Seal Size and Resolution Guide: Pixels, Millimeters, and DPI

The seal looked crisp in the editor. In the Word contract it is the size of a coaster. You shrink it, print the page, and the bottom text has turned into a pink smudge. Or the opposite: it is perfectly sized on screen, and the printed copy shows a soft, fuzzy ring that looks like a photocopy of a photocopy.

None of this is a design problem. It is a mismatch between three measurements, pixels, millimeters, and DPI, and a misunderstanding about which one each program pays attention to. Once you know which number controls which stage, sizing becomes predictable.

Pixels, millimeters, and DPI do different jobs

Pixels (px) describe how much detail a PNG contains. More pixels let the image stay crisp at a larger displayed or printed size.
Millimeters (mm) describe the physical size you want on paper.
DPI (dots per inch) connects the two. It tells print software how densely to place the pixels. It does not add detail that is not in the file.

The conversion is:

pixels = millimeters ÷ 25.4 × DPI

A 40 mm seal at 300 DPI needs about 472 pixels across. A 20 mm mark at the same resolution needs about 236. Rounding by a pixel or two is harmless.

Start from the physical size

Most sizing mistakes happen because the physical size was never decided. Before exporting, find out how large the seal should be on the page. If a recipient, printer, bank form, or internal guideline specifies a size, use it. Otherwise, these are common real-world sizes to start from:

| Seal type | Common physical size | |---|---| | Company seal, round (China) | about 40–42 mm diameter | | Invoice seal, oval (China) | about 40 × 30 mm | | Legal representative seal, square (China) | about 18 × 18 mm | | Japanese square company seal (角印) | roughly 18–30 mm square | | Japanese registered personal seal (実印) | about 13.5–18 mm | | Western embossed corporate seal | about 41–51 mm (1⅝–2 in) diameter | | Office rubber stamp, rectangular | about 50–60 × 20–30 mm | | Small approval or date stamp | about 30–40 mm wide |

These are typical ranges, not legal requirements. Registered seals in some countries do have rules; Japan's corporate registry, for instance, sets size limits for a company's registered seal. The regional guides go into local conventions.

If you design in the seal generator, switch the size control to mm + DPI and enter the physical size. The editor shows the resulting pixel dimensions, and its presets use common physical sizes for each seal type.

Choose the DPI for the output

300 DPI is the practical default for anything printed on an office printer or sent as a PDF that might be printed.
150 DPI is often enough for documents that will only ever be viewed on screen, and keeps files smaller.
600 DPI or vector artwork is worth it for very small seals with fine detail, or for professional print. Ask the printer what they need.

Higher is not automatically better. A 40 mm seal at 1200 DPI is nearly 1900 pixels across, which bloats every document it is placed in and gains nothing visible on an office printer.

Place it at an exact size, not by eye

This is the step most people skip. Image files can carry a DPI value in their metadata, but applications do not interpret it consistently. Word may place a 472-pixel image at one size, Google Docs at another, and a PDF editor at a third. Do not trust the size an image lands at. Set it.

Word: right-click › Size and Position, lock the aspect ratio, and type the width, for example 4 cm. See how to add a seal to a PDF or Word document.
Google Docs: Image options › Size & Rotation. Docs works in inches, so convert: 40 mm ≈ 1.57 in.
PDF tools: many work in points, where 1 point = 1/72 inch. A 40 mm seal is about 113 points wide. The PDF stamp tool offers standard physical sizes directly.
Layout software (InDesign, Affinity, Canva): set the frame width in millimeters.

Shortcut: the generator can also export a PDF or Word file that already contains the seal at its true physical size, which is handy when you want to copy it into another document without measuring.

Portals, forms, and file-size limits

Online forms that ask for a seal image usually care about pixel dimensions and file size, not millimeters. Typical instructions look like "PNG or JPG, under 200 KB, at least 300 × 300 pixels." Read the instructions rather than guessing:

If a minimum pixel size is given, export at least that large.
If a maximum file size is given, a transparent PNG of a simple seal is usually small enough at 300 to 600 pixels. If it is not, reduce the pixel dimensions slightly rather than converting to a heavily compressed JPG.
If the portal shows a preview, check that the text is still readable in it. That preview is often what the reviewer sees.

The guide to preparing a seal image for online forms covers the rest of the checklist.

A simple workflow for documents and print

Decide the finished size in millimeters, or in pixels for screen-only use.
Set millimeters and DPI before fine-tuning the design, because very small output can make thin lines and small text unreadable, and you will want to adjust for that.
Export with enough pixels. PNG suits office documents; SVG stays sharp at any size when the destination accepts vector artwork. SVG vs. PNG for seals explains the trade-offs.
Place it at the intended physical size, set explicitly in the application.
Check at 100% and print a proof on the printer that will actually be used.

Common sizing mistakes

Enlarging a small PNG in the document. It gets bigger but not sharper. Re-export at a larger pixel size instead.
Using a high-resolution export without changing the placed size. A 4× file is only useful if the document keeps the same physical width; otherwise the seal simply appears four times larger.
Treating DPI as quality by itself. A 160-pixel image labeled 600 DPI still contains only 160 pixels.
Printing with "Fit to page." Many print dialogs shrink the whole page slightly to fit printer margins, which shrinks the seal too. Print at 100% or "Actual size" when the physical size matters.
Judging from a zoomed preview. Lines that look fine at 300% zoom can disappear or fill in at actual size.
Adding a white box around the seal. For overlays, use a transparent background, as described in how to create a transparent PNG seal.

When the document will be scanned or faxed

A sealed document that gets printed, signed, and scanned back loses detail at each step. Scanners at 200 DPI, common for office multifunction devices, will blur thin rings and small text. If you know a document will make that round trip, choose a slightly larger seal, bolder lines, and fewer small elements. Test by printing and scanning one copy before you send the batch.

Quick pre-export checklist

Final physical size is known, or the pixel size for screen-only use.
PNG pixel dimensions are large enough for that size at 300 DPI.
Text and borders remain readable at actual size.
Transparent background is on when the seal must overlap content.
The destination accepts the chosen format and file size.
A proof has been checked in the real document and printed at 100%.

Size is part of the design. Set it early, place it exactly, and check it at the final scale, and most blurry and oversized seal problems never happen.