What the checker actually checks
The tool takes two numbers: the pixel dimensions of your image file and the finished size you want to print it at. From those it works out the resolution in dots per inch (dpi), which is the number of printed dots packed into each inch of the finished magnet. That single figure tells you whether the image will look sharp at that size or whether it will look soft, blurred or visibly blocky once it comes off the press.
It is a calculation, not a scan of the image itself. It cannot see whether the photo is in focus, whether the colours are right, or whether someone has cropped a head out of the frame. It can only tell you whether there are enough pixels for the size you have asked for.
What resolution means for a printed magnet
Dpi measures how finely an image is printed, and 300 dpi is the figure most UK print firms treat as the minimum for anything viewed close up, which a fridge magnet always is. Below that, individual pixels start to become visible as soft squares, especially along edges and text.
A photo saved at 72 dpi, the standard for web images, is not undersized because of some flaw in the file. It simply was not built with print in mind: 72 dpi is enough detail for a screen but not enough for paper. The checker exists because this catches people out constantly, because a photo can look perfectly sharp on a phone and still fail at magnet size.
A worked example
Take a photo that is 1200 pixels wide, a common size for an image saved from a phone gallery or downloaded from social media. Printed at 50 x 50 mm, a small square promotional magnet, that file works out to roughly 610 dpi, comfortably above the 300 dpi threshold. Print the same file at A5, 148 x 210 mm, and it drops to around 206 dpi, below the line, and the image will look softer than it did on screen.
Pixel count is what decides it
A large file in megabytes does not necessarily mean a high-resolution image. A photo can be 8 MB and still only 800 pixels wide, if it has been saved with heavy colour depth or minimal compression. What the checker reads is the pixel dimensions, usually reported as width times height, such as 3000 x 2000. Check that figure in your image editor or file properties before you upload anything, because it is the number that decides the outcome.
Bleed and safe area: what a resolution figure cannot show you
Resolution tells you whether an image is sharp enough. It says nothing about whether the right part of the image will actually end up on the finished magnet, and that is a separate problem with its own vocabulary.
Bleed is the printed area that extends beyond the finished edge of the magnet, so that small variations in trimming do not leave a thin white line around the border. Most magnet printers ask for 2 to 3 mm of bleed on each side, though this varies by supplier, so confirm the figure with your printer before you set up the file.
Safe area is the opposite: an inset zone, usually a similar 2 to 3 mm from the edge, within which no text, logo or face should sit, because that is the area most at risk from any trimming drift. A phone number set 1 mm from the edge of the artwork can end up cut clean off, even though the file itself was sharp enough to print.
A resolution checker has no way to see either of these, because bleed and safe area are about layout. An image can pass the resolution check completely and still print with a chopped-off logo if the safe area was never respected.
Typical pixel requirements at 300 dpi
The table below gives the pixel dimensions needed to hit 300 dpi at some common magnet sizes, for reference before you check your own artwork.
| Finished size | Pixels needed at 300 dpi |
|---|---|
| 50 x 50 mm (button magnet) | approx. 590 x 590 px |
| 90 x 55 mm (business card magnet) | approx. 1063 x 650 px |
| 148 x 210 mm (A5) | approx. 1748 x 2480 px |
These figures are for the finished size only and do not include bleed, so add the equivalent pixels for whatever bleed your printer specifies before you treat a file as ready.
If your artwork checks below 300 dpi
- Confirm the pixel dimensions of the original file, since resizing upward never adds real detail.
- Reduce the print size if the image cannot supply enough pixels at the size you first wanted.
- Ask your printer whether their process tolerates a lower figure, since some large-format or simpler designs print acceptably below 300 dpi where a photo would not.
- Source a higher-resolution original where one exists.
What the checker cannot tell you
The result is a calculation based on the numbers you enter, and it can only be as accurate as those numbers. It will not catch a wrongly cropped image, a colour that will shift on a different printer's press, or a file saved in a colour mode your printer cannot use. Treat a pass on the checker as one confirmed fact, not a guarantee that the artwork is ready to send.
For the layout side of things, bleed, safe area and how to specify a job to a printer, see Ordering Printed Promotional Magnets: What to Specify. If you are working from a photo for a personal or gift magnet rather than a promotional run, Custom and Photo Magnets: Getting One You Will Still Like in a Year covers what makes a photo file suitable in the first place. To run your own file through the checks described here, use the Artwork Size and Resolution Checker.