What forms usually ask for
Signature requirements come in three shapes. Some state pixel dimensions — a width and height the field expects. Some state a physical size such as a number of centimetres, which only matters if the signature will be printed. Some state nothing about dimensions at all and only enforce a byte limit, leaving you to choose a size that looks right in the box.
A physical size in millimetres only converts to pixels once a DPI is chosen — at 300 DPI, 35 mm is roughly 413 pixels. See DPI and pixels, DPI and print size if your requirement is given in millimetres.
Preparing the signature
- Get the cleanest source you can: flatbed scan, or a photo of the signature on plain white paper in even light with no shadow across it. A drawn signature from Signature Maker avoids the capture problem entirely.
- Crop tight around the signature with Crop Image. Tight cropping means fewer background pixels to compress, which directly helps any byte limit.
- Resize to the stated dimensions with Signature Resizer. Use fit-box or a single edge where possible so the aspect ratio is preserved; force exact dimensions only when the form demands both.
- If a byte limit applies, set it as a maximum in the same tool — it re-encodes until it fits while keeping the pixel size you chose.
- Check the result at 100% zoom for broken or thickened strokes, then verify format and size with Image Metadata.
PNG or JPEG?
- **PNG** when there is no byte limit, or when the signature must sit over other content. It is lossless, it keeps transparency, and thin strokes stay exactly as drawn.
- **JPEG** when a byte limit makes PNG impossible. Keep the quality high — thin strokes are the first casualty of lossy compression, as explained in why signatures go blurry.
- **Never JPEG if transparency matters.** JPEG has no alpha channel; a cut-out signature saved as JPEG gets a solid background and will show as a white or black block when placed on a form.
Using the signature on a document
When the signature belongs on a PDF rather than in an image field, keep it as a transparent PNG and place it with Sign a PDF — pick the page, position it left, centre or right, and add a name or date underneath if the document calls for one. If the signature needs to go onto many documents, prepare it once at a good size and reuse the same file rather than reprocessing it each time.
- Sign on a flat surface with a pen that does not bleed; feathered edges compress badly.
- Sign slightly larger than needed — reducing is safe, enlarging is not.
- Keep one master copy unmodified, and work only on derived copies.
Common questions
What if the form states a size in centimetres?
If the signature will only ever be displayed on screen, choose pixel dimensions that look correct in the box at normal zoom. If it will be printed at that physical size, convert using the DPI the document uses — 300 DPI is the usual print target.
Should the signature have a white background or a transparent one?
Transparent PNG if it will be placed over other content. White is fine when the form field itself has a white background. The one to avoid is JPEG with transparency already flattened into an unexpected colour.
Why does my signature look smaller than the box after upload?
The form scales images to fit its field. Choose dimensions with the correct aspect ratio and the box will fill sensibly; choose a mismatched ratio and it will letterbox or crop.