The Kuretake Fude Brush Pen Extra Fine (LS4-10S) gives you a bit of flexibility when it comes to line width, depending on how much pressure you use and your technique. The manufacturer describes it as 'Extra Fine,' so you can expect thin, precise strokes. But exact measurements? Not really available. It's the nature of brush pens, where line thickness can vary a bit.

The pen itself is flexible enough to go from very fine lines to slightly thicker ones if you apply more pressure. Our research suggests that folks like artists and calligraphers dig it for detailed work and precision line illustrations.

It's worth noting the pen isn’t waterproof. It works best for dry applications, offering clean and consistent lines that's great for sketching and writing.

Just keep in mind that the water-based ink can bleed if it gets wet, which makes it a poor fit for water-based art projects. If you’re working with watercolors or need something for a wetter environment, you might need a different pen—one that's specifically waterproof.

In essence, the Kuretake Fude Brush Pen Extra Fine is fantastic for dry, precise work. Whether you're into illustrating, sketching, or calligraphy, this pen handles dry media beautifully.