4 Inch to mm Conversion for Thermal Label Printers

When buying a 4-inch label printer or its label rolls, you may have noticed something confusing. Your thermal label paper might say “4 inches,” but your printer specs list a “print width of 108 mm.” If you've wondered, Wait—4 inches equals how many millimeters?, you're not alone.

Let's clear that up once and for all, and also explain what those 4-inch measurements mean in the world of label printers.

Quick Answer – 4 Inches to mm

The conversion is simple:

1 inch = 25.4 millimeters

The numbers below show the most common 4 inch to mm conversions you'll find in thermal label printing.

4 inches = 101.6 mm

4.125 inches = 104.775 mm

4.25 inches (or 4 1/4 inches) = 107.95 mm

4.75 inches (or 4 3/4 inches) = 120.65 mm

The 4-inch label printer is the most popular size today, particularly the 4-inch thermal shipping label printer. It perfectly fits standard 4×6-inch shipping labels and is widely used by e-commerce sellers, small businesses, and logistics carriers like FedEx, UPS, and Amazon.

Check the best shipping label printers for your business.

What “4-Inch” Really Means in Thermal Label Printers

Now that you know the math, here's the real-world part:

When a label printer is described as “4-inch,” it's not only about the label size, but also about the print width the printer can handle.

1. Print Width vs Media Width

• Print width = the maximum area the print head can actually print on.

• Media width = the total width of the label roll or paper the printer can physically hold.

When you look closely at the specifications of a 4-inch thermal printer, you'll often notice that the print width isn't exactly 4 inches — it's usually 4.09 inches or 4.25 inches, which converts to:

• 4.09 inches = 103.87 mm

• 4.25 inches (or 4 1/4 inches) = 107.95 mm

So a “4-inch thermal printer” typically:

• Prints up to 104 mm or 108 mm wide (effective print area)

• Compatible media width: around 108 mm, 112 mm, or even 118 mm (the total paper width, slightly wider than the print zone)

For example, if your printer's print width is 4.25 inches, it can print up to 108 mm wide—even if your label roll is a bit wider. The extra paper width won't expand the printable area.

2. Why the Print Width and Media Width Are Different

Why aren't print width and media width the same? It all comes down to mechanical design.

A printer's maximum print width depends on the number of heating dots on the printhead and the printer's DPI (dots per inch).

Meanwhile, media width depends on the feed path, guide rail width, and sensor placement.

The printhead is usually centered or slightly offset to ensure smooth media feeding. The printer also needs mechanical margins on both sides so that ribbons, liners, or label edges don't rub against the rails and cause jams.

There's more. Thermal printers use gap sensors or black mark sensors to detect label spacing and positioning. These sensors sit slightly off-center and require a small non-printable zone for accurate reading.

If the printhead extends beyond the paper's edge, it overheats—fast. Exposed heating elements can burn out over time.

That's why label printer manufacturers intentionally design the actual print width to be slightly smaller than the total paper width. It keeps the printhead covered, allows proper heat dissipation, and dramatically extends its lifespan.

Ready to find the right printer for your labels?

Explore Hanin's line of direct thermal and thermal transfer label printers—trusted by e-commerce sellers and logistics teams worldwide for consistent, high-speed performance.