date to 1786.

The brilliance of the A-series is its mathematical elegance. The aspect ratio of every A-size paper is 1:√2 (approximately 1:1.414). This specific ratio has a remarkable property: when you cut a sheet in half parallel to the shorter side, both halves have exactly the same aspect ratio as the original. This means A4 folded in half is perfectly A5. Two A4 sheets side by side equal A3.

A-series dimensions:

• A0: 841 × 1189 mm (33.1 × 46.8 inches) — area exactly 1 m²
• A1: 594 × 841 mm (23.4 × 33.1 inches) — half of A0
• A2: 420 × 594 mm (16.5 × 23.4 inches)
• A3: 297 × 420 mm (11.7 × 16.5 inches)
• A4: 210 × 297 mm (8.27 × 11.69 inches) — the global standard document size
• A5: 148 × 210 mm (5.83 × 8.27 inches) — common for notebooks and booklets
• A6: 105 × 148 mm (4.13 × 5.83 inches) — postcard size

ANSI/North American Sizes

The US Letter, Legal, and Tabloid sizes evolved from historical paper-making conventions without a unifying mathematical principle.

• Letter: 8.5 × 11 inches (215.9 × 279.4 mm) — the US standard document size
• Legal: 8.5 × 14 inches (215.9 × 355.6 mm) — contracts and legal documents
• Tabloid/Ledger: 11 × 17 inches (279.4 × 431.8 mm) — newspapers, large format

Unlike the A-series, folding US Letter in half does not produce a standard size. Two Letters side by side equal Tabloid (11 × 17), but the half-letter (5.5 × 8.5) is not a standard format.

A4 vs. Letter: The Critical Difference

The most important comparison in daily life is A4 versus US Letter, since these are the default document sizes in their respective regions.

Property	A4	US Letter
Width	210 mm (8.27”)	215.9 mm (8.5”)
Height	297 mm (11.69”)	279.4 mm (11”)
Area	62,370 mm²	60,322 mm²
Aspect ratio	1:1.414	1:1.294

Letter is slightly wider (by 6mm / 0.23 inches) and noticeably shorter (by 18mm / 0.69 inches) than A4. This difference causes real problems:

Printing A4 documents on Letter paper: Content may be cut off at the bottom. Margins may shift. Headers and footers positioned for A4 can disappear or overlap content.

Formatting for international audiences: If you send a PDF formatted for Letter to European recipients, their A4 printers may scale the content (making text slightly smaller) or add visible white space.

Solution: When creating documents for international distribution, use A4 with generous margins (at least 1 inch / 25mm all sides). This ensures content fits within both A4 and Letter printable areas without scaling.

Pixel Dimensions and DPI

For digital design and print preparation, you need pixel dimensions. The formula is:

Pixels = (mm × DPI) ÷ 25.4

Or equivalently: Pixels = inches × DPI

Standard DPI Values

72 DPI — Screen/web display. This was the original Macintosh screen resolution and remains the reference for web graphics. At 72 DPI, A4 = 595 × 842 pixels.

96 DPI — Windows screen reference. Some applications default to this. At 96 DPI, A4 = 794 × 1123 pixels.

150 DPI — Draft quality printing. Acceptable for internal documents, proofs, and large-format prints viewed from a distance. At 150 DPI, A4 = 1240 × 1754 pixels.

300 DPI — Standard print quality. The industry standard for professional documents, brochures, photos, and publications. At 300 DPI, A4 = 2480 × 3508 pixels. This is what most printers and print shops expect.

600 DPI — High-quality printing. Used for fine art reproduction, detailed technical drawings, and professional photography. At 600 DPI, A4 = 4960 × 7016 pixels. Files at this resolution are very large (100+ MB for full-color images).

Our paper size converter calculates pixel dimensions at all standard DPI values simultaneously, saving you from manual calculations for each resolution.

Choosing the Right Paper Size

For Documents and Reports

Use your region’s standard: A4 for international or European audiences, Letter for US/Canadian audiences. When uncertain, A4 with standard margins works acceptably on both.

For Posters and Presentations

• Small poster / handout: A3 (11.7 × 16.5”) or Tabloid (11 × 17”)
• Medium poster: A2 (16.5 × 23.4”)
• Large poster: A1 (23.4 × 33.1”)
• Conference poster: A0 (33.1 × 46.8”)

For Booklets and Brochures

A4 folded in half = A5 booklet (148 × 210 mm). Letter folded in half = 5.5 × 8.5” booklet. These are the most common brochure/booklet sizes in their respective markets.

For Business Cards

Standard US business card: 3.5 × 2 inches (88.9 × 50.8 mm). Standard European/international: 85 × 55 mm. Japanese: 91 × 55 mm. Always confirm the local standard for your market.

Print Design Best Practices

Bleed and Trim

Professional printing requires “bleed” — extending backgrounds and images 3mm (0.125”) beyond the trim edge. This ensures that when the paper is cut to final size, there are no unprinted white edges. Your design file should be:

• Final size + bleed on all sides
• A4 with 3mm bleed = 216 × 303 mm design area
• Include crop marks to guide the trimmer

Safe Zone (Margins)

Critical content (text, logos) should stay at least 5-10mm (0.25-0.5”) inside the trim edge. Content too close to edges risks being cut off due to minor trimming variations.

Color Mode

Print uses CMYK color (Cyan, Magenta, Yellow, Key/Black). Screen uses RGB. Always convert to CMYK for print files — colors shift noticeably between modes, particularly vibrant blues and greens that look different in CMYK.

File Formats for Print

• PDF/X-1a: Industry standard for print-ready files. Embeds fonts, flattens transparency, CMYK only.
• TIFF (300 DPI, CMYK): For image-heavy single pages.
• EPS: For vector graphics that scale to any size without quality loss.

Understanding Aspect Ratios

Aspect ratio describes the proportional relationship between width and height. It matters for scaling and cropping:

• A-series: 1:1.414 (all sizes identical ratio)
• US Letter: 1:1.294
• US Legal: 1:1.647
• Tabloid: 1:1.545

When scaling a design from one paper size to another with a different aspect ratio, you must either:

1. Crop content to fit the new ratio
2. Add white space (letterbox/pillarbox)
3. Distort the content (never recommended)

The A-series advantage: scaling between any A sizes requires no cropping or distortion. An A3 poster scales perfectly to an A4 handout — same content, half the size, no adjustments needed.

Paper Weight and Thickness

Paper weight is measured differently in different systems:

GSM (grams per square meter): International standard. Common weights:

• 80 gsm: Standard office/copy paper
• 100-120 gsm: Premium letterhead, presentations
• 160-200 gsm: Card stock, invitations
• 250-350 gsm: Business cards, postcards
• 400+ gsm: Heavy board, packaging

US Bond Weight: Measured based on 500 sheets at the paper’s “basic size.” Confusingly, the basic size varies by paper type, making direct comparison difficult. 20 lb bond ≈ 75 gsm (standard copy paper). 24 lb bond ≈ 90 gsm (nicer letterhead).

When specifying paper for printing, use GSM for clarity — it means the same thing regardless of paper type or size, making it the unambiguous global standard. Our paper size converter focuses on dimensional conversions while these weight specifications help complete your paper selection process for any printing project.