A4 vs Letter Paper Size

A4 vs Letter: Exact Dimensions

A4 paper measures 210 × 297 millimeters, or approximately 8.2677 × 11.6929 inches. US Letter measures exactly 8.5 × 11 inches, or 215.9 × 279.4 millimeters. The exact metric conversion for Letter uses 25.4 millimeters per inch. You may see 216 × 279 mm in printer documentation because those values are rounded; use the exact dimensions when calculating tiled prints.

A4 belongs to the A series of paper sizes. The series is designed around a square-root-of-two shape, with small rounding differences in the millimeter dimensions. Cutting an A3 sheet in half yields two A4 sheets; cutting A4 in half yields A5. Letter uses its own proportions, so cutting a Letter sheet in half does not create an A-series size.

In everyday use, Letter is common in the United States and Canada, while A4 is widely used elsewhere. Local offices, imported printers, and schools may use either, so geography is a clue rather than a setting to assume. Check the pack of paper and the printer tray. For an example of manufacturer-supported sizes, see Epson’s paper specifications; its converted dimensions are rounded.

Which Is Bigger — and By How Much?

A4 is 17.6 mm taller than Letter. That is about 0.693 inches, enough to matter for a footer, a full-page diagram, or the bottom of a poster tile. Letter is 5.9 mm wider than A4, about 0.232 inches. A design that reaches both sides of a Letter page therefore extends beyond the width of A4 when printed without scaling.

By area, A4 is larger: 62,370 square millimeters compared with 60,322.46 for Letter. The difference is 2,047.54 square millimeters, approximately 3.39% of Letter’s area. But area alone is not a useful fit test. A4’s extra height cannot compensate for its narrower width when you need to print a wide diagram. Neither sheet completely contains the other in the same orientation.

The distinction also explains why “these sizes are almost the same” can lead to a disappointing print. An office document with wide margins may transfer easily. A measured craft pattern, edge-to-edge photograph, or tiled poster is much less forgiving. Even a small automatic reduction can accumulate across several rows or columns.

Does It Matter When Printing a Poster at Home?

Yes, because every tile has physical dimensions. A PDF created for A4 expects a taller sheet than Letter. If your viewer shrinks each A4 page to fit Letter, the whole poster becomes smaller than the size shown in the tool. If it keeps the page at 100%, some content may extend beyond the printable area. Neither outcome is a reliable way to preserve a planned poster size.

Choose your actual paper before generating the PDF. PosterSheet subtracts the selected printer margin from both edges of every sheet, then accounts for the repeated overlap between neighboring tiles. Changing from A4 to Letter changes both the available width and the available height. The grid may need more rows, fewer columns, or the same count with different amounts of unused space.

A concrete six-sheet example

With portrait paper, 7 mm margins, and 10 mm overlap, a 3-column by 2-row A4 grid makes a 568 × 556 mm canvas. The same grid using Letter makes a 585.7 × 520.8 mm canvas. Letter makes this poster wider and shorter, even though the sheet count stays at six. The image is fitted inside that canvas without stretching; white space depends on its proportions.

In automatic size mode, the situation is different. If you request a 600 × 900 mm poster, the engine keeps that finished size and adjusts the grid to cover it. Final-row and final-column sheets may only be partly used. This is the right mode when you need a specific physical size. You can compare common options in the standard poster sizes chart.

Margins are separate from paper size

A printer usually cannot place ink across every millimeter of a sheet unless it supports the appropriate borderless mode. The printable area depends on the printer, stock, feed method, and settings. A 7 mm margin is a practical starting point, not a promise that every machine supports it. Increase it if a test print clips the image. Changing margin settings changes the tiling calculation, so export again after making that adjustment.

How to Print an A4 File on Letter Paper (and Vice Versa)

For an ordinary one-page document, selecting Fit in your PDF viewer often produces a readable result. The viewer scales the complete page into the available area and may add white space. This is suitable when exact measurements do not matter, such as reading a report or reviewing a draft. Check the preview for page breaks, tiny text, and content near the edges before printing a large batch.

Ignoring printer margins, fitting an A4 page onto portrait Letter requires reducing it to about 94.07%, because Letter’s height is the limiting dimension: 279.4 divided by 297. Fitting Letter onto portrait A4 requires about 97.27%, because A4’s width is the limiting dimension: 210 divided by 215.9. A real printer’s margins can require a greater reduction. These percentages describe full-page geometry, not a universal print-dialog value.

For a tiled poster or a template that must measure correctly, regenerate the document for the paper you have instead. Return to the poster tool, choose the correct paper, check the new layout, and download another PDF. Then print at Actual Size or 100%. Do not mix A4 and Letter tiles from different exports in the same assembly, because their positions and overlap may differ.

A short print-dialog checklist

1. Check the physical stock in the tray and choose that same paper in the tool.
2. Set the PDF viewer’s paper size to match the exported pages.
3. Select one PDF page per physical sheet and disable duplex.
4. For a tiled poster, choose Actual Size or 100%, with automatic Fit disabled.
5. Print one tile, check its edges and a known measurement, then print the remaining poster pages.

The optional assembly guide is a separate A4 page at the end of the poster PDF. You can view it on screen instead of printing it. If your poster uses Letter, choose only the poster-page range for the Letter print run. The guide’s 50 mm check line is useful only when the guide itself is printed at 100%; fitting that page changes the line’s length.

A4 vs Letter: Frequently Asked Practical Questions

Can I put A4 paper in a Letter printer?

Some printers accept both sizes; others have limits on feed length, tray settings, or supported stock. Check the model’s manual. Adjust the tray guides to hold the paper without bending it, and select the matching size in the printer driver. A physical sheet fitting in a tray does not by itself establish that the selected print mode supports it.

Does landscape make them interchangeable?

No. Rotating each sheet swaps width and height but preserves the same differences. Landscape A4 is 297 × 210 mm; landscape Letter is 279.4 × 215.9 mm. One is wider and the other taller. Landscape can make a particular image use fewer tiles, but it does not turn A4 into Letter.

Is Letter the same as Legal?

No. US Legal is 8.5 × 14 inches, while Letter is 8.5 × 11. They share a width but differ by three inches in height. If your printer supports Legal, you can enter 215.9 × 355.6 mm as custom paper in PosterSheet. Match the PDF and print driver to that stock rather than selecting Letter because the widths happen to match.

Which should I choose for a photo poster?

Start with what your printer supports and what you already have. A4 is not automatically better for photos, and Letter is not automatically cheaper in every location. Resolution, paper finish, correct print scale, and careful seam alignment have a larger practical effect than choosing one of these two formats. Use the live grid to compare your actual image and see the finished dimensions before downloading.

Keep the downloadable A4 vs Letter comparison chart near your printer, and use the step-by-step multi-page printing guide for assembly. The chart is a dimension reference, not a life-size sheet template.